kernel/futex/requeue.c

// SPDX-License-Identifier: GPL-2.0-or-later

#include <linux/sched/signal.h>

#include "futex.h"
#include "../locking/rtmutex_common.h"

/*
 * On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an
 * underlying rtmutex. The task which is about to be requeued could have
 * just woken up (timeout, signal). After the wake up the task has to
 * acquire hash bucket lock, which is held by the requeue code.  As a task
 * can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking
 * and the hash bucket lock blocking would collide and corrupt state.
 *
 * On !PREEMPT_RT this is not a problem and everything could be serialized
 * on hash bucket lock, but aside of having the benefit of common code,
 * this allows to avoid doing the requeue when the task is already on the
 * way out and taking the hash bucket lock of the original uaddr1 when the
 * requeue has been completed.
 *
 * The following state transitions are valid:
 *
 * On the waiter side:
 *   Q_REQUEUE_PI_NONE		-> Q_REQUEUE_PI_IGNORE
 *   Q_REQUEUE_PI_IN_PROGRESS	-> Q_REQUEUE_PI_WAIT
 *
 * On the requeue side:
 *   Q_REQUEUE_PI_NONE		-> Q_REQUEUE_PI_INPROGRESS
 *   Q_REQUEUE_PI_IN_PROGRESS	-> Q_REQUEUE_PI_DONE/LOCKED
 *   Q_REQUEUE_PI_IN_PROGRESS	-> Q_REQUEUE_PI_NONE (requeue failed)
 *   Q_REQUEUE_PI_WAIT		-> Q_REQUEUE_PI_DONE/LOCKED
 *   Q_REQUEUE_PI_WAIT		-> Q_REQUEUE_PI_IGNORE (requeue failed)
 *
 * The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this
 * signals that the waiter is already on the way out. It also means that
 * the waiter is still on the 'wait' futex, i.e. uaddr1.
 *
 * The waiter side signals early wakeup to the requeue side either through
 * setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending
 * on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately
 * proceed to take the hash bucket lock of uaddr1. If it set state to WAIT,
 * which means the wakeup is interleaving with a requeue in progress it has
 * to wait for the requeue side to change the state. Either to DONE/LOCKED
 * or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex
 * and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by
 * the requeue side when the requeue attempt failed via deadlock detection
 * and therefore the waiter q is still on the uaddr1 futex.
 */
enum {
	Q_REQUEUE_PI_NONE		=  0,
	Q_REQUEUE_PI_IGNORE,
	Q_REQUEUE_PI_IN_PROGRESS,
	Q_REQUEUE_PI_WAIT,
	Q_REQUEUE_PI_DONE,
	Q_REQUEUE_PI_LOCKED,
};

const struct futex_q futex_q_init = {
	/* list gets initialized in futex_queue()*/
	.wake		= futex_wake_mark,
	.key		= FUTEX_KEY_INIT,
	.bitset		= FUTEX_BITSET_MATCH_ANY,
	.requeue_state	= ATOMIC_INIT(Q_REQUEUE_PI_NONE),
};

/**
 * requeue_futex() - Requeue a futex_q from one hb to another
 * @q:		the futex_q to requeue
 * @hb1:	the source hash_bucket
 * @hb2:	the target hash_bucket
 * @key2:	the new key for the requeued futex_q
 */
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
		   struct futex_hash_bucket *hb2, union futex_key *key2)
{

	/*
	 * If key1 and key2 hash to the same bucket, no need to
	 * requeue.
	 */
	if (likely(&hb1->chain != &hb2->chain)) {
		plist_del(&q->list, &hb1->chain);
		futex_hb_waiters_dec(hb1);
		futex_hb_waiters_inc(hb2);
		plist_add(&q->list, &hb2->chain);
		q->lock_ptr = &hb2->lock;
	}
	q->key = *key2;
}

static inline bool futex_requeue_pi_prepare(struct futex_q *q,
					    struct futex_pi_state *pi_state)
{
	int old, new;

	/*
	 * Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has
	 * already set Q_REQUEUE_PI_IGNORE to signal that requeue should
	 * ignore the waiter.
	 */
	old = atomic_read_acquire(&q->requeue_state);
	do {
		if (old == Q_REQUEUE_PI_IGNORE)
			return false;

		/*
		 * futex_proxy_trylock_atomic() might have set it to
		 * IN_PROGRESS and a interleaved early wake to WAIT.
		 *
		 * It was considered to have an extra state for that
		 * trylock, but that would just add more conditionals
		 * all over the place for a dubious value.
		 */
		if (old != Q_REQUEUE_PI_NONE)
			break;

		new = Q_REQUEUE_PI_IN_PROGRESS;
	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

	q->pi_state = pi_state;
	return true;
}

static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
{
	int old, new;

	old = atomic_read_acquire(&q->requeue_state);
	do {
		if (old == Q_REQUEUE_PI_IGNORE)
			return;

		if (locked >= 0) {
			/* Requeue succeeded. Set DONE or LOCKED */
			WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
				     old != Q_REQUEUE_PI_WAIT);
			new = Q_REQUEUE_PI_DONE + locked;
		} else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
			/* Deadlock, no early wakeup interleave */
			new = Q_REQUEUE_PI_NONE;
		} else {
			/* Deadlock, early wakeup interleave. */
			WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);
			new = Q_REQUEUE_PI_IGNORE;
		}
	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

#ifdef CONFIG_PREEMPT_RT
	/* If the waiter interleaved with the requeue let it know */
	if (unlikely(old == Q_REQUEUE_PI_WAIT))
		rcuwait_wake_up(&q->requeue_wait);
#endif
}

static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
{
	int old, new;

	old = atomic_read_acquire(&q->requeue_state);
	do {
		/* Is requeue done already? */
		if (old >= Q_REQUEUE_PI_DONE)
			return old;

		/*
		 * If not done, then tell the requeue code to either ignore
		 * the waiter or to wake it up once the requeue is done.
		 */
		new = Q_REQUEUE_PI_WAIT;
		if (old == Q_REQUEUE_PI_NONE)
			new = Q_REQUEUE_PI_IGNORE;
	} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));

	/* If the requeue was in progress, wait for it to complete */
	if (old == Q_REQUEUE_PI_IN_PROGRESS) {
#ifdef CONFIG_PREEMPT_RT
		rcuwait_wait_event(&q->requeue_wait,
				   atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
				   TASK_UNINTERRUPTIBLE);
#else
		(void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
#endif
	}

	/*
	 * Requeue is now either prohibited or complete. Reread state
	 * because during the wait above it might have changed. Nothing
	 * will modify q->requeue_state after this point.
	 */
	return atomic_read(&q->requeue_state);
}

/**
 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
 * @q:		the futex_q
 * @key:	the key of the requeue target futex
 * @hb:		the hash_bucket of the requeue target futex
 *
 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
 * target futex if it is uncontended or via a lock steal.
 *
 * 1) Set @q::key to the requeue target futex key so the waiter can detect
 *    the wakeup on the right futex.
 *
 * 2) Dequeue @q from the hash bucket.
 *
 * 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
 *    acquisition.
 *
 * 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
 *    the waiter has to fixup the pi state.
 *
 * 5) Complete the requeue state so the waiter can make progress. After
 *    this point the waiter task can return from the syscall immediately in
 *    case that the pi state does not have to be fixed up.
 *
 * 6) Wake the waiter task.
 *
 * Must be called with both q->lock_ptr and hb->lock held.
 */
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
			   struct futex_hash_bucket *hb)
{
	q->key = *key;

	__futex_unqueue(q);

	WARN_ON(!q->rt_waiter);
	q->rt_waiter = NULL;

	q->lock_ptr = &hb->lock;

	/* Signal locked state to the waiter */
	futex_requeue_pi_complete(q, 1);
	wake_up_state(q->task, TASK_NORMAL);
}

/**
 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
 * @pifutex:		the user address of the to futex
 * @hb1:		the from futex hash bucket, must be locked by the caller
 * @hb2:		the to futex hash bucket, must be locked by the caller
 * @key1:		the from futex key
 * @key2:		the to futex key
 * @ps:			address to store the pi_state pointer
 * @exiting:		Pointer to store the task pointer of the owner task
 *			which is in the middle of exiting
 * @set_waiters:	force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Try and get the lock on behalf of the top waiter if we can do it atomically.
 * Wake the top waiter if we succeed.  If the caller specified set_waiters,
 * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
 * hb1 and hb2 must be held by the caller.
 *
 * @exiting is only set when the return value is -EBUSY. If so, this holds
 * a refcount on the exiting task on return and the caller needs to drop it
 * after waiting for the exit to complete.
 *
 * Return:
 *  -  0 - failed to acquire the lock atomically;
 *  - >0 - acquired the lock, return value is vpid of the top_waiter
 *  - <0 - error
 */
static int
futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
			   struct futex_hash_bucket *hb2, union futex_key *key1,
			   union futex_key *key2, struct futex_pi_state **ps,
			   struct task_struct **exiting, int set_waiters)
{
	struct futex_q *top_waiter;
	u32 curval;
	int ret;

	if (futex_get_value_locked(&curval, pifutex))
		return -EFAULT;

	if (unlikely(should_fail_futex(true)))
		return -EFAULT;

	/*
	 * Find the top_waiter and determine if there are additional waiters.
	 * If the caller intends to requeue more than 1 waiter to pifutex,
	 * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
	 * as we have means to handle the possible fault.  If not, don't set
	 * the bit unnecessarily as it will force the subsequent unlock to enter
	 * the kernel.
	 */
	top_waiter = futex_top_waiter(hb1, key1);

	/* There are no waiters, nothing for us to do. */
	if (!top_waiter)
		return 0;

	/*
	 * Ensure that this is a waiter sitting in futex_wait_requeue_pi()
	 * and waiting on the 'waitqueue' futex which is always !PI.
	 */
	if (!top_waiter->rt_waiter || top_waiter->pi_state)
		return -EINVAL;

	/* Ensure we requeue to the expected futex. */
	if (!futex_match(top_waiter->requeue_pi_key, key2))
		return -EINVAL;

	/* Ensure that this does not race against an early wakeup */
	if (!futex_requeue_pi_prepare(top_waiter, NULL))
		return -EAGAIN;

	/*
	 * Try to take the lock for top_waiter and set the FUTEX_WAITERS bit
	 * in the contended case or if @set_waiters is true.
	 *
	 * In the contended case PI state is attached to the lock owner. If
	 * the user space lock can be acquired then PI state is attached to
	 * the new owner (@top_waiter->task) when @set_waiters is true.
	 */
	ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
				   exiting, set_waiters);
	if (ret == 1) {
		/*
		 * Lock was acquired in user space and PI state was
		 * attached to @top_waiter->task. That means state is fully
		 * consistent and the waiter can return to user space
		 * immediately after the wakeup.
		 */
		requeue_pi_wake_futex(top_waiter, key2, hb2);
	} else if (ret < 0) {
		/* Rewind top_waiter::requeue_state */
		futex_requeue_pi_complete(top_waiter, ret);
	} else {
		/*
		 * futex_lock_pi_atomic() did not acquire the user space
		 * futex, but managed to establish the proxy lock and pi
		 * state. top_waiter::requeue_state cannot be fixed up here
		 * because the waiter is not enqueued on the rtmutex
		 * yet. This is handled at the callsite depending on the
		 * result of rt_mutex_start_proxy_lock() which is
		 * guaranteed to be reached with this function returning 0.
		 */
	}
	return ret;
}

/**
 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
 * @uaddr1:	source futex user address
 * @flags1:	futex flags (FLAGS_SHARED, etc.)
 * @uaddr2:	target futex user address
 * @flags2:	futex flags (FLAGS_SHARED, etc.)
 * @nr_wake:	number of waiters to wake (must be 1 for requeue_pi)
 * @nr_requeue:	number of waiters to requeue (0-INT_MAX)
 * @cmpval:	@uaddr1 expected value (or %NULL)
 * @requeue_pi:	if we are attempting to requeue from a non-pi futex to a
 *		pi futex (pi to pi requeue is not supported)
 *
 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
 * uaddr2 atomically on behalf of the top waiter.
 *
 * Return:
 *  - >=0 - on success, the number of tasks requeued or woken;
 *  -  <0 - on error
 */
int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
		  u32 __user *uaddr2, unsigned int flags2,
		  int nr_wake, int nr_requeue, u32 *cmpval, int requeue_pi)
{
	union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
	int task_count = 0, ret;
	struct futex_pi_state *pi_state = NULL;
	struct futex_hash_bucket *hb1, *hb2;
	struct futex_q *this, *next;
	DEFINE_WAKE_Q(wake_q);

	if (nr_wake < 0 || nr_requeue < 0)
		return -EINVAL;

	/*
	 * When PI not supported: return -ENOSYS if requeue_pi is true,
	 * consequently the compiler knows requeue_pi is always false past
	 * this point which will optimize away all the conditional code
	 * further down.
	 */
	if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
		return -ENOSYS;

	if (requeue_pi) {
		/*
		 * Requeue PI only works on two distinct uaddrs. This
		 * check is only valid for private futexes. See below.
		 */
		if (uaddr1 == uaddr2)
			return -EINVAL;

		/*
		 * futex_requeue() allows the caller to define the number
		 * of waiters to wake up via the @nr_wake argument. With
		 * REQUEUE_PI, waking up more than one waiter is creating
		 * more problems than it solves. Waking up a waiter makes
		 * only sense if the PI futex @uaddr2 is uncontended as
		 * this allows the requeue code to acquire the futex
		 * @uaddr2 before waking the waiter. The waiter can then
		 * return to user space without further action. A secondary
		 * wakeup would just make the futex_wait_requeue_pi()
		 * handling more complex, because that code would have to
		 * look up pi_state and do more or less all the handling
		 * which the requeue code has to do for the to be requeued
		 * waiters. So restrict the number of waiters to wake to
		 * one, and only wake it up when the PI futex is
		 * uncontended. Otherwise requeue it and let the unlock of
		 * the PI futex handle the wakeup.
		 *
		 * All REQUEUE_PI users, e.g. pthread_cond_signal() and
		 * pthread_cond_broadcast() must use nr_wake=1.
		 */
		if (nr_wake != 1)
			return -EINVAL;

		/*
		 * requeue_pi requires a pi_state, try to allocate it now
		 * without any locks in case it fails.
		 */
		if (refill_pi_state_cache())
			return -ENOMEM;
	}

retry:
	ret = get_futex_key(uaddr1, flags1, &key1, FUTEX_READ);
	if (unlikely(ret != 0))
		return ret;
	ret = get_futex_key(uaddr2, flags2, &key2,
			    requeue_pi ? FUTEX_WRITE : FUTEX_READ);
	if (unlikely(ret != 0))
		return ret;

	/*
	 * The check above which compares uaddrs is not sufficient for
	 * shared futexes. We need to compare the keys:
	 */
	if (requeue_pi && futex_match(&key1, &key2))
		return -EINVAL;

	hb1 = futex_hash(&key1);
	hb2 = futex_hash(&key2);

retry_private:
	futex_hb_waiters_inc(hb2);
	double_lock_hb(hb1, hb2);

	if (likely(cmpval != NULL)) {
		u32 curval;

		ret = futex_get_value_locked(&curval, uaddr1);

		if (unlikely(ret)) {
			double_unlock_hb(hb1, hb2);
			futex_hb_waiters_dec(hb2);

			ret = get_user(curval, uaddr1);
			if (ret)
				return ret;

			if (!(flags1 & FLAGS_SHARED))
				goto retry_private;

			goto retry;
		}
		if (curval != *cmpval) {
			ret = -EAGAIN;
			goto out_unlock;
		}
	}

	if (requeue_pi) {
		struct task_struct *exiting = NULL;

		/*
		 * Attempt to acquire uaddr2 and wake the top waiter. If we
		 * intend to requeue waiters, force setting the FUTEX_WAITERS
		 * bit.  We force this here where we are able to easily handle
		 * faults rather in the requeue loop below.
		 *
		 * Updates topwaiter::requeue_state if a top waiter exists.
		 */
		ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
						 &key2, &pi_state,
						 &exiting, nr_requeue);

		/*
		 * At this point the top_waiter has either taken uaddr2 or
		 * is waiting on it. In both cases pi_state has been
		 * established and an initial refcount on it. In case of an
		 * error there's nothing.
		 *
		 * The top waiter's requeue_state is up to date:
		 *
		 *  - If the lock was acquired atomically (ret == 1), then
		 *    the state is Q_REQUEUE_PI_LOCKED.
		 *
		 *    The top waiter has been dequeued and woken up and can
		 *    return to user space immediately. The kernel/user
		 *    space state is consistent. In case that there must be
		 *    more waiters requeued the WAITERS bit in the user
		 *    space futex is set so the top waiter task has to go
		 *    into the syscall slowpath to unlock the futex. This
		 *    will block until this requeue operation has been
		 *    completed and the hash bucket locks have been
		 *    dropped.
		 *
		 *  - If the trylock failed with an error (ret < 0) then
		 *    the state is either Q_REQUEUE_PI_NONE, i.e. "nothing
		 *    happened", or Q_REQUEUE_PI_IGNORE when there was an
		 *    interleaved early wakeup.
		 *
		 *  - If the trylock did not succeed (ret == 0) then the
		 *    state is either Q_REQUEUE_PI_IN_PROGRESS or
		 *    Q_REQUEUE_PI_WAIT if an early wakeup interleaved.
		 *    This will be cleaned up in the loop below, which
		 *    cannot fail because futex_proxy_trylock_atomic() did
		 *    the same sanity checks for requeue_pi as the loop
		 *    below does.
		 */
		switch (ret) {
		case 0:
			/* We hold a reference on the pi state. */
			break;

		case 1:
			/*
			 * futex_proxy_trylock_atomic() acquired the user space
			 * futex. Adjust task_count.
			 */
			task_count++;
			ret = 0;
			break;

		/*
		 * If the above failed, then pi_state is NULL and
		 * waiter::requeue_state is correct.
		 */
		case -EFAULT:
			double_unlock_hb(hb1, hb2);
			futex_hb_waiters_dec(hb2);
			ret = fault_in_user_writeable(uaddr2);
			if (!ret)
				goto retry;
			return ret;
		case -EBUSY:
		case -EAGAIN:
			/*
			 * Two reasons for this:
			 * - EBUSY: Owner is exiting and we just wait for the
			 *   exit to complete.
			 * - EAGAIN: The user space value changed.
			 */
			double_unlock_hb(hb1, hb2);
			futex_hb_waiters_dec(hb2);
			/*
			 * Handle the case where the owner is in the middle of
			 * exiting. Wait for the exit to complete otherwise
			 * this task might loop forever, aka. live lock.
			 */
			wait_for_owner_exiting(ret, exiting);
			cond_resched();
			goto retry;
		default:
			goto out_unlock;
		}
	}

	plist_for_each_entry_safe(this, next, &hb1->chain, list) {
		if (task_count - nr_wake >= nr_requeue)
			break;

		if (!futex_match(&this->key, &key1))
			continue;

		/*
		 * FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI should always
		 * be paired with each other and no other futex ops.
		 *
		 * We should never be requeueing a futex_q with a pi_state,
		 * which is awaiting a futex_unlock_pi().
		 */
		if ((requeue_pi && !this->rt_waiter) ||
		    (!requeue_pi && this->rt_waiter) ||
		    this->pi_state) {
			ret = -EINVAL;
			break;
		}

		/* Plain futexes just wake or requeue and are done */
		if (!requeue_pi) {
			if (++task_count <= nr_wake)
				this->wake(&wake_q, this);
			else
				requeue_futex(this, hb1, hb2, &key2);
			continue;
		}

		/* Ensure we requeue to the expected futex for requeue_pi. */
		if (!futex_match(this->requeue_pi_key, &key2)) {
			ret = -EINVAL;
			break;
		}

		/*
		 * Requeue nr_requeue waiters and possibly one more in the case
		 * of requeue_pi if we couldn't acquire the lock atomically.
		 *
		 * Prepare the waiter to take the rt_mutex. Take a refcount
		 * on the pi_state and store the pointer in the futex_q
		 * object of the waiter.
		 */
		get_pi_state(pi_state);

		/* Don't requeue when the waiter is already on the way out. */
		if (!futex_requeue_pi_prepare(this, pi_state)) {
			/*
			 * Early woken waiter signaled that it is on the
			 * way out. Drop the pi_state reference and try the
			 * next waiter. @this->pi_state is still NULL.
			 */
			put_pi_state(pi_state);
			continue;
		}

		ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
						this->rt_waiter,
						this->task);

		if (ret == 1) {
			/*
			 * We got the lock. We do neither drop the refcount
			 * on pi_state nor clear this->pi_state because the
			 * waiter needs the pi_state for cleaning up the
			 * user space value. It will drop the refcount
			 * after doing so. this::requeue_state is updated
			 * in the wakeup as well.
			 */
			requeue_pi_wake_futex(this, &key2, hb2);
			task_count++;
		} else if (!ret) {
			/* Waiter is queued, move it to hb2 */
			requeue_futex(this, hb1, hb2, &key2);
			futex_requeue_pi_complete(this, 0);
			task_count++;
		} else {
			/*
			 * rt_mutex_start_proxy_lock() detected a potential
			 * deadlock when we tried to queue that waiter.
			 * Drop the pi_state reference which we took above
			 * and remove the pointer to the state from the
			 * waiters futex_q object.
			 */
			this->pi_state = NULL;
			put_pi_state(pi_state);
			futex_requeue_pi_complete(this, ret);
			/*
			 * We stop queueing more waiters and let user space
			 * deal with the mess.
			 */
			break;
		}
	}

	/*
	 * We took an extra initial reference to the pi_state in
	 * futex_proxy_trylock_atomic(). We need to drop it here again.
	 */
	put_pi_state(pi_state);

out_unlock:
	double_unlock_hb(hb1, hb2);
	wake_up_q(&wake_q);
	futex_hb_waiters_dec(hb2);
	return ret ? ret : task_count;
}

/**
 * handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
 * @hb:		the hash_bucket futex_q was original enqueued on
 * @q:		the futex_q woken while waiting to be requeued
 * @timeout:	the timeout associated with the wait (NULL if none)
 *
 * Determine the cause for the early wakeup.
 *
 * Return:
 *  -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
 */
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
				   struct futex_q *q,
				   struct hrtimer_sleeper *timeout)
{
	int ret;

	/*
	 * With the hb lock held, we avoid races while we process the wakeup.
	 * We only need to hold hb (and not hb2) to ensure atomicity as the
	 * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
	 * It can't be requeued from uaddr2 to something else since we don't
	 * support a PI aware source futex for requeue.
	 */
	WARN_ON_ONCE(&hb->lock != q->lock_ptr);

	/*
	 * We were woken prior to requeue by a timeout or a signal.
	 * Unqueue the futex_q and determine which it was.
	 */
	plist_del(&q->list, &hb->chain);
	futex_hb_waiters_dec(hb);

	/* Handle spurious wakeups gracefully */
	ret = -EWOULDBLOCK;
	if (timeout && !timeout->task)
		ret = -ETIMEDOUT;
	else if (signal_pending(current))
		ret = -ERESTARTNOINTR;
	return ret;
}

/**
 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
 * @uaddr:	the futex we initially wait on (non-pi)
 * @flags:	futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
 *		the same type, no requeueing from private to shared, etc.
 * @val:	the expected value of uaddr
 * @abs_time:	absolute timeout
 * @bitset:	32 bit wakeup bitset set by userspace, defaults to all
 * @uaddr2:	the pi futex we will take prior to returning to user-space
 *
 * The caller will wait on uaddr and will be requeued by futex_requeue() to
 * uaddr2 which must be PI aware and unique from uaddr.  Normal wakeup will wake
 * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
 * userspace.  This ensures the rt_mutex maintains an owner when it has waiters;
 * without one, the pi logic would not know which task to boost/deboost, if
 * there was a need to.
 *
 * We call schedule in futex_wait_queue() when we enqueue and return there
 * via the following--
 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
 * 2) wakeup on uaddr2 after a requeue
 * 3) signal
 * 4) timeout
 *
 * If 3, cleanup and return -ERESTARTNOINTR.
 *
 * If 2, we may then block on trying to take the rt_mutex and return via:
 * 5) successful lock
 * 6) signal
 * 7) timeout
 * 8) other lock acquisition failure
 *
 * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
 *
 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
 *
 * Return:
 *  -  0 - On success;
 *  - <0 - On error
 */
int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
			  u32 val, ktime_t *abs_time, u32 bitset,
			  u32 __user *uaddr2)
{
	struct hrtimer_sleeper timeout, *to;
	struct rt_mutex_waiter rt_waiter;
	struct futex_hash_bucket *hb;
	union futex_key key2 = FUTEX_KEY_INIT;
	struct futex_q q = futex_q_init;
	struct rt_mutex_base *pi_mutex;
	int res, ret;

	if (!IS_ENABLED(CONFIG_FUTEX_PI))
		return -ENOSYS;

	if (uaddr == uaddr2)
		return -EINVAL;

	if (!bitset)
		return -EINVAL;

	to = futex_setup_timer(abs_time, &timeout, flags,
			       current->timer_slack_ns);

	/*
	 * The waiter is allocated on our stack, manipulated by the requeue
	 * code while we sleep on uaddr.
	 */
	rt_mutex_init_waiter(&rt_waiter);

	ret = get_futex_key(uaddr2, flags, &key2, FUTEX_WRITE);
	if (unlikely(ret != 0))
		goto out;

	q.bitset = bitset;
	q.rt_waiter = &rt_waiter;
	q.requeue_pi_key = &key2;

	/*
	 * Prepare to wait on uaddr. On success, it holds hb->lock and q
	 * is initialized.
	 */
	ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
	if (ret)
		goto out;

	/*
	 * The check above which compares uaddrs is not sufficient for
	 * shared futexes. We need to compare the keys:
	 */
	if (futex_match(&q.key, &key2)) {
		futex_q_unlock(hb);
		ret = -EINVAL;
		goto out;
	}

	/* Queue the futex_q, drop the hb lock, wait for wakeup. */
	futex_wait_queue(hb, &q, to);

	switch (futex_requeue_pi_wakeup_sync(&q)) {
	case Q_REQUEUE_PI_IGNORE:
		/* The waiter is still on uaddr1 */
		spin_lock(&hb->lock);
		ret = handle_early_requeue_pi_wakeup(hb, &q, to);
		spin_unlock(&hb->lock);
		break;

	case Q_REQUEUE_PI_LOCKED:
		/* The requeue acquired the lock */
		if (q.pi_state && (q.pi_state->owner != current)) {
			spin_lock(q.lock_ptr);
			ret = fixup_pi_owner(uaddr2, &q, true);
			/*
			 * Drop the reference to the pi state which the
			 * requeue_pi() code acquired for us.
			 */
			put_pi_state(q.pi_state);
			spin_unlock(q.lock_ptr);
			/*
			 * Adjust the return value. It's either -EFAULT or
			 * success (1) but the caller expects 0 for success.
			 */
			ret = ret < 0 ? ret : 0;
		}
		break;

	case Q_REQUEUE_PI_DONE:
		/* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
		pi_mutex = &q.pi_state->pi_mutex;
		ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);

		/*
		 * See futex_unlock_pi()'s cleanup: comment.
		 */
		if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
			ret = 0;

		spin_lock(q.lock_ptr);
		debug_rt_mutex_free_waiter(&rt_waiter);
		/*
		 * Fixup the pi_state owner and possibly acquire the lock if we
		 * haven't already.
		 */
		res = fixup_pi_owner(uaddr2, &q, !ret);
		/*
		 * If fixup_pi_owner() returned an error, propagate that.  If it
		 * acquired the lock, clear -ETIMEDOUT or -EINTR.
		 */
		if (res)
			ret = (res < 0) ? res : 0;

		futex_unqueue_pi(&q);
		spin_unlock(q.lock_ptr);

		if (ret == -EINTR) {
			/*
			 * We've already been requeued, but cannot restart
			 * by calling futex_lock_pi() directly. We could
			 * restart this syscall, but it would detect that
			 * the user space "val" changed and return
			 * -EWOULDBLOCK.  Save the overhead of the restart
			 * and return -EWOULDBLOCK directly.
			 */
			ret = -EWOULDBLOCK;
		}
		break;
	default:
		BUG();
	}

out:
	if (to) {
		hrtimer_cancel(&to->timer);
		destroy_hrtimer_on_stack(&to->timer);
	}
	return ret;
}