kernel/printk/nmi.c - linux - Git at Google

 /*
  * nmi.c - Safe printk in NMI context
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
  * as published by the Free Software Foundation; either version 2
  * of the License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */

 #include <linux/preempt.h>
 #include <linux/spinlock.h>
 #include <linux/debug_locks.h>
 #include <linux/smp.h>
 #include <linux/cpumask.h>
 #include <linux/irq_work.h>
 #include <linux/printk.h>

 #include "internal.h"

 /*
  * printk() could not take logbuf_lock in NMI context. Instead,
  * it uses an alternative implementation that temporary stores
  * the strings into a per-CPU buffer. The content of the buffer
  * is later flushed into the main ring buffer via IRQ work.
  *
  * The alternative implementation is chosen transparently
  * via @printk_func per-CPU variable.
  *
  * The implementation allows to flush the strings also from another CPU.
  * There are situations when we want to make sure that all buffers
  * were handled or when IRQs are blocked.
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
 atomic_t nmi_message_lost;

 #define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -		\
 			 sizeof(atomic_t) - sizeof(struct irq_work))

 struct nmi_seq_buf {
 	atomic_t		len;	/* length of written data */
 	struct irq_work		work;	/* IRQ work that flushes the buffer */
 	unsigned char		buffer[NMI_LOG_BUF_LEN];
 };
 static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);

 /*
  * Safe printk() for NMI context. It uses a per-CPU buffer to
  * store the message. NMIs are not nested, so there is always only
  * one writer running. But the buffer might get flushed from another
  * CPU, so we need to be careful.
  */
 static int vprintk_nmi(const char *fmt, va_list args)
 {
 	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
 	int add = 0;
 	size_t len;

 again:
 	len = atomic_read(&s->len);

 	/* The trailing '\0' is not counted into len. */
 	if (len >= sizeof(s->buffer) - 1) {
 		atomic_inc(&nmi_message_lost);
 		return 0;
 	}

 	/*
 	 * Make sure that all old data have been read before the buffer was
 	 * reseted. This is not needed when we just append data.
 	 */
 	if (!len)
 		smp_rmb();

 	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);

 	/*
 	 * Do it once again if the buffer has been flushed in the meantime.
 	 * Note that atomic_cmpxchg() is an implicit memory barrier that
 	 * makes sure that the data were written before updating s->len.
 	 */
 	if (atomic_cmpxchg(&s->len, len, len + add) != len)
 		goto again;

 	/* Get flushed in a more safe context. */
 	if (add && printk_nmi_irq_ready) {
 		/* Make sure that IRQ work is really initialized. */
 		smp_rmb();
 		irq_work_queue(&s->work);
 	}

 	return add;
 }

 static void printk_nmi_flush_line(const char *text, int len)
 {
 	/*
 	 * The buffers are flushed in NMI only on panic.  The messages must
 	 * go only into the ring buffer at this stage.  Consoles will get
 	 * explicitly called later when a crashdump is not generated.
 	 */
 	if (in_nmi())
 		printk_deferred("%.*s", len, text);
 	else
 		printk("%.*s", len, text);

 }

 /* printk part of the temporary buffer line by line */
 static int printk_nmi_flush_buffer(const char *start, size_t len)
 {
 	const char *c, *end;
 	bool header;

 	c = start;
 	end = start + len;
 	header = true;

 	/* Print line by line. */
 	while (c < end) {
 		if (*c == '\n') {
 			printk_nmi_flush_line(start, c - start + 1);
 			start = ++c;
 			header = true;
 			continue;
 		}

 		/* Handle continuous lines or missing new line. */
 		if ((c + 1 < end) && printk_get_level(c)) {
 			if (header) {
 				c = printk_skip_level(c);
 				continue;
 			}

 			printk_nmi_flush_line(start, c - start);
 			start = c++;
 			header = true;
 			continue;
 		}

 		header = false;
 		c++;
 	}

 	/* Check if there was a partial line. Ignore pure header. */
 	if (start < end && !header) {
 		static const char newline[] = KERN_CONT "\n";

 		printk_nmi_flush_line(start, end - start);
 		printk_nmi_flush_line(newline, strlen(newline));
 	}

 	return len;
 }

 /*
  * Flush data from the associated per_CPU buffer. The function
  * can be called either via IRQ work or independently.
  */
 static void __printk_nmi_flush(struct irq_work *work)
 {
 	static raw_spinlock_t read_lock =
 		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
 	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
 	unsigned long flags;
 	size_t len;
 	int i;

 	/*
 	 * The lock has two functions. First, one reader has to flush all
 	 * available message to make the lockless synchronization with
 	 * writers easier. Second, we do not want to mix messages from
 	 * different CPUs. This is especially important when printing
 	 * a backtrace.
 	 */
 	raw_spin_lock_irqsave(&read_lock, flags);

 	i = 0;
 more:
 	len = atomic_read(&s->len);

 	/*
 	 * This is just a paranoid check that nobody has manipulated
 	 * the buffer an unexpected way. If we printed something then
 	 * @len must only increase. Also it should never overflow the
 	 * buffer size.
 	 */
 	if ((i && i >= len) || len > sizeof(s->buffer)) {
 		const char *msg = "printk_nmi_flush: internal error\n";

 		printk_nmi_flush_line(msg, strlen(msg));
 		len = 0;
 	}

 	if (!len)
 		goto out; /* Someone else has already flushed the buffer. */

 	/* Make sure that data has been written up to the @len */
 	smp_rmb();
 	i += printk_nmi_flush_buffer(s->buffer + i, len - i);

 	/*
 	 * Check that nothing has got added in the meantime and truncate
 	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
 	 * barrier that makes sure that the data were copied before
 	 * updating s->len.
 	 */
 	if (atomic_cmpxchg(&s->len, len, 0) != len)
 		goto more;

 out:
 	raw_spin_unlock_irqrestore(&read_lock, flags);
 }

 /**
  * printk_nmi_flush - flush all per-cpu nmi buffers.
  *
  * The buffers are flushed automatically via IRQ work. This function
  * is useful only when someone wants to be sure that all buffers have
  * been flushed at some point.
  */
 void printk_nmi_flush(void)
 {
 	int cpu;

 	for_each_possible_cpu(cpu)
 		__printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
 }

 /**
  * printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
  *	goes down.
  *
  * Similar to printk_nmi_flush() but it can be called even in NMI context when
  * the system goes down. It does the best effort to get NMI messages into
  * the main ring buffer.
  *
  * Note that it could try harder when there is only one CPU online.
  */
 void printk_nmi_flush_on_panic(void)
 {
 	/*
 	 * Make sure that we could access the main ring buffer.
 	 * Do not risk a double release when more CPUs are up.
 	 */
 	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
 		if (num_online_cpus() > 1)
 			return;

 		debug_locks_off();
 		raw_spin_lock_init(&logbuf_lock);
 	}

 	printk_nmi_flush();
 }

 void __init printk_nmi_init(void)
 {
 	int cpu;

 	for_each_possible_cpu(cpu) {
 		struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);

 		init_irq_work(&s->work, __printk_nmi_flush);
 	}

 	/* Make sure that IRQ works are initialized before enabling. */
 	smp_wmb();
 	printk_nmi_irq_ready = 1;

 	/* Flush pending messages that did not have scheduled IRQ works. */
 	printk_nmi_flush();
 }

 void printk_nmi_enter(void)
 {
 	this_cpu_write(printk_func, vprintk_nmi);
 }

 void printk_nmi_exit(void)
 {
 	this_cpu_write(printk_func, vprintk_default);
 }
	/*
	* nmi.c - Safe printk in NMI context
	*
	* This program is free software; you can redistribute it and/or
	* modify it under the terms of the GNU General Public License
	* as published by the Free Software Foundation; either version 2
	* of the License, or (at your option) any later version.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, see <http://www.gnu.org/licenses/>.
	*/

	#include <linux/preempt.h>
	#include <linux/spinlock.h>
	#include <linux/debug_locks.h>
	#include <linux/smp.h>
	#include <linux/cpumask.h>
	#include <linux/irq_work.h>
	#include <linux/printk.h>

	#include "internal.h"

	/*
	* printk() could not take logbuf_lock in NMI context. Instead,
	* it uses an alternative implementation that temporary stores
	* the strings into a per-CPU buffer. The content of the buffer
	* is later flushed into the main ring buffer via IRQ work.
	*
	* The alternative implementation is chosen transparently
	* via @printk_func per-CPU variable.
	*
	* The implementation allows to flush the strings also from another CPU.
	* There are situations when we want to make sure that all buffers
	* were handled or when IRQs are blocked.
	*/
	DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
	static int printk_nmi_irq_ready;
	atomic_t nmi_message_lost;

	#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) - \
	sizeof(atomic_t) - sizeof(struct irq_work))

	struct nmi_seq_buf {
	atomic_t len; /* length of written data */
	struct irq_work work; /* IRQ work that flushes the buffer */
	unsigned char buffer[NMI_LOG_BUF_LEN];
	};
	static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);

	/*
	* Safe printk() for NMI context. It uses a per-CPU buffer to
	* store the message. NMIs are not nested, so there is always only
	* one writer running. But the buffer might get flushed from another
	* CPU, so we need to be careful.
	*/
	static int vprintk_nmi(const char *fmt, va_list args)
	{
	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
	int add = 0;
	size_t len;

	again:
	len = atomic_read(&s->len);

	/* The trailing '\0' is not counted into len. */
	if (len >= sizeof(s->buffer) - 1) {
	atomic_inc(&nmi_message_lost);
	return 0;
	}

	/*
	* Make sure that all old data have been read before the buffer was
	* reseted. This is not needed when we just append data.
	*/
	if (!len)
	smp_rmb();

	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);

	/*
	* Do it once again if the buffer has been flushed in the meantime.
	* Note that atomic_cmpxchg() is an implicit memory barrier that
	* makes sure that the data were written before updating s->len.
	*/
	if (atomic_cmpxchg(&s->len, len, len + add) != len)
	goto again;

	/* Get flushed in a more safe context. */
	if (add && printk_nmi_irq_ready) {
	/* Make sure that IRQ work is really initialized. */
	smp_rmb();
	irq_work_queue(&s->work);
	}

	return add;
	}

	static void printk_nmi_flush_line(const char *text, int len)
	{
	/*
	* The buffers are flushed in NMI only on panic. The messages must
	* go only into the ring buffer at this stage. Consoles will get
	* explicitly called later when a crashdump is not generated.
	*/
	if (in_nmi())
	printk_deferred("%.*s", len, text);
	else
	printk("%.*s", len, text);

	}

	/* printk part of the temporary buffer line by line */
	static int printk_nmi_flush_buffer(const char *start, size_t len)
	{
	const char c, end;
	bool header;

	c = start;
	end = start + len;
	header = true;

	/* Print line by line. */
	while (c < end) {
	if (*c == '\n') {
	printk_nmi_flush_line(start, c - start + 1);
	start = ++c;
	header = true;
	continue;
	}

	/* Handle continuous lines or missing new line. */
	if ((c + 1 < end) && printk_get_level(c)) {
	if (header) {
	c = printk_skip_level(c);
	continue;
	}

	printk_nmi_flush_line(start, c - start);
	start = c++;
	header = true;
	continue;
	}

	header = false;
	c++;
	}

	/* Check if there was a partial line. Ignore pure header. */
	if (start < end && !header) {
	static const char newline[] = KERN_CONT "\n";

	printk_nmi_flush_line(start, end - start);
	printk_nmi_flush_line(newline, strlen(newline));
	}

	return len;
	}

	/*
	* Flush data from the associated per_CPU buffer. The function
	* can be called either via IRQ work or independently.
	*/
	static void __printk_nmi_flush(struct irq_work *work)
	{
	static raw_spinlock_t read_lock =
	__RAW_SPIN_LOCK_INITIALIZER(read_lock);
	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
	unsigned long flags;
	size_t len;
	int i;

	/*
	* The lock has two functions. First, one reader has to flush all
	* available message to make the lockless synchronization with
	* writers easier. Second, we do not want to mix messages from
	* different CPUs. This is especially important when printing
	* a backtrace.
	*/
	raw_spin_lock_irqsave(&read_lock, flags);

	i = 0;
	more:
	len = atomic_read(&s->len);

	/*
	* This is just a paranoid check that nobody has manipulated
	* the buffer an unexpected way. If we printed something then
	* @len must only increase. Also it should never overflow the
	* buffer size.
	*/
	if ((i && i >= len) \|\| len > sizeof(s->buffer)) {
	const char *msg = "printk_nmi_flush: internal error\n";

	printk_nmi_flush_line(msg, strlen(msg));
	len = 0;
	}

	if (!len)
	goto out; /* Someone else has already flushed the buffer. */

	/* Make sure that data has been written up to the @len */
	smp_rmb();
	i += printk_nmi_flush_buffer(s->buffer + i, len - i);

	/*
	* Check that nothing has got added in the meantime and truncate
	* the buffer. Note that atomic_cmpxchg() is an implicit memory
	* barrier that makes sure that the data were copied before
	* updating s->len.
	*/
	if (atomic_cmpxchg(&s->len, len, 0) != len)
	goto more;

	out:
	raw_spin_unlock_irqrestore(&read_lock, flags);
	}

	/**
	* printk_nmi_flush - flush all per-cpu nmi buffers.
	*
	* The buffers are flushed automatically via IRQ work. This function
	* is useful only when someone wants to be sure that all buffers have
	* been flushed at some point.
	*/
	void printk_nmi_flush(void)
	{
	int cpu;

	for_each_possible_cpu(cpu)
	__printk_nmi_flush(&per_cpu(nmi_print_seq, cpu).work);
	}

	/**
	* printk_nmi_flush_on_panic - flush all per-cpu nmi buffers when the system
	* goes down.
	*
	* Similar to printk_nmi_flush() but it can be called even in NMI context when
	* the system goes down. It does the best effort to get NMI messages into
	* the main ring buffer.
	*
	* Note that it could try harder when there is only one CPU online.
	*/
	void printk_nmi_flush_on_panic(void)
	{
	/*
	* Make sure that we could access the main ring buffer.
	* Do not risk a double release when more CPUs are up.
	*/
	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
	if (num_online_cpus() > 1)
	return;

	debug_locks_off();
	raw_spin_lock_init(&logbuf_lock);
	}

	printk_nmi_flush();
	}

	void __init printk_nmi_init(void)
	{
	int cpu;

	for_each_possible_cpu(cpu) {
	struct nmi_seq_buf *s = &per_cpu(nmi_print_seq, cpu);

	init_irq_work(&s->work, __printk_nmi_flush);
	}

	/* Make sure that IRQ works are initialized before enabling. */
	smp_wmb();
	printk_nmi_irq_ready = 1;

	/* Flush pending messages that did not have scheduled IRQ works. */
	printk_nmi_flush();
	}

	void printk_nmi_enter(void)
	{
	this_cpu_write(printk_func, vprintk_nmi);
	}

	void printk_nmi_exit(void)
	{
	this_cpu_write(printk_func, vprintk_default);
	}