// 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()*/
        .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 = NULL;
        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
 * @flags:      futex flags (FLAGS_SHARED, etc.)
 * @uaddr2:     target futex user address
 * @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 flags, u32 __user *uaddr2,
                  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, flags & FLAGS_SHARED, &key1, FUTEX_READ);
        if (unlikely(ret != 0))
                return ret;
        ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &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 (!(flags & 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)
                                futex_wake_mark(&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 & FLAGS_SHARED, &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);

                /* Current is not longer pi_blocked_on */
                spin_lock(q.lock_ptr);
                if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
                        ret = 0;

                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;
}