kernel/trace/ring_buffer.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Generic ring buffer
 *
 * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
 */
#include <linux/trace_recursion.h>
#include <linux/trace_events.h>
#include <linux/ring_buffer.h>
#include <linux/trace_clock.h>
#include <linux/sched/clock.h>
#include <linux/cacheflush.h>
#include <linux/trace_seq.h>
#include <linux/spinlock.h>
#include <linux/irq_work.h>
#include <linux/security.h>
#include <linux/uaccess.h>
#include <linux/hardirq.h>
#include <linux/kthread.h>	/* for self test */
#include <linux/module.h>
#include <linux/percpu.h>
#include <linux/mutex.h>
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/hash.h>
#include <linux/list.h>
#include <linux/cpu.h>
#include <linux/oom.h>
#include <linux/mm.h>

#include <asm/local64.h>
#include <asm/local.h>

/*
 * The "absolute" timestamp in the buffer is only 59 bits.
 * If a clock has the 5 MSBs set, it needs to be saved and
 * reinserted.
 */
#define TS_MSB		(0xf8ULL << 56)
#define ABS_TS_MASK	(~TS_MSB)

static void update_pages_handler(struct work_struct *work);

/*
 * The ring buffer header is special. We must manually up keep it.
 */
int ring_buffer_print_entry_header(struct trace_seq *s)
{
	trace_seq_puts(s, "# compressed entry header\n");
	trace_seq_puts(s, "\ttype_len    :    5 bits\n");
	trace_seq_puts(s, "\ttime_delta  :   27 bits\n");
	trace_seq_puts(s, "\tarray       :   32 bits\n");
	trace_seq_putc(s, '\n');
	trace_seq_printf(s, "\tpadding     : type == %d\n",
			 RINGBUF_TYPE_PADDING);
	trace_seq_printf(s, "\ttime_extend : type == %d\n",
			 RINGBUF_TYPE_TIME_EXTEND);
	trace_seq_printf(s, "\ttime_stamp : type == %d\n",
			 RINGBUF_TYPE_TIME_STAMP);
	trace_seq_printf(s, "\tdata max type_len  == %d\n",
			 RINGBUF_TYPE_DATA_TYPE_LEN_MAX);

	return !trace_seq_has_overflowed(s);
}

/*
 * The ring buffer is made up of a list of pages. A separate list of pages is
 * allocated for each CPU. A writer may only write to a buffer that is
 * associated with the CPU it is currently executing on.  A reader may read
 * from any per cpu buffer.
 *
 * The reader is special. For each per cpu buffer, the reader has its own
 * reader page. When a reader has read the entire reader page, this reader
 * page is swapped with another page in the ring buffer.
 *
 * Now, as long as the writer is off the reader page, the reader can do what
 * ever it wants with that page. The writer will never write to that page
 * again (as long as it is out of the ring buffer).
 *
 * Here's some silly ASCII art.
 *
 *   +------+
 *   |reader|          RING BUFFER
 *   |page  |
 *   +------+        +---+   +---+   +---+
 *                   |   |-->|   |-->|   |
 *                   +---+   +---+   +---+
 *                     ^               |
 *                     |               |
 *                     +---------------+
 *
 *
 *   +------+
 *   |reader|          RING BUFFER
 *   |page  |------------------v
 *   +------+        +---+   +---+   +---+
 *                   |   |-->|   |-->|   |
 *                   +---+   +---+   +---+
 *                     ^               |
 *                     |               |
 *                     +---------------+
 *
 *
 *   +------+
 *   |reader|          RING BUFFER
 *   |page  |------------------v
 *   +------+        +---+   +---+   +---+
 *      ^            |   |-->|   |-->|   |
 *      |            +---+   +---+   +---+
 *      |                              |
 *      |                              |
 *      +------------------------------+
 *
 *
 *   +------+
 *   |buffer|          RING BUFFER
 *   |page  |------------------v
 *   +------+        +---+   +---+   +---+
 *      ^            |   |   |   |-->|   |
 *      |   New      +---+   +---+   +---+
 *      |  Reader------^               |
 *      |   page                       |
 *      +------------------------------+
 *
 *
 * After we make this swap, the reader can hand this page off to the splice
 * code and be done with it. It can even allocate a new page if it needs to
 * and swap that into the ring buffer.
 *
 * We will be using cmpxchg soon to make all this lockless.
 *
 */

/* Used for individual buffers (after the counter) */
#define RB_BUFFER_OFF		(1 << 20)

#define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)

#define RB_EVNT_HDR_SIZE (offsetof(struct ring_buffer_event, array))
#define RB_ALIGNMENT		4U
#define RB_MAX_SMALL_DATA	(RB_ALIGNMENT * RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
#define RB_EVNT_MIN_SIZE	8U	/* two 32bit words */

#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
# define RB_FORCE_8BYTE_ALIGNMENT	0
# define RB_ARCH_ALIGNMENT		RB_ALIGNMENT
#else
# define RB_FORCE_8BYTE_ALIGNMENT	1
# define RB_ARCH_ALIGNMENT		8U
#endif

#define RB_ALIGN_DATA		__aligned(RB_ARCH_ALIGNMENT)

/* define RINGBUF_TYPE_DATA for 'case RINGBUF_TYPE_DATA:' */
#define RINGBUF_TYPE_DATA 0 ... RINGBUF_TYPE_DATA_TYPE_LEN_MAX

enum {
	RB_LEN_TIME_EXTEND = 8,
	RB_LEN_TIME_STAMP =  8,
};

#define skip_time_extend(event) \
	((struct ring_buffer_event *)((char *)event + RB_LEN_TIME_EXTEND))

#define extended_time(event) \
	(event->type_len >= RINGBUF_TYPE_TIME_EXTEND)

static inline bool rb_null_event(struct ring_buffer_event *event)
{
	return event->type_len == RINGBUF_TYPE_PADDING && !event->time_delta;
}

static void rb_event_set_padding(struct ring_buffer_event *event)
{
	/* padding has a NULL time_delta */
	event->type_len = RINGBUF_TYPE_PADDING;
	event->time_delta = 0;
}

static unsigned
rb_event_data_length(struct ring_buffer_event *event)
{
	unsigned length;

	if (event->type_len)
		length = event->type_len * RB_ALIGNMENT;
	else
		length = event->array[0];
	return length + RB_EVNT_HDR_SIZE;
}

/*
 * Return the length of the given event. Will return
 * the length of the time extend if the event is a
 * time extend.
 */
static inline unsigned
rb_event_length(struct ring_buffer_event *event)
{
	switch (event->type_len) {
	case RINGBUF_TYPE_PADDING:
		if (rb_null_event(event))
			/* undefined */
			return -1;
		return  event->array[0] + RB_EVNT_HDR_SIZE;

	case RINGBUF_TYPE_TIME_EXTEND:
		return RB_LEN_TIME_EXTEND;

	case RINGBUF_TYPE_TIME_STAMP:
		return RB_LEN_TIME_STAMP;

	case RINGBUF_TYPE_DATA:
		return rb_event_data_length(event);
	default:
		WARN_ON_ONCE(1);
	}
	/* not hit */
	return 0;
}

/*
 * Return total length of time extend and data,
 *   or just the event length for all other events.
 */
static inline unsigned
rb_event_ts_length(struct ring_buffer_event *event)
{
	unsigned len = 0;

	if (extended_time(event)) {
		/* time extends include the data event after it */
		len = RB_LEN_TIME_EXTEND;
		event = skip_time_extend(event);
	}
	return len + rb_event_length(event);
}

/**
 * ring_buffer_event_length - return the length of the event
 * @event: the event to get the length of
 *
 * Returns the size of the data load of a data event.
 * If the event is something other than a data event, it
 * returns the size of the event itself. With the exception
 * of a TIME EXTEND, where it still returns the size of the
 * data load of the data event after it.
 */
unsigned ring_buffer_event_length(struct ring_buffer_event *event)
{
	unsigned length;

	if (extended_time(event))
		event = skip_time_extend(event);

	length = rb_event_length(event);
	if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
		return length;
	length -= RB_EVNT_HDR_SIZE;
	if (length > RB_MAX_SMALL_DATA + sizeof(event->array[0]))
                length -= sizeof(event->array[0]);
	return length;
}
EXPORT_SYMBOL_GPL(ring_buffer_event_length);

/* inline for ring buffer fast paths */
static __always_inline void *
rb_event_data(struct ring_buffer_event *event)
{
	if (extended_time(event))
		event = skip_time_extend(event);
	WARN_ON_ONCE(event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX);
	/* If length is in len field, then array[0] has the data */
	if (event->type_len)
		return (void *)&event->array[0];
	/* Otherwise length is in array[0] and array[1] has the data */
	return (void *)&event->array[1];
}

/**
 * ring_buffer_event_data - return the data of the event
 * @event: the event to get the data from
 */
void *ring_buffer_event_data(struct ring_buffer_event *event)
{
	return rb_event_data(event);
}
EXPORT_SYMBOL_GPL(ring_buffer_event_data);

#define for_each_buffer_cpu(buffer, cpu)		\
	for_each_cpu(cpu, buffer->cpumask)

#define for_each_online_buffer_cpu(buffer, cpu)		\
	for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask)

#define TS_SHIFT	27
#define TS_MASK		((1ULL << TS_SHIFT) - 1)
#define TS_DELTA_TEST	(~TS_MASK)

static u64 rb_event_time_stamp(struct ring_buffer_event *event)
{
	u64 ts;

	ts = event->array[0];
	ts <<= TS_SHIFT;
	ts += event->time_delta;

	return ts;
}

/* Flag when events were overwritten */
#define RB_MISSED_EVENTS	(1 << 31)
/* Missed count stored at end */
#define RB_MISSED_STORED	(1 << 30)

#define RB_MISSED_MASK		(3 << 30)

struct buffer_data_page {
	u64		 time_stamp;	/* page time stamp */
	local_t		 commit;	/* write committed index */
	unsigned char	 data[] RB_ALIGN_DATA;	/* data of buffer page */
};

struct buffer_data_read_page {
	unsigned		order;	/* order of the page */
	struct buffer_data_page	*data;	/* actual data, stored in this page */
};

/*
 * Note, the buffer_page list must be first. The buffer pages
 * are allocated in cache lines, which means that each buffer
 * page will be at the beginning of a cache line, and thus
 * the least significant bits will be zero. We use this to
 * add flags in the list struct pointers, to make the ring buffer
 * lockless.
 */
struct buffer_page {
	struct list_head list;		/* list of buffer pages */
	local_t		 write;		/* index for next write */
	unsigned	 read;		/* index for next read */
	local_t		 entries;	/* entries on this page */
	unsigned long	 real_end;	/* real end of data */
	unsigned	 order;		/* order of the page */
	u32		 id;		/* ID for external mapping */
	struct buffer_data_page *page;	/* Actual data page */
};

/*
 * The buffer page counters, write and entries, must be reset
 * atomically when crossing page boundaries. To synchronize this
 * update, two counters are inserted into the number. One is
 * the actual counter for the write position or count on the page.
 *
 * The other is a counter of updaters. Before an update happens
 * the update partition of the counter is incremented. This will
 * allow the updater to update the counter atomically.
 *
 * The counter is 20 bits, and the state data is 12.
 */
#define RB_WRITE_MASK		0xfffff
#define RB_WRITE_INTCNT		(1 << 20)

static void rb_init_page(struct buffer_data_page *bpage)
{
	local_set(&bpage->commit, 0);
}

static __always_inline unsigned int rb_page_commit(struct buffer_page *bpage)
{
	return local_read(&bpage->page->commit);
}

static void free_buffer_page(struct buffer_page *bpage)
{
	free_pages((unsigned long)bpage->page, bpage->order);
	kfree(bpage);
}

/*
 * We need to fit the time_stamp delta into 27 bits.
 */
static inline bool test_time_stamp(u64 delta)
{
	return !!(delta & TS_DELTA_TEST);
}

struct rb_irq_work {
	struct irq_work			work;
	wait_queue_head_t		waiters;
	wait_queue_head_t		full_waiters;
	atomic_t			seq;
	bool				waiters_pending;
	bool				full_waiters_pending;
	bool				wakeup_full;
};

/*
 * Structure to hold event state and handle nested events.
 */
struct rb_event_info {
	u64			ts;
	u64			delta;
	u64			before;
	u64			after;
	unsigned long		length;
	struct buffer_page	*tail_page;
	int			add_timestamp;
};

/*
 * Used for the add_timestamp
 *  NONE
 *  EXTEND - wants a time extend
 *  ABSOLUTE - the buffer requests all events to have absolute time stamps
 *  FORCE - force a full time stamp.
 */
enum {
	RB_ADD_STAMP_NONE		= 0,
	RB_ADD_STAMP_EXTEND		= BIT(1),
	RB_ADD_STAMP_ABSOLUTE		= BIT(2),
	RB_ADD_STAMP_FORCE		= BIT(3)
};
/*
 * Used for which event context the event is in.
 *  TRANSITION = 0
 *  NMI     = 1
 *  IRQ     = 2
 *  SOFTIRQ = 3
 *  NORMAL  = 4
 *
 * See trace_recursive_lock() comment below for more details.
 */
enum {
	RB_CTX_TRANSITION,
	RB_CTX_NMI,
	RB_CTX_IRQ,
	RB_CTX_SOFTIRQ,
	RB_CTX_NORMAL,
	RB_CTX_MAX
};

struct rb_time_struct {
	local64_t	time;
};
typedef struct rb_time_struct rb_time_t;

#define MAX_NEST	5

/*
 * head_page == tail_page && head == tail then buffer is empty.
 */
struct ring_buffer_per_cpu {
	int				cpu;
	atomic_t			record_disabled;
	atomic_t			resize_disabled;
	struct trace_buffer	*buffer;
	raw_spinlock_t			reader_lock;	/* serialize readers */
	arch_spinlock_t			lock;
	struct lock_class_key		lock_key;
	struct buffer_data_page		*free_page;
	unsigned long			nr_pages;
	unsigned int			current_context;
	struct list_head		*pages;
	struct buffer_page		*head_page;	/* read from head */
	struct buffer_page		*tail_page;	/* write to tail */
	struct buffer_page		*commit_page;	/* committed pages */
	struct buffer_page		*reader_page;
	unsigned long			lost_events;
	unsigned long			last_overrun;
	unsigned long			nest;
	local_t				entries_bytes;
	local_t				entries;
	local_t				overrun;
	local_t				commit_overrun;
	local_t				dropped_events;
	local_t				committing;
	local_t				commits;
	local_t				pages_touched;
	local_t				pages_lost;
	local_t				pages_read;
	long				last_pages_touch;
	size_t				shortest_full;
	unsigned long			read;
	unsigned long			read_bytes;
	rb_time_t			write_stamp;
	rb_time_t			before_stamp;
	u64				event_stamp[MAX_NEST];
	u64				read_stamp;
	/* pages removed since last reset */
	unsigned long			pages_removed;

	unsigned int			mapped;
	struct mutex			mapping_lock;
	unsigned long			*subbuf_ids;	/* ID to subbuf VA */
	struct trace_buffer_meta	*meta_page;

	/* ring buffer pages to update, > 0 to add, < 0 to remove */
	long				nr_pages_to_update;
	struct list_head		new_pages; /* new pages to add */
	struct work_struct		update_pages_work;
	struct completion		update_done;

	struct rb_irq_work		irq_work;
};

struct trace_buffer {
	unsigned			flags;
	int				cpus;
	atomic_t			record_disabled;
	atomic_t			resizing;
	cpumask_var_t			cpumask;

	struct lock_class_key		*reader_lock_key;

	struct mutex			mutex;

	struct ring_buffer_per_cpu	**buffers;

	struct hlist_node		node;
	u64				(*clock)(void);

	struct rb_irq_work		irq_work;
	bool				time_stamp_abs;

	unsigned int			subbuf_size;
	unsigned int			subbuf_order;
	unsigned int			max_data_size;
};

struct ring_buffer_iter {
	struct ring_buffer_per_cpu	*cpu_buffer;
	unsigned long			head;
	unsigned long			next_event;
	struct buffer_page		*head_page;
	struct buffer_page		*cache_reader_page;
	unsigned long			cache_read;
	unsigned long			cache_pages_removed;
	u64				read_stamp;
	u64				page_stamp;
	struct ring_buffer_event	*event;
	size_t				event_size;
	int				missed_events;
};

int ring_buffer_print_page_header(struct trace_buffer *buffer, struct trace_seq *s)
{
	struct buffer_data_page field;

	trace_seq_printf(s, "\tfield: u64 timestamp;\t"
			 "offset:0;\tsize:%u;\tsigned:%u;\n",
			 (unsigned int)sizeof(field.time_stamp),
			 (unsigned int)is_signed_type(u64));

	trace_seq_printf(s, "\tfield: local_t commit;\t"
			 "offset:%u;\tsize:%u;\tsigned:%u;\n",
			 (unsigned int)offsetof(typeof(field), commit),
			 (unsigned int)sizeof(field.commit),
			 (unsigned int)is_signed_type(long));

	trace_seq_printf(s, "\tfield: int overwrite;\t"
			 "offset:%u;\tsize:%u;\tsigned:%u;\n",
			 (unsigned int)offsetof(typeof(field), commit),
			 1,
			 (unsigned int)is_signed_type(long));

	trace_seq_printf(s, "\tfield: char data;\t"
			 "offset:%u;\tsize:%u;\tsigned:%u;\n",
			 (unsigned int)offsetof(typeof(field), data),
			 (unsigned int)buffer->subbuf_size,
			 (unsigned int)is_signed_type(char));

	return !trace_seq_has_overflowed(s);
}

static inline void rb_time_read(rb_time_t *t, u64 *ret)
{
	*ret = local64_read(&t->time);
}
static void rb_time_set(rb_time_t *t, u64 val)
{
	local64_set(&t->time, val);
}

/*
 * Enable this to make sure that the event passed to
 * ring_buffer_event_time_stamp() is not committed and also
 * is on the buffer that it passed in.
 */
//#define RB_VERIFY_EVENT
#ifdef RB_VERIFY_EVENT
static struct list_head *rb_list_head(struct list_head *list);
static void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
			 void *event)
{
	struct buffer_page *page = cpu_buffer->commit_page;
	struct buffer_page *tail_page = READ_ONCE(cpu_buffer->tail_page);
	struct list_head *next;
	long commit, write;
	unsigned long addr = (unsigned long)event;
	bool done = false;
	int stop = 0;

	/* Make sure the event exists and is not committed yet */
	do {
		if (page == tail_page || WARN_ON_ONCE(stop++ > 100))
			done = true;
		commit = local_read(&page->page->commit);
		write = local_read(&page->write);
		if (addr >= (unsigned long)&page->page->data[commit] &&
		    addr < (unsigned long)&page->page->data[write])
			return;

		next = rb_list_head(page->list.next);
		page = list_entry(next, struct buffer_page, list);
	} while (!done);
	WARN_ON_ONCE(1);
}
#else
static inline void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
			 void *event)
{
}
#endif

/*
 * The absolute time stamp drops the 5 MSBs and some clocks may
 * require them. The rb_fix_abs_ts() will take a previous full
 * time stamp, and add the 5 MSB of that time stamp on to the
 * saved absolute time stamp. Then they are compared in case of
 * the unlikely event that the latest time stamp incremented
 * the 5 MSB.
 */
static inline u64 rb_fix_abs_ts(u64 abs, u64 save_ts)
{
	if (save_ts & TS_MSB) {
		abs |= save_ts & TS_MSB;
		/* Check for overflow */
		if (unlikely(abs < save_ts))
			abs += 1ULL << 59;
	}
	return abs;
}

static inline u64 rb_time_stamp(struct trace_buffer *buffer);

/**
 * ring_buffer_event_time_stamp - return the event's current time stamp
 * @buffer: The buffer that the event is on
 * @event: the event to get the time stamp of
 *
 * Note, this must be called after @event is reserved, and before it is
 * committed to the ring buffer. And must be called from the same
 * context where the event was reserved (normal, softirq, irq, etc).
 *
 * Returns the time stamp associated with the current event.
 * If the event has an extended time stamp, then that is used as
 * the time stamp to return.
 * In the highly unlikely case that the event was nested more than
 * the max nesting, then the write_stamp of the buffer is returned,
 * otherwise  current time is returned, but that really neither of
 * the last two cases should ever happen.
 */
u64 ring_buffer_event_time_stamp(struct trace_buffer *buffer,
				 struct ring_buffer_event *event)
{
	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[smp_processor_id()];
	unsigned int nest;
	u64 ts;

	/* If the event includes an absolute time, then just use that */
	if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
		ts = rb_event_time_stamp(event);
		return rb_fix_abs_ts(ts, cpu_buffer->tail_page->page->time_stamp);
	}

	nest = local_read(&cpu_buffer->committing);
	verify_event(cpu_buffer, event);
	if (WARN_ON_ONCE(!nest))
		goto fail;

	/* Read the current saved nesting level time stamp */
	if (likely(--nest < MAX_NEST))
		return cpu_buffer->event_stamp[nest];

	/* Shouldn't happen, warn if it does */
	WARN_ONCE(1, "nest (%d) greater than max", nest);

 fail:
	rb_time_read(&cpu_buffer->write_stamp, &ts);

	return ts;
}

/**
 * ring_buffer_nr_dirty_pages - get the number of used pages in the ring buffer
 * @buffer: The ring_buffer to get the number of pages from
 * @cpu: The cpu of the ring_buffer to get the number of pages from
 *
 * Returns the number of pages that have content in the ring buffer.
 */
size_t ring_buffer_nr_dirty_pages(struct trace_buffer *buffer, int cpu)
{
	size_t read;
	size_t lost;
	size_t cnt;

	read = local_read(&buffer->buffers[cpu]->pages_read);
	lost = local_read(&buffer->buffers[cpu]->pages_lost);
	cnt = local_read(&buffer->buffers[cpu]->pages_touched);

	if (WARN_ON_ONCE(cnt < lost))
		return 0;

	cnt -= lost;

	/* The reader can read an empty page, but not more than that */
	if (cnt < read) {
		WARN_ON_ONCE(read > cnt + 1);
		return 0;
	}

	return cnt - read;
}

static __always_inline bool full_hit(struct trace_buffer *buffer, int cpu, int full)
{
	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
	size_t nr_pages;
	size_t dirty;

	nr_pages = cpu_buffer->nr_pages;
	if (!nr_pages || !full)
		return true;

	/*
	 * Add one as dirty will never equal nr_pages, as the sub-buffer
	 * that the writer is on is not counted as dirty.
	 * This is needed if "buffer_percent" is set to 100.
	 */
	dirty = ring_buffer_nr_dirty_pages(buffer, cpu) + 1;

	return (dirty * 100) >= (full * nr_pages);
}

/*
 * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
 *
 * Schedules a delayed work to wake up any task that is blocked on the
 * ring buffer waiters queue.
 */
static void rb_wake_up_waiters(struct irq_work *work)
{
	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);

	/* For waiters waiting for the first wake up */
	(void)atomic_fetch_inc_release(&rbwork->seq);

	wake_up_all(&rbwork->waiters);
	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
		/* Only cpu_buffer sets the above flags */
		struct ring_buffer_per_cpu *cpu_buffer =
			container_of(rbwork, struct ring_buffer_per_cpu, irq_work);

		/* Called from interrupt context */
		raw_spin_lock(&cpu_buffer->reader_lock);
		rbwork->wakeup_full = false;
		rbwork->full_waiters_pending = false;

		/* Waking up all waiters, they will reset the shortest full */
		cpu_buffer->shortest_full = 0;
		raw_spin_unlock(&cpu_buffer->reader_lock);

		wake_up_all(&rbwork->full_waiters);
	}
}

/**
 * ring_buffer_wake_waiters - wake up any waiters on this ring buffer
 * @buffer: The ring buffer to wake waiters on
 * @cpu: The CPU buffer to wake waiters on
 *
 * In the case of a file that represents a ring buffer is closing,
 * it is prudent to wake up any waiters that are on this.
 */
void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	struct rb_irq_work *rbwork;

	if (!buffer)
		return;

	if (cpu == RING_BUFFER_ALL_CPUS) {

		/* Wake up individual ones too. One level recursion */
		for_each_buffer_cpu(buffer, cpu)
			ring_buffer_wake_waiters(buffer, cpu);

		rbwork = &buffer->irq_work;
	} else {
		if (WARN_ON_ONCE(!buffer->buffers))
			return;
		if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
			return;

		cpu_buffer = buffer->buffers[cpu];
		/* The CPU buffer may not have been initialized yet */
		if (!cpu_buffer)
			return;
		rbwork = &cpu_buffer->irq_work;
	}

	/* This can be called in any context */
	irq_work_queue(&rbwork->work);
}

static bool rb_watermark_hit(struct trace_buffer *buffer, int cpu, int full)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	bool ret = false;

	/* Reads of all CPUs always waits for any data */
	if (cpu == RING_BUFFER_ALL_CPUS)
		return !ring_buffer_empty(buffer);

	cpu_buffer = buffer->buffers[cpu];

	if (!ring_buffer_empty_cpu(buffer, cpu)) {
		unsigned long flags;
		bool pagebusy;

		if (!full)
			return true;

		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
		pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
		ret = !pagebusy && full_hit(buffer, cpu, full);

		if (!ret && (!cpu_buffer->shortest_full ||
			     cpu_buffer->shortest_full > full)) {
		    cpu_buffer->shortest_full = full;
		}
		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
	}
	return ret;
}

static inline bool
rb_wait_cond(struct rb_irq_work *rbwork, struct trace_buffer *buffer,
	     int cpu, int full, ring_buffer_cond_fn cond, void *data)
{
	if (rb_watermark_hit(buffer, cpu, full))
		return true;

	if (cond(data))
		return true;

	/*
	 * The events can happen in critical sections where
	 * checking a work queue can cause deadlocks.
	 * After adding a task to the queue, this flag is set
	 * only to notify events to try to wake up the queue
	 * using irq_work.
	 *
	 * We don't clear it even if the buffer is no longer
	 * empty. The flag only causes the next event to run
	 * irq_work to do the work queue wake up. The worse
	 * that can happen if we race with !trace_empty() is that
	 * an event will cause an irq_work to try to wake up
	 * an empty queue.
	 *
	 * There's no reason to protect this flag either, as
	 * the work queue and irq_work logic will do the necessary
	 * synchronization for the wake ups. The only thing
	 * that is necessary is that the wake up happens after
	 * a task has been queued. It's OK for spurious wake ups.
	 */
	if (full)
		rbwork->full_waiters_pending = true;
	else
		rbwork->waiters_pending = true;

	return false;
}

struct rb_wait_data {
	struct rb_irq_work		*irq_work;
	int				seq;
};

/*
 * The default wait condition for ring_buffer_wait() is to just to exit the
 * wait loop the first time it is woken up.
 */
static bool rb_wait_once(void *data)
{
	struct rb_wait_data *rdata = data;
	struct rb_irq_work *rbwork = rdata->irq_work;

	return atomic_read_acquire(&rbwork->seq) != rdata->seq;
}

/**
 * ring_buffer_wait - wait for input to the ring buffer
 * @buffer: buffer to wait on
 * @cpu: the cpu buffer to wait on
 * @full: wait until the percentage of pages are available, if @cpu != RING_BUFFER_ALL_CPUS
 * @cond: condition function to break out of wait (NULL to run once)
 * @data: the data to pass to @cond.
 *
 * If @cpu == RING_BUFFER_ALL_CPUS then the task will wake up as soon
 * as data is added to any of the @buffer's cpu buffers. Otherwise
 * it will wait for data to be added to a specific cpu buffer.
 */
int ring_buffer_wait(struct trace_buffer *buffer, int cpu, int full,
		     ring_buffer_cond_fn cond, void *data)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	struct wait_queue_head *waitq;
	struct rb_irq_work *rbwork;
	struct rb_wait_data rdata;
	int ret = 0;

	/*
	 * Depending on what the caller is waiting for, either any
	 * data in any cpu buffer, or a specific buffer, put the
	 * caller on the appropriate wait queue.
	 */
	if (cpu == RING_BUFFER_ALL_CPUS) {
		rbwork = &buffer->irq_work;
		/* Full only makes sense on per cpu reads */
		full = 0;
	} else {
		if (!cpumask_test_cpu(cpu, buffer->cpumask))
			return -ENODEV;
		cpu_buffer = buffer->buffers[cpu];
		rbwork = &cpu_buffer->irq_work;
	}

	if (full)
		waitq = &rbwork->full_waiters;
	else
		waitq = &rbwork->waiters;

	/* Set up to exit loop as soon as it is woken */
	if (!cond) {
		cond = rb_wait_once;
		rdata.irq_work = rbwork;
		rdata.seq = atomic_read_acquire(&rbwork->seq);
		data = &rdata;
	}

	ret = wait_event_interruptible((*waitq),
				rb_wait_cond(rbwork, buffer, cpu, full, cond, data));

	return ret;
}

/**
 * ring_buffer_poll_wait - poll on buffer input
 * @buffer: buffer to wait on
 * @cpu: the cpu buffer to wait on
 * @filp: the file descriptor
 * @poll_table: The poll descriptor
 * @full: wait until the percentage of pages are available, if @cpu != RING_BUFFER_ALL_CPUS
 *
 * If @cpu == RING_BUFFER_ALL_CPUS then the task will wake up as soon
 * as data is added to any of the @buffer's cpu buffers. Otherwise
 * it will wait for data to be added to a specific cpu buffer.
 *
 * Returns EPOLLIN | EPOLLRDNORM if data exists in the buffers,
 * zero otherwise.
 */
__poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
			  struct file *filp, poll_table *poll_table, int full)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	struct rb_irq_work *rbwork;

	if (cpu == RING_BUFFER_ALL_CPUS) {
		rbwork = &buffer->irq_work;
		full = 0;
	} else {
		if (!cpumask_test_cpu(cpu, buffer->cpumask))
			return EPOLLERR;

		cpu_buffer = buffer->buffers[cpu];
		rbwork = &cpu_buffer->irq_work;
	}

	if (full) {
		poll_wait(filp, &rbwork->full_waiters, poll_table);

		if (rb_watermark_hit(buffer, cpu, full))
			return EPOLLIN | EPOLLRDNORM;
		/*
		 * Only allow full_waiters_pending update to be seen after
		 * the shortest_full is set (in rb_watermark_hit). If the
		 * writer sees the full_waiters_pending flag set, it will
		 * compare the amount in the ring buffer to shortest_full.
		 * If the amount in the ring buffer is greater than the
		 * shortest_full percent, it will call the irq_work handler
		 * to wake up this list. The irq_handler will reset shortest_full
		 * back to zero. That's done under the reader_lock, but
		 * the below smp_mb() makes sure that the update to
		 * full_waiters_pending doesn't leak up into the above.
		 */
		smp_mb();
		rbwork->full_waiters_pending = true;
		return 0;
	}

	poll_wait(filp, &rbwork->waiters, poll_table);
	rbwork->waiters_pending = true;

	/*
	 * There's a tight race between setting the waiters_pending and
	 * checking if the ring buffer is empty.  Once the waiters_pending bit
	 * is set, the next event will wake the task up, but we can get stuck
	 * if there's only a single event in.
	 *
	 * FIXME: Ideally, we need a memory barrier on the writer side as well,
	 * but adding a memory barrier to all events will cause too much of a
	 * performance hit in the fast path.  We only need a memory barrier when
	 * the buffer goes from empty to having content.  But as this race is
	 * extremely small, and it's not a problem if another event comes in, we
	 * will fix it later.
	 */
	smp_mb();

	if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) ||
	    (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))
		return EPOLLIN | EPOLLRDNORM;
	return 0;
}

/* buffer may be either ring_buffer or ring_buffer_per_cpu */
#define RB_WARN_ON(b, cond)						\
	({								\
		int _____ret = unlikely(cond);				\
		if (_____ret) {						\
			if (__same_type(*(b), struct ring_buffer_per_cpu)) { \
				struct ring_buffer_per_cpu *__b =	\
					(void *)b;			\
				atomic_inc(&__b->buffer->record_disabled); \
			} else						\
				atomic_inc(&b->record_disabled);	\
			WARN_ON(1);					\
		}							\
		_____ret;						\
	})

/* Up this if you want to test the TIME_EXTENTS and normalization */
#define DEBUG_SHIFT 0

static inline u64 rb_time_stamp(struct trace_buffer *buffer)
{
	u64 ts;

	/* Skip retpolines :-( */
	if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && likely(buffer->clock == trace_clock_local))
		ts = trace_clock_local();
	else
		ts = buffer->clock();

	/* shift to debug/test normalization and TIME_EXTENTS */
	return ts << DEBUG_SHIFT;
}

u64 ring_buffer_time_stamp(struct trace_buffer *buffer)
{
	u64 time;

	preempt_disable_notrace();
	time = rb_time_stamp(buffer);
	preempt_enable_notrace();

	return time;
}
EXPORT_SYMBOL_GPL(ring_buffer_time_stamp);

void ring_buffer_normalize_time_stamp(struct trace_buffer *buffer,
				      int cpu, u64 *ts)
{
	/* Just stupid testing the normalize function and deltas */
	*ts >>= DEBUG_SHIFT;
}
EXPORT_SYMBOL_GPL(ring_buffer_normalize_time_stamp);

/*
 * Making the ring buffer lockless makes things tricky.
 * Although writes only happen on the CPU that they are on,
 * and they only need to worry about interrupts. Reads can
 * happen on any CPU.
 *
 * The reader page is always off the ring buffer, but when the
 * reader finishes with a page, it needs to swap its page with
 * a new one from the buffer. The reader needs to take from
 * the head (writes go to the tail). But if a writer is in overwrite
 * mode and wraps, it must push the head page forward.
 *
 * Here lies the problem.
 *
 * The reader must be careful to replace only the head page, and
 * not another one. As described at the top of the file in the
 * ASCII art, the reader sets its old page to point to the next
 * page after head. It then sets the page after head to point to
 * the old reader page. But if the writer moves the head page
 * during this operation, the reader could end up with the tail.
 *
 * We use cmpxchg to help prevent this race. We also do something
 * special with the page before head. We set the LSB to 1.
 *
 * When the writer must push the page forward, it will clear the
 * bit that points to the head page, move the head, and then set
 * the bit that points to the new head page.
 *
 * We also don't want an interrupt coming in and moving the head
 * page on another writer. Thus we use the second LSB to catch
 * that too. Thus:
 *
 * head->list->prev->next        bit 1          bit 0
 *                              -------        -------
 * Normal page                     0              0
 * Points to head page             0              1
 * New head page                   1              0
 *
 * Note we can not trust the prev pointer of the head page, because:
 *
 * +----+       +-----+        +-----+
 * |    |------>|  T  |---X--->|  N  |
 * |    |<------|     |        |     |
 * +----+       +-----+        +-----+
 *   ^                           ^ |
 *   |          +-----+          | |
 *   +----------|  R  |----------+ |
 *              |     |<-----------+
 *              +-----+
 *
 * Key:  ---X-->  HEAD flag set in pointer
 *         T      Tail page
 *         R      Reader page
 *         N      Next page
 *
 * (see __rb_reserve_next() to see where this happens)
 *
 *  What the above shows is that the reader just swapped out
 *  the reader page with a page in the buffer, but before it
 *  could make the new header point back to the new page added
 *  it was preempted by a writer. The writer moved forward onto
 *  the new page added by the reader and is about to move forward
 *  again.
 *
 *  You can see, it is legitimate for the previous pointer of
 *  the head (or any page) not to point back to itself. But only
 *  temporarily.
 */

#define RB_PAGE_NORMAL		0UL
#define RB_PAGE_HEAD		1UL
#define RB_PAGE_UPDATE		2UL


#define RB_FLAG_MASK		3UL

/* PAGE_MOVED is not part of the mask */
#define RB_PAGE_MOVED		4UL

/*
 * rb_list_head - remove any bit
 */
static struct list_head *rb_list_head(struct list_head *list)
{
	unsigned long val = (unsigned long)list;

	return (struct list_head *)(val & ~RB_FLAG_MASK);
}

/*
 * rb_is_head_page - test if the given page is the head page
 *
 * Because the reader may move the head_page pointer, we can
 * not trust what the head page is (it may be pointing to
 * the reader page). But if the next page is a header page,
 * its flags will be non zero.
 */
static inline int
rb_is_head_page(struct buffer_page *page, struct list_head *list)
{
	unsigned long val;

	val = (unsigned long)list->next;

	if ((val & ~RB_FLAG_MASK) != (unsigned long)&page->list)
		return RB_PAGE_MOVED;

	return val & RB_FLAG_MASK;
}

/*
 * rb_is_reader_page
 *
 * The unique thing about the reader page, is that, if the
 * writer is ever on it, the previous pointer never points
 * back to the reader page.
 */
static bool rb_is_reader_page(struct buffer_page *page)
{
	struct list_head *list = page->list.prev;

	return rb_list_head(list->next) != &page->list;
}

/*
 * rb_set_list_to_head - set a list_head to be pointing to head.
 */
static void rb_set_list_to_head(struct list_head *list)
{
	unsigned long *ptr;

	ptr = (unsigned long *)&list->next;
	*ptr |= RB_PAGE_HEAD;
	*ptr &= ~RB_PAGE_UPDATE;
}

/*
 * rb_head_page_activate - sets up head page
 */
static void rb_head_page_activate(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct buffer_page *head;

	head = cpu_buffer->head_page;
	if (!head)
		return;

	/*
	 * Set the previous list pointer to have the HEAD flag.
	 */
	rb_set_list_to_head(head->list.prev);
}

static void rb_list_head_clear(struct list_head *list)
{
	unsigned long *ptr = (unsigned long *)&list->next;

	*ptr &= ~RB_FLAG_MASK;
}

/*
 * rb_head_page_deactivate - clears head page ptr (for free list)
 */
static void
rb_head_page_deactivate(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct list_head *hd;

	/* Go through the whole list and clear any pointers found. */
	rb_list_head_clear(cpu_buffer->pages);

	list_for_each(hd, cpu_buffer->pages)
		rb_list_head_clear(hd);
}

static int rb_head_page_set(struct ring_buffer_per_cpu *cpu_buffer,
			    struct buffer_page *head,
			    struct buffer_page *prev,
			    int old_flag, int new_flag)
{
	struct list_head *list;
	unsigned long val = (unsigned long)&head->list;
	unsigned long ret;

	list = &prev->list;

	val &= ~RB_FLAG_MASK;

	ret = cmpxchg((unsigned long *)&list->next,
		      val | old_flag, val | new_flag);

	/* check if the reader took the page */
	if ((ret & ~RB_FLAG_MASK) != val)
		return RB_PAGE_MOVED;

	return ret & RB_FLAG_MASK;
}

static int rb_head_page_set_update(struct ring_buffer_per_cpu *cpu_buffer,
				   struct buffer_page *head,
				   struct buffer_page *prev,
				   int old_flag)
{
	return rb_head_page_set(cpu_buffer, head, prev,
				old_flag, RB_PAGE_UPDATE);
}

static int rb_head_page_set_head(struct ring_buffer_per_cpu *cpu_buffer,
				 struct buffer_page *head,
				 struct buffer_page *prev,
				 int old_flag)
{
	return rb_head_page_set(cpu_buffer, head, prev,
				old_flag, RB_PAGE_HEAD);
}

static int rb_head_page_set_normal(struct ring_buffer_per_cpu *cpu_buffer,
				   struct buffer_page *head,
				   struct buffer_page *prev,
				   int old_flag)
{
	return rb_head_page_set(cpu_buffer, head, prev,
				old_flag, RB_PAGE_NORMAL);
}

static inline void rb_inc_page(struct buffer_page **bpage)
{
	struct list_head *p = rb_list_head((*bpage)->list.next);

	*bpage = list_entry(p, struct buffer_page, list);
}

static struct buffer_page *
rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct buffer_page *head;
	struct buffer_page *page;
	struct list_head *list;
	int i;

	if (RB_WARN_ON(cpu_buffer, !cpu_buffer->head_page))
		return NULL;

	/* sanity check */
	list = cpu_buffer->pages;
	if (RB_WARN_ON(cpu_buffer, rb_list_head(list->prev->next) != list))
		return NULL;

	page = head = cpu_buffer->head_page;
	/*
	 * It is possible that the writer moves the header behind
	 * where we started, and we miss in one loop.
	 * A second loop should grab the header, but we'll do
	 * three loops just because I'm paranoid.
	 */
	for (i = 0; i < 3; i++) {
		do {
			if (rb_is_head_page(page, page->list.prev)) {
				cpu_buffer->head_page = page;
				return page;
			}
			rb_inc_page(&page);
		} while (page != head);
	}

	RB_WARN_ON(cpu_buffer, 1);

	return NULL;
}

static bool rb_head_page_replace(struct buffer_page *old,
				struct buffer_page *new)
{
	unsigned long *ptr = (unsigned long *)&old->list.prev->next;
	unsigned long val;

	val = *ptr & ~RB_FLAG_MASK;
	val |= RB_PAGE_HEAD;

	return try_cmpxchg(ptr, &val, (unsigned long)&new->list);
}

/*
 * rb_tail_page_update - move the tail page forward
 */
static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
			       struct buffer_page *tail_page,
			       struct buffer_page *next_page)
{
	unsigned long old_entries;
	unsigned long old_write;

	/*
	 * The tail page now needs to be moved forward.
	 *
	 * We need to reset the tail page, but without messing
	 * with possible erasing of data brought in by interrupts
	 * that have moved the tail page and are currently on it.
	 *
	 * We add a counter to the write field to denote this.
	 */
	old_write = local_add_return(RB_WRITE_INTCNT, &next_page->write);
	old_entries = local_add_return(RB_WRITE_INTCNT, &next_page->entries);

	/*
	 * Just make sure we have seen our old_write and synchronize
	 * with any interrupts that come in.
	 */
	barrier();

	/*
	 * If the tail page is still the same as what we think
	 * it is, then it is up to us to update the tail
	 * pointer.
	 */
	if (tail_page == READ_ONCE(cpu_buffer->tail_page)) {
		/* Zero the write counter */
		unsigned long val = old_write & ~RB_WRITE_MASK;
		unsigned long eval = old_entries & ~RB_WRITE_MASK;

		/*
		 * This will only succeed if an interrupt did
		 * not come in and change it. In which case, we
		 * do not want to modify it.
		 *
		 * We add (void) to let the compiler know that we do not care
		 * about the return value of these functions. We use the
		 * cmpxchg to only update if an interrupt did not already
		 * do it for us. If the cmpxchg fails, we don't care.
		 */
		(void)local_cmpxchg(&next_page->write, old_write, val);
		(void)local_cmpxchg(&next_page->entries, old_entries, eval);

		/*
		 * No need to worry about races with clearing out the commit.
		 * it only can increment when a commit takes place. But that
		 * only happens in the outer most nested commit.
		 */
		local_set(&next_page->page->commit, 0);

		/* Either we update tail_page or an interrupt does */
		if (try_cmpxchg(&cpu_buffer->tail_page, &tail_page, next_page))
			local_inc(&cpu_buffer->pages_touched);
	}
}

static void rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer,
			  struct buffer_page *bpage)
{
	unsigned long val = (unsigned long)bpage;

	RB_WARN_ON(cpu_buffer, val & RB_FLAG_MASK);
}

/**
 * rb_check_pages - integrity check of buffer pages
 * @cpu_buffer: CPU buffer with pages to test
 *
 * As a safety measure we check to make sure the data pages have not
 * been corrupted.
 *
 * Callers of this function need to guarantee that the list of pages doesn't get
 * modified during the check. In particular, if it's possible that the function
 * is invoked with concurrent readers which can swap in a new reader page then
 * the caller should take cpu_buffer->reader_lock.
 */
static void rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct list_head *head = rb_list_head(cpu_buffer->pages);
	struct list_head *tmp;

	if (RB_WARN_ON(cpu_buffer,
			rb_list_head(rb_list_head(head->next)->prev) != head))
		return;

	if (RB_WARN_ON(cpu_buffer,
			rb_list_head(rb_list_head(head->prev)->next) != head))
		return;

	for (tmp = rb_list_head(head->next); tmp != head; tmp = rb_list_head(tmp->next)) {
		if (RB_WARN_ON(cpu_buffer,
				rb_list_head(rb_list_head(tmp->next)->prev) != tmp))
			return;

		if (RB_WARN_ON(cpu_buffer,
				rb_list_head(rb_list_head(tmp->prev)->next) != tmp))
			return;
	}
}

static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
		long nr_pages, struct list_head *pages)
{
	struct buffer_page *bpage, *tmp;
	bool user_thread = current->mm != NULL;
	gfp_t mflags;
	long i;

	/*
	 * Check if the available memory is there first.
	 * Note, si_mem_available() only gives us a rough estimate of available
	 * memory. It may not be accurate. But we don't care, we just want
	 * to prevent doing any allocation when it is obvious that it is
	 * not going to succeed.
	 */
	i = si_mem_available();
	if (i < nr_pages)
		return -ENOMEM;

	/*
	 * __GFP_RETRY_MAYFAIL flag makes sure that the allocation fails
	 * gracefully without invoking oom-killer and the system is not
	 * destabilized.
	 */
	mflags = GFP_KERNEL | __GFP_RETRY_MAYFAIL;

	/*
	 * If a user thread allocates too much, and si_mem_available()
	 * reports there's enough memory, even though there is not.
	 * Make sure the OOM killer kills this thread. This can happen
	 * even with RETRY_MAYFAIL because another task may be doing
	 * an allocation after this task has taken all memory.
	 * This is the task the OOM killer needs to take out during this
	 * loop, even if it was triggered by an allocation somewhere else.
	 */
	if (user_thread)
		set_current_oom_origin();
	for (i = 0; i < nr_pages; i++) {
		struct page *page;

		bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
				    mflags, cpu_to_node(cpu_buffer->cpu));
		if (!bpage)
			goto free_pages;

		rb_check_bpage(cpu_buffer, bpage);

		list_add(&bpage->list, pages);

		page = alloc_pages_node(cpu_to_node(cpu_buffer->cpu),
					mflags | __GFP_COMP | __GFP_ZERO,
					cpu_buffer->buffer->subbuf_order);
		if (!page)
			goto free_pages;
		bpage->page = page_address(page);
		bpage->order = cpu_buffer->buffer->subbuf_order;
		rb_init_page(bpage->page);

		if (user_thread && fatal_signal_pending(current))
			goto free_pages;
	}
	if (user_thread)
		clear_current_oom_origin();

	return 0;

free_pages:
	list_for_each_entry_safe(bpage, tmp, pages, list) {
		list_del_init(&bpage->list);
		free_buffer_page(bpage);
	}
	if (user_thread)
		clear_current_oom_origin();

	return -ENOMEM;
}

static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
			     unsigned long nr_pages)
{
	LIST_HEAD(pages);

	WARN_ON(!nr_pages);

	if (__rb_allocate_pages(cpu_buffer, nr_pages, &pages))
		return -ENOMEM;

	/*
	 * The ring buffer page list is a circular list that does not
	 * start and end with a list head. All page list items point to
	 * other pages.
	 */
	cpu_buffer->pages = pages.next;
	list_del(&pages);

	cpu_buffer->nr_pages = nr_pages;

	rb_check_pages(cpu_buffer);

	return 0;
}

static struct ring_buffer_per_cpu *
rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	struct buffer_page *bpage;
	struct page *page;
	int ret;

	cpu_buffer = kzalloc_node(ALIGN(sizeof(*cpu_buffer), cache_line_size()),
				  GFP_KERNEL, cpu_to_node(cpu));
	if (!cpu_buffer)
		return NULL;

	cpu_buffer->cpu = cpu;
	cpu_buffer->buffer = buffer;
	raw_spin_lock_init(&cpu_buffer->reader_lock);
	lockdep_set_class(&cpu_buffer->reader_lock, buffer->reader_lock_key);
	cpu_buffer->lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
	INIT_WORK(&cpu_buffer->update_pages_work, update_pages_handler);
	init_completion(&cpu_buffer->update_done);
	init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
	init_waitqueue_head(&cpu_buffer->irq_work.waiters);
	init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
	mutex_init(&cpu_buffer->mapping_lock);

	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
			    GFP_KERNEL, cpu_to_node(cpu));
	if (!bpage)
		goto fail_free_buffer;

	rb_check_bpage(cpu_buffer, bpage);

	cpu_buffer->reader_page = bpage;

	page = alloc_pages_node(cpu_to_node(cpu), GFP_KERNEL | __GFP_COMP | __GFP_ZERO,
				cpu_buffer->buffer->subbuf_order);
	if (!page)
		goto fail_free_reader;
	bpage->page = page_address(page);
	rb_init_page(bpage->page);

	INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
	INIT_LIST_HEAD(&cpu_buffer->new_pages);

	ret = rb_allocate_pages(cpu_buffer, nr_pages);
	if (ret < 0)
		goto fail_free_reader;

	cpu_buffer->head_page
		= list_entry(cpu_buffer->pages, struct buffer_page, list);
	cpu_buffer->tail_page = cpu_buffer->commit_page = cpu_buffer->head_page;

	rb_head_page_activate(cpu_buffer);

	return cpu_buffer;

 fail_free_reader:
	free_buffer_page(cpu_buffer->reader_page);

 fail_free_buffer:
	kfree(cpu_buffer);
	return NULL;
}

static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct list_head *head = cpu_buffer->pages;
	struct buffer_page *bpage, *tmp;

	irq_work_sync(&cpu_buffer->irq_work.work);

	free_buffer_page(cpu_buffer->reader_page);

	if (head) {
		rb_head_page_deactivate(cpu_buffer);

		list_for_each_entry_safe(bpage, tmp, head, list) {
			list_del_init(&bpage->list);
			free_buffer_page(bpage);
		}
		bpage = list_entry(head, struct buffer_page, list);
		free_buffer_page(bpage);
	}

	free_page((unsigned long)cpu_buffer->free_page);

	kfree(cpu_buffer);
}

/**
 * __ring_buffer_alloc - allocate a new ring_buffer
 * @size: the size in bytes per cpu that is needed.
 * @flags: attributes to set for the ring buffer.
 * @key: ring buffer reader_lock_key.
 *
 * Currently the only flag that is available is the RB_FL_OVERWRITE
 * flag. This flag means that the buffer will overwrite old data
 * when the buffer wraps. If this flag is not set, the buffer will
 * drop data when the tail hits the head.
 */
struct trace_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
					struct lock_class_key *key)
{
	struct trace_buffer *buffer;
	long nr_pages;
	int bsize;
	int cpu;
	int ret;

	/* keep it in its own cache line */
	buffer = kzalloc(ALIGN(sizeof(*buffer), cache_line_size()),
			 GFP_KERNEL);
	if (!buffer)
		return NULL;

	if (!zalloc_cpumask_var(&buffer->cpumask, GFP_KERNEL))
		goto fail_free_buffer;

	/* Default buffer page size - one system page */
	buffer->subbuf_order = 0;
	buffer->subbuf_size = PAGE_SIZE - BUF_PAGE_HDR_SIZE;

	/* Max payload is buffer page size - header (8bytes) */
	buffer->max_data_size = buffer->subbuf_size - (sizeof(u32) * 2);

	nr_pages = DIV_ROUND_UP(size, buffer->subbuf_size);
	buffer->flags = flags;
	buffer->clock = trace_clock_local;
	buffer->reader_lock_key = key;

	init_irq_work(&buffer->irq_work.work, rb_wake_up_waiters);
	init_waitqueue_head(&buffer->irq_work.waiters);

	/* need at least two pages */
	if (nr_pages < 2)
		nr_pages = 2;

	buffer->cpus = nr_cpu_ids;

	bsize = sizeof(void *) * nr_cpu_ids;
	buffer->buffers = kzalloc(ALIGN(bsize, cache_line_size()),
				  GFP_KERNEL);
	if (!buffer->buffers)
		goto fail_free_cpumask;

	cpu = raw_smp_processor_id();
	cpumask_set_cpu(cpu, buffer->cpumask);
	buffer->buffers[cpu] = rb_allocate_cpu_buffer(buffer, nr_pages, cpu);
	if (!buffer->buffers[cpu])
		goto fail_free_buffers;

	ret = cpuhp_state_add_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
	if (ret < 0)
		goto fail_free_buffers;

	mutex_init(&buffer->mutex);

	return buffer;

 fail_free_buffers:
	for_each_buffer_cpu(buffer, cpu) {
		if (buffer->buffers[cpu])
			rb_free_cpu_buffer(buffer->buffers[cpu]);
	}
	kfree(buffer->buffers);

 fail_free_cpumask:
	free_cpumask_var(buffer->cpumask);

 fail_free_buffer:
	kfree(buffer);
	return NULL;
}
EXPORT_SYMBOL_GPL(__ring_buffer_alloc);

/**
 * ring_buffer_free - free a ring buffer.
 * @buffer: the buffer to free.
 */
void
ring_buffer_free(struct trace_buffer *buffer)
{
	int cpu;

	cpuhp_state_remove_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);

	irq_work_sync(&buffer->irq_work.work);

	for_each_buffer_cpu(buffer, cpu)
		rb_free_cpu_buffer(buffer->buffers[cpu]);

	kfree(buffer->buffers);
	free_cpumask_var(buffer->cpumask);

	kfree(buffer);
}
EXPORT_SYMBOL_GPL(ring_buffer_free);

void ring_buffer_set_clock(struct trace_buffer *buffer,
			   u64 (*clock)(void))
{
	buffer->clock = clock;
}

void ring_buffer_set_time_stamp_abs(struct trace_buffer *buffer, bool abs)
{
	buffer->time_stamp_abs = abs;
}

bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
{
	return buffer->time_stamp_abs;
}

static inline unsigned long rb_page_entries(struct buffer_page *bpage)
{
	return local_read(&bpage->entries) & RB_WRITE_MASK;
}

static inline unsigned long rb_page_write(struct buffer_page *bpage)
{
	return local_read(&bpage->write) & RB_WRITE_MASK;
}

static bool
rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
{
	struct list_head *tail_page, *to_remove, *next_page;
	struct buffer_page *to_remove_page, *tmp_iter_page;
	struct buffer_page *last_page, *first_page;
	unsigned long nr_removed;
	unsigned long head_bit;
	int page_entries;

	head_bit = 0;

	raw_spin_lock_irq(&cpu_buffer->reader_lock);
	atomic_inc(&cpu_buffer->record_disabled);
	/*
	 * We don't race with the readers since we have acquired the reader
	 * lock. We also don't race with writers after disabling recording.
	 * This makes it easy to figure out the first and the last page to be
	 * removed from the list. We unlink all the pages in between including
	 * the first and last pages. This is done in a busy loop so that we
	 * lose the least number of traces.
	 * The pages are freed after we restart recording and unlock readers.
	 */
	tail_page = &cpu_buffer->tail_page->list;

	/*
	 * tail page might be on reader page, we remove the next page
	 * from the ring buffer
	 */
	if (cpu_buffer->tail_page == cpu_buffer->reader_page)
		tail_page = rb_list_head(tail_page->next);
	to_remove = tail_page;

	/* start of pages to remove */
	first_page = list_entry(rb_list_head(to_remove->next),
				struct buffer_page, list);

	for (nr_removed = 0; nr_removed < nr_pages; nr_removed++) {
		to_remove = rb_list_head(to_remove)->next;
		head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;
	}
	/* Read iterators need to reset themselves when some pages removed */
	cpu_buffer->pages_removed += nr_removed;

	next_page = rb_list_head(to_remove)->next;

	/*
	 * Now we remove all pages between tail_page and next_page.
	 * Make sure that we have head_bit value preserved for the
	 * next page
	 */
	tail_page->next = (struct list_head *)((unsigned long)next_page |
						head_bit);
	next_page = rb_list_head(next_page);
	next_page->prev = tail_page;

	/* make sure pages points to a valid page in the ring buffer */
	cpu_buffer->pages = next_page;

	/* update head page */
	if (head_bit)
		cpu_buffer->head_page = list_entry(next_page,
						struct buffer_page, list);

	/* pages are removed, resume tracing and then free the pages */
	atomic_dec(&cpu_buffer->record_disabled);
	raw_spin_unlock_irq(&cpu_buffer->reader_lock);

	RB_WARN_ON(cpu_buffer, list_empty(cpu_buffer->pages));

	/* last buffer page to remove */
	last_page = list_entry(rb_list_head(to_remove), struct buffer_page,
				list);
	tmp_iter_page = first_page;

	do {
		cond_resched();

		to_remove_page = tmp_iter_page;
		rb_inc_page(&tmp_iter_page);

		/* update the counters */
		page_entries = rb_page_entries(to_remove_page);
		if (page_entries) {
			/*
			 * If something was added to this page, it was full
			 * since it is not the tail page. So we deduct the
			 * bytes consumed in ring buffer from here.
			 * Increment overrun to account for the lost events.
			 */
			local_add(page_entries, &cpu_buffer->overrun);
			local_sub(rb_page_commit(to_remove_page), &cpu_buffer->entries_bytes);
			local_inc(&cpu_buffer->pages_lost);
		}

		/*
		 * We have already removed references to this list item, just
		 * free up the buffer_page and its page
		 */
		free_buffer_page(to_remove_page);
		nr_removed--;

	} while (to_remove_page != last_page);

	RB_WARN_ON(cpu_buffer, nr_removed);

	return nr_removed == 0;
}

static bool
rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct list_head *pages = &cpu_buffer->new_pages;
	unsigned long flags;
	bool success;
	int retries;

	/* Can be called at early boot up, where interrupts must not been enabled */
	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
	/*
	 * We are holding the reader lock, so the reader page won't be swapped
	 * in the ring buffer. Now we are racing with the writer trying to
	 * move head page and the tail page.
	 * We are going to adapt the reader page update process where:
	 * 1. We first splice the start and end of list of new pages between
	 *    the head page and its previous page.
	 * 2. We cmpxchg the prev_page->next to point from head page to the
	 *    start of new pages list.
	 * 3. Finally, we update the head->prev to the end of new list.
	 *
	 * We will try this process 10 times, to make sure that we don't keep
	 * spinning.
	 */
	retries = 10;
	success = false;
	while (retries--) {
		struct list_head *head_page, *prev_page;
		struct list_head *last_page, *first_page;
		struct list_head *head_page_with_bit;
		struct buffer_page *hpage = rb_set_head_page(cpu_buffer);

		if (!hpage)
			break;
		head_page = &hpage->list;
		prev_page = head_page->prev;

		first_page = pages->next;
		last_page  = pages->prev;

		head_page_with_bit = (struct list_head *)
				     ((unsigned long)head_page | RB_PAGE_HEAD);

		last_page->next = head_page_with_bit;
		first_page->prev = prev_page;

		/* caution: head_page_with_bit gets updated on cmpxchg failure */
		if (try_cmpxchg(&prev_page->next,
				&head_page_with_bit, first_page)) {
			/*
			 * yay, we replaced the page pointer to our new list,
			 * now, we just have to update to head page's prev
			 * pointer to point to end of list
			 */
			head_page->prev = last_page;
			success = true;
			break;
		}
	}

	if (success)
		INIT_LIST_HEAD(pages);
	/*
	 * If we weren't successful in adding in new pages, warn and stop
	 * tracing
	 */
	RB_WARN_ON(cpu_buffer, !success);
	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

	/* free pages if they weren't inserted */
	if (!success) {
		struct buffer_page *bpage, *tmp;
		list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
					 list) {
			list_del_init(&bpage->list);
			free_buffer_page(bpage);
		}
	}
	return success;
}

static void rb_update_pages(struct ring_buffer_per_cpu *cpu_buffer)
{
	bool success;

	if (cpu_buffer->nr_pages_to_update > 0)
		success = rb_insert_pages(cpu_buffer);
	else
		success = rb_remove_pages(cpu_buffer,
					-cpu_buffer->nr_pages_to_update);

	if (success)
		cpu_buffer->nr_pages += cpu_buffer->nr_pages_to_update;
}

static void update_pages_handler(struct work_struct *work)
{
	struct ring_buffer_per_cpu *cpu_buffer = container_of(work,
			struct ring_buffer_per_cpu, update_pages_work);
	rb_update_pages(cpu_buffer);
	complete(&cpu_buffer->update_done);
}

/**
 * ring_buffer_resize - resize the ring buffer
 * @buffer: the buffer to resize.
 * @size: the new size.
 * @cpu_id: the cpu buffer to resize
 *
 * Minimum size is 2 * buffer->subbuf_size.
 *
 * Returns 0 on success and < 0 on failure.
 */
int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
			int cpu_id)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	unsigned long nr_pages;
	int cpu, err;

	/*
	 * Always succeed at resizing a non-existent buffer:
	 */
	if (!buffer)
		return 0;

	/* Make sure the requested buffer exists */
	if (cpu_id != RING_BUFFER_ALL_CPUS &&
	    !cpumask_test_cpu(cpu_id, buffer->cpumask))
		return 0;

	nr_pages = DIV_ROUND_UP(size, buffer->subbuf_size);

	/* we need a minimum of two pages */
	if (nr_pages < 2)
		nr_pages = 2;

	/* prevent another thread from changing buffer sizes */
	mutex_lock(&buffer->mutex);
	atomic_inc(&buffer->resizing);

	if (cpu_id == RING_BUFFER_ALL_CPUS) {
		/*
		 * Don't succeed if resizing is disabled, as a reader might be
		 * manipulating the ring buffer and is expecting a sane state while
		 * this is true.
		 */
		for_each_buffer_cpu(buffer, cpu) {
			cpu_buffer = buffer->buffers[cpu];
			if (atomic_read(&cpu_buffer->resize_disabled)) {
				err = -EBUSY;
				goto out_err_unlock;
			}
		}

		/* calculate the pages to update */
		for_each_buffer_cpu(buffer, cpu) {
			cpu_buffer = buffer->buffers[cpu];

			cpu_buffer->nr_pages_to_update = nr_pages -
							cpu_buffer->nr_pages;
			/*
			 * nothing more to do for removing pages or no update
			 */
			if (cpu_buffer->nr_pages_to_update <= 0)
				continue;
			/*
			 * to add pages, make sure all new pages can be
			 * allocated without receiving ENOMEM
			 */
			INIT_LIST_HEAD(&cpu_buffer->new_pages);
			if (__rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
						&cpu_buffer->new_pages)) {
				/* not enough memory for new pages */
				err = -ENOMEM;
				goto out_err;
			}

			cond_resched();
		}

		cpus_read_lock();
		/*
		 * Fire off all the required work handlers
		 * We can't schedule on offline CPUs, but it's not necessary
		 * since we can change their buffer sizes without any race.
		 */
		for_each_buffer_cpu(buffer, cpu) {
			cpu_buffer = buffer->buffers[cpu];
			if (!cpu_buffer->nr_pages_to_update)
				continue;

			/* Can't run something on an offline CPU. */
			if (!cpu_online(cpu)) {
				rb_update_pages(cpu_buffer);
				cpu_buffer->nr_pages_to_update = 0;
			} else {
				/* Run directly if possible. */
				migrate_disable();
				if (cpu != smp_processor_id()) {
					migrate_enable();
					schedule_work_on(cpu,
							 &cpu_buffer->update_pages_work);
				} else {
					update_pages_handler(&cpu_buffer->update_pages_work);
					migrate_enable();
				}
			}
		}

		/* wait for all the updates to complete */
		for_each_buffer_cpu(buffer, cpu) {
			cpu_buffer = buffer->buffers[cpu];
			if (!cpu_buffer->nr_pages_to_update)
				continue;

			if (cpu_online(cpu))
				wait_for_completion(&cpu_buffer->update_done);
			cpu_buffer->nr_pages_to_update = 0;
		}

		cpus_read_unlock();
	} else {
		cpu_buffer = buffer->buffers[cpu_id];

		if (nr_pages == cpu_buffer->nr_pages)
			goto out;

		/*
		 * Don't succeed if resizing is disabled, as a reader might be
		 * manipulating the ring buffer and is expecting a sane state while
		 * this is true.
		 */
		if (atomic_read(&cpu_buffer->resize_disabled)) {
			err = -EBUSY;
			goto out_err_unlock;
		}

		cpu_buffer->nr_pages_to_update = nr_pages -
						cpu_buffer->nr_pages;

		INIT_LIST_HEAD(&cpu_buffer->new_pages);
		if (cpu_buffer->nr_pages_to_update > 0 &&
			__rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
					    &cpu_buffer->new_pages)) {
			err = -ENOMEM;
			goto out_err;
		}

		cpus_read_lock();

		/* Can't run something on an offline CPU. */
		if (!cpu_online(cpu_id))
			rb_update_pages(cpu_buffer);
		else {
			/* Run directly if possible. */
			migrate_disable();
			if (cpu_id == smp_processor_id()) {
				rb_update_pages(cpu_buffer);
				migrate_enable();
			} else {
				migrate_enable();
				schedule_work_on(cpu_id,
						 &cpu_buffer->update_pages_work);
				wait_for_completion(&cpu_buffer->update_done);
			}
		}

		cpu_buffer->nr_pages_to_update = 0;
		cpus_read_unlock();
	}

 out:
	/*
	 * The ring buffer resize can happen with the ring buffer
	 * enabled, so that the update disturbs the tracing as little
	 * as possible. But if the buffer is disabled, we do not need
	 * to worry about that, and we can take the time to verify
	 * that the buffer is not corrupt.
	 */
	if (atomic_read(&buffer->record_disabled)) {
		atomic_inc(&buffer->record_disabled);
		/*
		 * Even though the buffer was disabled, we must make sure
		 * that it is truly disabled before calling rb_check_pages.
		 * There could have been a race between checking
		 * record_disable and incrementing it.
		 */
		synchronize_rcu();
		for_each_buffer_cpu(buffer, cpu) {
			unsigned long flags;

			cpu_buffer = buffer->buffers[cpu];
			raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
			rb_check_pages(cpu_buffer);
			raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
		}
		atomic_dec(&buffer->record_disabled);
	}

	atomic_dec(&buffer->resizing);
	mutex_unlock(&buffer->mutex);
	return 0;

 out_err:
	for_each_buffer_cpu(buffer, cpu) {
		struct buffer_page *bpage, *tmp;

		cpu_buffer = buffer->buffers[cpu];
		cpu_buffer->nr_pages_to_update = 0;

		if (list_empty(&cpu_buffer->new_pages))
			continue;

		list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
					list) {
			list_del_init(&bpage->list);
			free_buffer_page(bpage);
		}
	}
 out_err_unlock:
	atomic_dec(&buffer->resizing);
	mutex_unlock(&buffer->mutex);
	return err;
}
EXPORT_SYMBOL_GPL(ring_buffer_resize);

void ring_buffer_change_overwrite(struct trace_buffer *buffer, int val)
{
	mutex_lock(&buffer->mutex);
	if (val)
		buffer->flags |= RB_FL_OVERWRITE;
	else
		buffer->flags &= ~RB_FL_OVERWRITE;
	mutex_unlock(&buffer->mutex);
}
EXPORT_SYMBOL_GPL(ring_buffer_change_overwrite);

static __always_inline void *__rb_page_index(struct buffer_page *bpage, unsigned index)
{
	return bpage->page->data + index;
}

static __always_inline struct ring_buffer_event *
rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
{
	return __rb_page_index(cpu_buffer->reader_page,
			       cpu_buffer->reader_page->read);
}

static struct ring_buffer_event *
rb_iter_head_event(struct ring_buffer_iter *iter)
{
	struct ring_buffer_event *event;
	struct buffer_page *iter_head_page = iter->head_page;
	unsigned long commit;
	unsigned length;

	if (iter->head != iter->next_event)
		return iter->event;

	/*
	 * When the writer goes across pages, it issues a cmpxchg which
	 * is a mb(), which will synchronize with the rmb here.
	 * (see rb_tail_page_update() and __rb_reserve_next())
	 */
	commit = rb_page_commit(iter_head_page);
	smp_rmb();

	/* An event needs to be at least 8 bytes in size */
	if (iter->head > commit - 8)
		goto reset;

	event = __rb_page_index(iter_head_page, iter->head);
	length = rb_event_length(event);

	/*
	 * READ_ONCE() doesn't work on functions and we don't want the
	 * compiler doing any crazy optimizations with length.
	 */
	barrier();

	if ((iter->head + length) > commit || length > iter->event_size)
		/* Writer corrupted the read? */
		goto reset;

	memcpy(iter->event, event, length);
	/*
	 * If the page stamp is still the same after this rmb() then the
	 * event was safely copied without the writer entering the page.
	 */
	smp_rmb();

	/* Make sure the page didn't change since we read this */
	if (iter->page_stamp != iter_head_page->page->time_stamp ||
	    commit > rb_page_commit(iter_head_page))
		goto reset;

	iter->next_event = iter->head + length;
	return iter->event;
 reset:
	/* Reset to the beginning */
	iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
	iter->head = 0;
	iter->next_event = 0;
	iter->missed_events = 1;
	return NULL;
}

/* Size is determined by what has been committed */
static __always_inline unsigned rb_page_size(struct buffer_page *bpage)
{
	return rb_page_commit(bpage) & ~RB_MISSED_MASK;
}

static __always_inline unsigned
rb_commit_index(struct ring_buffer_per_cpu *cpu_buffer)
{
	return rb_page_commit(cpu_buffer->commit_page);
}

static __always_inline unsigned
rb_event_index(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event *event)
{
	unsigned long addr = (unsigned long)event;

	addr &= (PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1;

	return addr - BUF_PAGE_HDR_SIZE;
}

static void rb_inc_iter(struct ring_buffer_iter *iter)
{
	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;

	/*
	 * The iterator could be on the reader page (it starts there).
	 * But the head could have moved, since the reader was
	 * found. Check for this case and assign the iterator
	 * to the head page instead of next.
	 */
	if (iter->head_page == cpu_buffer->reader_page)
		iter->head_page = rb_set_head_page(cpu_buffer);
	else
		rb_inc_page(&iter->head_page);

	iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
	iter->head = 0;
	iter->next_event = 0;
}

/*
 * rb_handle_head_page - writer hit the head page
 *
 * Returns: +1 to retry page
 *           0 to continue
 *          -1 on error
 */
static int
rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
		    struct buffer_page *tail_page,
		    struct buffer_page *next_page)
{
	struct buffer_page *new_head;
	int entries;
	int type;
	int ret;

	entries = rb_page_entries(next_page);

	/*
	 * The hard part is here. We need to move the head
	 * forward, and protect against both readers on
	 * other CPUs and writers coming in via interrupts.
	 */
	type = rb_head_page_set_update(cpu_buffer, next_page, tail_page,
				       RB_PAGE_HEAD);

	/*
	 * type can be one of four:
	 *  NORMAL - an interrupt already moved it for us
	 *  HEAD   - we are the first to get here.
	 *  UPDATE - we are the interrupt interrupting
	 *           a current move.
	 *  MOVED  - a reader on another CPU moved the next
	 *           pointer to its reader page. Give up
	 *           and try again.
	 */

	switch (type) {
	case RB_PAGE_HEAD:
		/*
		 * We changed the head to UPDATE, thus
		 * it is our responsibility to update
		 * the counters.
		 */
		local_add(entries, &cpu_buffer->overrun);
		local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);
		local_inc(&cpu_buffer->pages_lost);

		/*
		 * The entries will be zeroed out when we move the
		 * tail page.
		 */

		/* still more to do */
		break;

	case RB_PAGE_UPDATE:
		/*
		 * This is an interrupt that interrupt the
		 * previous update. Still more to do.
		 */
		break;
	case RB_PAGE_NORMAL:
		/*
		 * An interrupt came in before the update
		 * and processed this for us.
		 * Nothing left to do.
		 */
		return 1;
	case RB_PAGE_MOVED:
		/*
		 * The reader is on another CPU and just did
		 * a swap with our next_page.
		 * Try again.
		 */
		return 1;
	default:
		RB_WARN_ON(cpu_buffer, 1); /* WTF??? */
		return -1;
	}

	/*
	 * Now that we are here, the old head pointer is
	 * set to UPDATE. This will keep the reader from
	 * swapping the head page with the reader page.
	 * The reader (on another CPU) will spin till
	 * we are finished.
	 *
	 * We just need to protect against interrupts
	 * doing the job. We will set the next pointer
	 * to HEAD. After that, we set the old pointer
	 * to NORMAL, but only if it was HEAD before.
	 * otherwise we are an interrupt, and only
	 * want the outer most commit to reset it.
	 */
	new_head = next_page;
	rb_inc_page(&new_head);

	ret = rb_head_page_set_head(cpu_buffer, new_head, next_page,
				    RB_PAGE_NORMAL);

	/*
	 * Valid returns are:
	 *  HEAD   - an interrupt came in and already set it.
	 *  NORMAL - One of two things:
	 *            1) We really set it.
	 *            2) A bunch of interrupts came in and moved
	 *               the page forward again.
	 */
	switch (ret) {
	case RB_PAGE_HEAD:
	case RB_PAGE_NORMAL:
		/* OK */
		break;
	default:
		RB_WARN_ON(cpu_buffer, 1);
		return -1;
	}

	/*
	 * It is possible that an interrupt came in,
	 * set the head up, then more interrupts came in
	 * and moved it again. When we get back here,
	 * the page would have been set to NORMAL but we
	 * just set it back to HEAD.
	 *
	 * How do you detect this? Well, if that happened
	 * the tail page would have moved.
	 */
	if (ret == RB_PAGE_NORMAL) {
		struct buffer_page *buffer_tail_page;

		buffer_tail_page = READ_ONCE(cpu_buffer->tail_page);
		/*
		 * If the tail had moved passed next, then we need
		 * to reset the pointer.
		 */
		if (buffer_tail_page != tail_page &&
		    buffer_tail_page != next_page)
			rb_head_page_set_normal(cpu_buffer, new_head,
						next_page,
						RB_PAGE_HEAD);
	}

	/*
	 * If this was the outer most commit (the one that
	 * changed the original pointer from HEAD to UPDATE),
	 * then it is up to us to reset it to NORMAL.
	 */
	if (type == RB_PAGE_HEAD) {
		ret = rb_head_page_set_normal(cpu_buffer, next_page,
					      tail_page,
					      RB_PAGE_UPDATE);
		if (RB_WARN_ON(cpu_buffer,
			       ret != RB_PAGE_UPDATE))
			return -1;
	}

	return 0;
}

static inline void
rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
	      unsigned long tail, struct rb_event_info *info)
{
	unsigned long bsize = READ_ONCE(cpu_buffer->buffer->subbuf_size);
	struct buffer_page *tail_page = info->tail_page;
	struct ring_buffer_event *event;
	unsigned long length = info->length;

	/*
	 * Only the event that crossed the page boundary
	 * must fill the old tail_page with padding.
	 */
	if (tail >= bsize) {
		/*
		 * If the page was filled, then we still need
		 * to update the real_end. Reset it to zero
		 * and the reader will ignore it.
		 */
		if (tail == bsize)
			tail_page->real_end = 0;

		local_sub(length, &tail_page->write);
		return;
	}

	event = __rb_page_index(tail_page, tail);

	/*
	 * Save the original length to the meta data.
	 * This will be used by the reader to add lost event
	 * counter.
	 */
	tail_page->real_end = tail;

	/*
	 * If this event is bigger than the minimum size, then
	 * we need to be careful that we don't subtract the
	 * write counter enough to allow another writer to slip
	 * in on this page.
	 * We put in a discarded commit instead, to make sure
	 * that this space is not used again, and this space will
	 * not be accounted into 'entries_bytes'.
	 *
	 * If we are less than the minimum size, we don't need to
	 * worry about it.
	 */
	if (tail > (bsize - RB_EVNT_MIN_SIZE)) {
		/* No room for any events */

		/* Mark the rest of the page with padding */
		rb_event_set_padding(event);

		/* Make sure the padding is visible before the write update */
		smp_wmb();

		/* Set the write back to the previous setting */
		local_sub(length, &tail_page->write);
		return;
	}

	/* Put in a discarded event */
	event->array[0] = (bsize - tail) - RB_EVNT_HDR_SIZE;
	event->type_len = RINGBUF_TYPE_PADDING;
	/* time delta must be non zero */
	event->time_delta = 1;

	/* account for padding bytes */
	local_add(bsize - tail, &cpu_buffer->entries_bytes);

	/* Make sure the padding is visible before the tail_page->write update */
	smp_wmb();

	/* Set write to end of buffer */
	length = (tail + length) - bsize;
	local_sub(length, &tail_page->write);
}

static inline void rb_end_commit(struct ring_buffer_per_cpu *cpu_buffer);

/*
 * This is the slow path, force gcc not to inline it.
 */
static noinline struct ring_buffer_event *
rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
	     unsigned long tail, struct rb_event_info *info)
{
	struct buffer_page *tail_page = info->tail_page;
	struct buffer_page *commit_page = cpu_buffer->commit_page;
	struct trace_buffer *buffer = cpu_buffer->buffer;
	struct buffer_page *next_page;
	int ret;

	next_page = tail_page;

	rb_inc_page(&next_page);

	/*
	 * If for some reason, we had an interrupt storm that made
	 * it all the way around the buffer, bail, and warn
	 * about it.
	 */
	if (unlikely(next_page == commit_page)) {
		local_inc(&cpu_buffer->commit_overrun);
		goto out_reset;
	}

	/*
	 * This is where the fun begins!
	 *
	 * We are fighting against races between a reader that
	 * could be on another CPU trying to swap its reader
	 * page with the buffer head.
	 *
	 * We are also fighting against interrupts coming in and
	 * moving the head or tail on us as well.
	 *
	 * If the next page is the head page then we have filled
	 * the buffer, unless the commit page is still on the
	 * reader page.
	 */
	if (rb_is_head_page(next_page, &tail_page->list)) {

		/*
		 * If the commit is not on the reader page, then
		 * move the header page.
		 */
		if (!rb_is_reader_page(cpu_buffer->commit_page)) {
			/*
			 * If we are not in overwrite mode,
			 * this is easy, just stop here.
			 */
			if (!(buffer->flags & RB_FL_OVERWRITE)) {
				local_inc(&cpu_buffer->dropped_events);
				goto out_reset;
			}

			ret = rb_handle_head_page(cpu_buffer,
						  tail_page,
						  next_page);
			if (ret < 0)
				goto out_reset;
			if (ret)
				goto out_again;
		} else {
			/*
			 * We need to be careful here too. The
			 * commit page could still be on the reader
			 * page. We could have a small buffer, and
			 * have filled up the buffer with events
			 * from interrupts and such, and wrapped.
			 *
			 * Note, if the tail page is also on the
			 * reader_page, we let it move out.
			 */
			if (unlikely((cpu_buffer->commit_page !=
				      cpu_buffer->tail_page) &&
				     (cpu_buffer->commit_page ==
				      cpu_buffer->reader_page))) {
				local_inc(&cpu_buffer->commit_overrun);
				goto out_reset;
			}
		}
	}

	rb_tail_page_update(cpu_buffer, tail_page, next_page);

 out_again:

	rb_reset_tail(cpu_buffer, tail, info);

	/* Commit what we have for now. */
	rb_end_commit(cpu_buffer);
	/* rb_end_commit() decs committing */
	local_inc(&cpu_buffer->committing);

	/* fail and let the caller try again */
	return ERR_PTR(-EAGAIN);

 out_reset:
	/* reset write */
	rb_reset_tail(cpu_buffer, tail, info);

	return NULL;
}

/* Slow path */
static struct ring_buffer_event *
rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
		  struct ring_buffer_event *event, u64 delta, bool abs)
{
	if (abs)
		event->type_len = RINGBUF_TYPE_TIME_STAMP;
	else
		event->type_len = RINGBUF_TYPE_TIME_EXTEND;

	/* Not the first event on the page, or not delta? */
	if (abs || rb_event_index(cpu_buffer, event)) {
		event->time_delta = delta & TS_MASK;
		event->array[0] = delta >> TS_SHIFT;
	} else {
		/* nope, just zero it */
		event->time_delta = 0;
		event->array[0] = 0;
	}

	return skip_time_extend(event);
}

#ifndef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
static inline bool sched_clock_stable(void)
{
	return true;
}
#endif

static void
rb_check_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
		   struct rb_event_info *info)
{
	u64 write_stamp;

	WARN_ONCE(1, "Delta way too big! %llu ts=%llu before=%llu after=%llu write stamp=%llu\n%s",
		  (unsigned long long)info->delta,
		  (unsigned long long)info->ts,
		  (unsigned long long)info->before,
		  (unsigned long long)info->after,
		  (unsigned long long)({rb_time_read(&cpu_buffer->write_stamp, &write_stamp); write_stamp;}),
		  sched_clock_stable() ? "" :
		  "If you just came from a suspend/resume,\n"
		  "please switch to the trace global clock:\n"
		  "  echo global > /sys/kernel/tracing/trace_clock\n"
		  "or add trace_clock=global to the kernel command line\n");
}

static void rb_add_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
				      struct ring_buffer_event **event,
				      struct rb_event_info *info,
				      u64 *delta,
				      unsigned int *length)
{
	bool abs = info->add_timestamp &
		(RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE);

	if (unlikely(info->delta > (1ULL << 59))) {
		/*
		 * Some timers can use more than 59 bits, and when a timestamp
		 * is added to the buffer, it will lose those bits.
		 */
		if (abs && (info->ts & TS_MSB)) {
			info->delta &= ABS_TS_MASK;

		/* did the clock go backwards */
		} else if (info->before == info->after && info->before > info->ts) {
			/* not interrupted */
			static int once;

			/*
			 * This is possible with a recalibrating of the TSC.
			 * Do not produce a call stack, but just report it.
			 */
			if (!once) {
				once++;
				pr_warn("Ring buffer clock went backwards: %llu -> %llu\n",
					info->before, info->ts);
			}
		} else
			rb_check_timestamp(cpu_buffer, info);
		if (!abs)
			info->delta = 0;
	}
	*event = rb_add_time_stamp(cpu_buffer, *event, info->delta, abs);
	*length -= RB_LEN_TIME_EXTEND;
	*delta = 0;
}

/**
 * rb_update_event - update event type and data
 * @cpu_buffer: The per cpu buffer of the @event
 * @event: the event to update
 * @info: The info to update the @event with (contains length and delta)
 *
 * Update the type and data fields of the @event. The length
 * is the actual size that is written to the ring buffer,
 * and with this, we can determine what to place into the
 * data field.
 */
static void
rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
		struct ring_buffer_event *event,
		struct rb_event_info *info)
{
	unsigned length = info->length;
	u64 delta = info->delta;
	unsigned int nest = local_read(&cpu_buffer->committing) - 1;

	if (!WARN_ON_ONCE(nest >= MAX_NEST))
		cpu_buffer->event_stamp[nest] = info->ts;

	/*
	 * If we need to add a timestamp, then we
	 * add it to the start of the reserved space.
	 */
	if (unlikely(info->add_timestamp))
		rb_add_timestamp(cpu_buffer, &event, info, &delta, &length);

	event->time_delta = delta;
	length -= RB_EVNT_HDR_SIZE;
	if (length > RB_MAX_SMALL_DATA || RB_FORCE_8BYTE_ALIGNMENT) {
		event->type_len = 0;
		event->array[0] = length;
	} else
		event->type_len = DIV_ROUND_UP(length, RB_ALIGNMENT);
}

static unsigned rb_calculate_event_length(unsigned length)
{
	struct ring_buffer_event event; /* Used only for sizeof array */

	/* zero length can cause confusions */
	if (!length)
		length++;

	if (length > RB_MAX_SMALL_DATA || RB_FORCE_8BYTE_ALIGNMENT)
		length += sizeof(event.array[0]);

	length += RB_EVNT_HDR_SIZE;
	length = ALIGN(length, RB_ARCH_ALIGNMENT);

	/*
	 * In case the time delta is larger than the 27 bits for it
	 * in the header, we need to add a timestamp. If another
	 * event comes in when trying to discard this one to increase
	 * the length, then the timestamp will be added in the allocated
	 * space of this event. If length is bigger than the size needed
	 * for the TIME_EXTEND, then padding has to be used. The events
	 * length must be either RB_LEN_TIME_EXTEND, or greater than or equal
	 * to RB_LEN_TIME_EXTEND + 8, as 8 is the minimum size for padding.
	 * As length is a multiple of 4, we only need to worry if it
	 * is 12 (RB_LEN_TIME_EXTEND + 4).
	 */
	if (length == RB_LEN_TIME_EXTEND + RB_ALIGNMENT)
		length += RB_ALIGNMENT;

	return length;
}

static inline bool
rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
		  struct ring_buffer_event *event)
{
	unsigned long new_index, old_index;
	struct buffer_page *bpage;
	unsigned long addr;

	new_index = rb_event_index(cpu_buffer, event);
	old_index = new_index + rb_event_ts_length(event);
	addr = (unsigned long)event;
	addr &= ~((PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1);

	bpage = READ_ONCE(cpu_buffer->tail_page);

	/*
	 * Make sure the tail_page is still the same and
	 * the next write location is the end of this event
	 */
	if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
		unsigned long write_mask =
			local_read(&bpage->write) & ~RB_WRITE_MASK;
		unsigned long event_length = rb_event_length(event);

		/*
		 * For the before_stamp to be different than the write_stamp
		 * to make sure that the next event adds an absolute
		 * value and does not rely on the saved write stamp, which
		 * is now going to be bogus.
		 *
		 * By setting the before_stamp to zero, the next event
		 * is not going to use the write_stamp and will instead
		 * create an absolute timestamp. This means there's no
		 * reason to update the wirte_stamp!
		 */
		rb_time_set(&cpu_buffer->before_stamp, 0);

		/*
		 * If an event were to come in now, it would see that the
		 * write_stamp and the before_stamp are different, and assume
		 * that this event just added itself before updating
		 * the write stamp. The interrupting event will fix the
		 * write stamp for us, and use an absolute timestamp.
		 */

		/*
		 * This is on the tail page. It is possible that
		 * a write could come in and move the tail page
		 * and write to the next page. That is fine
		 * because we just shorten what is on this page.
		 */
		old_index += write_mask;
		new_index += write_mask;

		/* caution: old_index gets updated on cmpxchg failure */
		if (local_try_cmpxchg(&bpage->write, &old_index, new_index)) {
			/* update counters */
			local_sub(event_length, &cpu_buffer->entries_bytes);
			return true;
		}
	}

	/* could not discard */
	return false;
}

static void rb_start_commit(struct ring_buffer_per_cpu *cpu_buffer)
{
	local_inc(&cpu_buffer->committing);
	local_inc(&cpu_buffer->commits);
}

static __always_inline void
rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
{
	unsigned long max_count;

	/*
	 * We only race with interrupts and NMIs on this CPU.
	 * If we own the commit event, then we can commit
	 * all others that interrupted us, since the interruptions
	 * are in stack format (they finish before they come
	 * back to us). This allows us to do a simple loop to
	 * assign the commit to the tail.
	 */
 again:
	max_count = cpu_buffer->nr_pages * 100;

	while (cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)) {
		if (RB_WARN_ON(cpu_buffer, !(--max_count)))
			return;
		if (RB_WARN_ON(cpu_buffer,
			       rb_is_reader_page(cpu_buffer->tail_page)))
			return;
		/*
		 * No need for a memory barrier here, as the update
		 * of the tail_page did it for this page.
		 */
		local_set(&cpu_buffer->commit_page->page->commit,
			  rb_page_write(cpu_buffer->commit_page));
		rb_inc_page(&cpu_buffer->commit_page);
		/* add barrier to keep gcc from optimizing too much */
		barrier();
	}
	while (rb_commit_index(cpu_buffer) !=
	       rb_page_write(cpu_buffer->commit_page)) {

		/* Make sure the readers see the content of what is committed. */
		smp_wmb();
		local_set(&cpu_buffer->commit_page->page->commit,
			  rb_page_write(cpu_buffer->commit_page));
		RB_WARN_ON(cpu_buffer,
			   local_read(&cpu_buffer->commit_page->page->commit) &
			   ~RB_WRITE_MASK);
		barrier();
	}

	/* again, keep gcc from optimizing */
	barrier();

	/*
	 * If an interrupt came in just after the first while loop
	 * and pushed the tail page forward, we will be left with
	 * a dangling commit that will never go forward.
	 */
	if (unlikely(cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)))
		goto again;
}

static __always_inline void rb_end_commit(struct ring_buffer_per_cpu *cpu_buffer)
{
	unsigned long commits;

	if (RB_WARN_ON(cpu_buffer,
		       !local_read(&cpu_buffer->committing)))
		return;

 again:
	commits = local_read(&cpu_buffer->commits);
	/* synchronize with interrupts */
	barrier();
	if (local_read(&cpu_buffer->committing) == 1)
		rb_set_commit_to_write(cpu_buffer);

	local_dec(&cpu_buffer->committing);

	/* synchronize with interrupts */
	barrier();

	/*
	 * Need to account for interrupts coming in between the
	 * updating of the commit page and the clearing of the
	 * committing counter.
	 */
	if (unlikely(local_read(&cpu_buffer->commits) != commits) &&
	    !local_read(&cpu_buffer->committing)) {
		local_inc(&cpu_buffer->committing);
		goto again;
	}
}

static inline void rb_event_discard(struct ring_buffer_event *event)
{
	if (extended_time(event))
		event = skip_time_extend(event);

	/* array[0] holds the actual length for the discarded event */
	event->array[0] = rb_event_data_length(event) - RB_EVNT_HDR_SIZE;
	event->type_len = RINGBUF_TYPE_PADDING;
	/* time delta must be non zero */
	if (!event->time_delta)
		event->time_delta = 1;
}

static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer)
{
	local_inc(&cpu_buffer->entries);
	rb_end_commit(cpu_buffer);
}

static __always_inline void
rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
{
	if (buffer->irq_work.waiters_pending) {
		buffer->irq_work.waiters_pending = false;
		/* irq_work_queue() supplies it's own memory barriers */
		irq_work_queue(&buffer->irq_work.work);
	}

	if (cpu_buffer->irq_work.waiters_pending) {
		cpu_buffer->irq_work.waiters_pending = false;
		/* irq_work_queue() supplies it's own memory barriers */
		irq_work_queue(&cpu_buffer->irq_work.work);
	}

	if (cpu_buffer->last_pages_touch == local_read(&cpu_buffer->pages_touched))
		return;

	if (cpu_buffer->reader_page == cpu_buffer->commit_page)
		return;

	if (!cpu_buffer->irq_work.full_waiters_pending)
		return;

	cpu_buffer->last_pages_touch = local_read(&cpu_buffer->pages_touched);

	if (!full_hit(buffer, cpu_buffer->cpu, cpu_buffer->shortest_full))
		return;

	cpu_buffer->irq_work.wakeup_full = true;
	cpu_buffer->irq_work.full_waiters_pending = false;
	/* irq_work_queue() supplies it's own memory barriers */
	irq_work_queue(&cpu_buffer->irq_work.work);
}

#ifdef CONFIG_RING_BUFFER_RECORD_RECURSION
# define do_ring_buffer_record_recursion()	\
	do_ftrace_record_recursion(_THIS_IP_, _RET_IP_)
#else
# define do_ring_buffer_record_recursion() do { } while (0)
#endif

/*
 * The lock and unlock are done within a preempt disable section.
 * The current_context per_cpu variable can only be modified
 * by the current task between lock and unlock. But it can
 * be modified more than once via an interrupt. To pass this
 * information from the lock to the unlock without having to
 * access the 'in_interrupt()' functions again (which do show
 * a bit of overhead in something as critical as function tracing,
 * we use a bitmask trick.
 *
 *  bit 1 =  NMI context
 *  bit 2 =  IRQ context
 *  bit 3 =  SoftIRQ context
 *  bit 4 =  normal context.
 *
 * This works because this is the order of contexts that can
 * preempt other contexts. A SoftIRQ never preempts an IRQ
 * context.
 *
 * When the context is determined, the corresponding bit is
 * checked and set (if it was set, then a recursion of that context
 * happened).
 *
 * On unlock, we need to clear this bit. To do so, just subtract
 * 1 from the current_context and AND it to itself.
 *
 * (binary)
 *  101 - 1 = 100
 *  101 & 100 = 100 (clearing bit zero)
 *
 *  1010 - 1 = 1001
 *  1010 & 1001 = 1000 (clearing bit 1)
 *
 * The least significant bit can be cleared this way, and it
 * just so happens that it is the same bit corresponding to
 * the current context.
 *
 * Now the TRANSITION bit breaks the above slightly. The TRANSITION bit
 * is set when a recursion is detected at the current context, and if
 * the TRANSITION bit is already set, it will fail the recursion.
 * This is needed because there's a lag between the changing of
 * interrupt context and updating the preempt count. In this case,
 * a false positive will be found. To handle this, one extra recursion
 * is allowed, and this is done by the TRANSITION bit. If the TRANSITION
 * bit is already set, then it is considered a recursion and the function
 * ends. Otherwise, the TRANSITION bit is set, and that bit is returned.
 *
 * On the trace_recursive_unlock(), the TRANSITION bit will be the first
 * to be cleared. Even if it wasn't the context that set it. That is,
 * if an interrupt comes in while NORMAL bit is set and the ring buffer
 * is called before preempt_count() is updated, since the check will
 * be on the NORMAL bit, the TRANSITION bit will then be set. If an
 * NMI then comes in, it will set the NMI bit, but when the NMI code
 * does the trace_recursive_unlock() it will clear the TRANSITION bit
 * and leave the NMI bit set. But this is fine, because the interrupt
 * code that set the TRANSITION bit will then clear the NMI bit when it
 * calls trace_recursive_unlock(). If another NMI comes in, it will
 * set the TRANSITION bit and continue.
 *
 * Note: The TRANSITION bit only handles a single transition between context.
 */

static __always_inline bool
trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
{
	unsigned int val = cpu_buffer->current_context;
	int bit = interrupt_context_level();

	bit = RB_CTX_NORMAL - bit;

	if (unlikely(val & (1 << (bit + cpu_buffer->nest)))) {
		/*
		 * It is possible that this was called by transitioning
		 * between interrupt context, and preempt_count() has not
		 * been updated yet. In this case, use the TRANSITION bit.
		 */
		bit = RB_CTX_TRANSITION;
		if (val & (1 << (bit + cpu_buffer->nest))) {
			do_ring_buffer_record_recursion();
			return true;
		}
	}

	val |= (1 << (bit + cpu_buffer->nest));
	cpu_buffer->current_context = val;

	return false;
}

static __always_inline void
trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
{
	cpu_buffer->current_context &=
		cpu_buffer->current_context - (1 << cpu_buffer->nest);
}

/* The recursive locking above uses 5 bits */
#define NESTED_BITS 5

/**
 * ring_buffer_nest_start - Allow to trace while nested
 * @buffer: The ring buffer to modify
 *
 * The ring buffer has a safety mechanism to prevent recursion.
 * But there may be a case where a trace needs to be done while
 * tracing something else. In this case, calling this function
 * will allow this function to nest within a currently active
 * ring_buffer_lock_reserve().
 *
 * Call this function before calling another ring_buffer_lock_reserve() and
 * call ring_buffer_nest_end() after the nested ring_buffer_unlock_commit().
 */
void ring_buffer_nest_start(struct trace_buffer *buffer)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	int cpu;

	/* Enabled by ring_buffer_nest_end() */
	preempt_disable_notrace();
	cpu = raw_smp_processor_id();
	cpu_buffer = buffer->buffers[cpu];
	/* This is the shift value for the above recursive locking */
	cpu_buffer->nest += NESTED_BITS;
}

/**
 * ring_buffer_nest_end - Allow to trace while nested
 * @buffer: The ring buffer to modify
 *
 * Must be called after ring_buffer_nest_start() and after the
 * ring_buffer_unlock_commit().
 */
void ring_buffer_nest_end(struct trace_buffer *buffer)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	int cpu;

	/* disabled by ring_buffer_nest_start() */
	cpu = raw_smp_processor_id();
	cpu_buffer = buffer->buffers[cpu];
	/* This is the shift value for the above recursive locking */
	cpu_buffer->nest -= NESTED_BITS;
	preempt_enable_notrace();
}

/**
 * ring_buffer_unlock_commit - commit a reserved
 * @buffer: The buffer to commit to
 *
 * This commits the data to the ring buffer, and releases any locks held.
 *
 * Must be paired with ring_buffer_lock_reserve.
 */
int ring_buffer_unlock_commit(struct trace_buffer *buffer)
{
	struct ring_buffer_per_cpu *cpu_buffer;
	int cpu = raw_smp_processor_id();

	cpu_buffer = buffer->buffers[cpu];

	rb_commit(cpu_buffer);

	rb_wakeups(buffer, cpu_buffer);

	trace_recursive_unlock(cpu_buffer);

	preempt_enable_notrace();

	return 0;
}
EXPORT_SYMBOL_GPL(ring_buffer_unlock_commit);

/* Special value to validate all deltas on a page. */
#define CHECK_FULL_PAGE		1L

#ifdef CONFIG_RING_BUFFER_VALIDATE_TIME_DELTAS

static const char *show_irq_str(int bits)
{
	const char *type[] = {
		".",	// 0
		"s",	// 1
		"h",	// 2
		"Hs",	// 3
		"n",	// 4
		"Ns",	// 5
		"Nh",	// 6
		"NHs",	// 7
	};

	return type[bits];
}

/* Assume this is an trace event */
static const char *show_flags(struct ring_buffer_event *event)
{
	struct trace_entry *entry;
	int bits = 0;

	if (rb_event_data_length(event) - RB_EVNT_HDR_SIZE < sizeof(*entry))
		return "X";

	entry = ring_buffer_event_data(event);

	if (entry->flags & TRACE_FLAG_SOFTIRQ)
		bits |= 1;

	if (entry->flags & TRACE_FLAG_HARDIRQ)
		bits |= 2;

	if (entry->flags & TRACE_FLAG_NMI)
		bits |= 4;

	return show_irq_str(bits);
}

static const char *show_irq(struct ring_buffer_event *event)
{
	struct trace_entry *entry;

	if (rb_event_data_length(event) - RB_EVNT_HDR_SIZE < sizeof(*entry))
		return "";

	entry = ring_buffer_event_data(event);
	if (entry->flags & TRACE_FLAG_IRQS_OFF)
		return "d";
	return "";
}

static const char *show_interrupt_level(void)
{
	unsigned long pc = preempt_count();
	unsigned char level = 0;

	if (pc & SOFTIRQ_OFFSET)
		level |= 1;

	if (pc & HARDIRQ_MASK)
		level |= 2;

	if (pc & NMI_MASK)
		level |= 4;

	return show_irq_str(level);
}

static void dump_buffer_page(struct buffer_data_page *bpage,
			     struct rb_event_info *info,
			     unsigned long tail)
{
	struct ring_buffer_event *event;
	u64 ts, delta;
	int e;

	ts = bpage->time_stamp;
	pr_warn("  [%lld] PAGE TIME STAMP\n", ts);

	for (e = 0; e < tail; e += rb_event_length(event)) {

		event = (struct ring_buffer_event *)(bpage->data + e);

		switch (event->type_len) {

		case RINGBUF_TYPE_TIME_EXTEND:
			delta = rb_event_time_stamp(event);
			ts += delta;
			pr_warn(" 0x%x: [%lld] delta:%lld TIME EXTEND\n",
				e, ts, delta);
			break;

		case RINGBUF_TYPE_TIME_STAMP:
			delta = rb_event_time_stamp(event);
			ts = rb_fix_abs_ts(delta, ts);
			pr_warn(" 0x%x:  [%lld] absolute:%lld TIME STAMP\n",
				e, ts, delta);
			break;

		case RINGBUF_TYPE_PADDING:
			ts += event->time_delta;
			pr_warn(" 0x%x:  [%lld] delta:%d PADDING\n",
				e, ts, event->time_delta);
			break;

		case RINGBUF_TYPE_DATA:
			ts += event->time_delta;
			pr_warn(" 0x%x:  [%lld] delta:%d %s%s\n",
				e, ts, event->time_delta,
				show_flags(event), show_irq(event));
			break;

		default:
			break;
		}
	}
	pr_warn("expected end:0x%lx last event actually ended at:0x%x\n",