locking/ww_mutex: Add rt_mutex based lock type and accessors

[linux-2.6-microblaze.git] / kernel / locking / rtmutex.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * RT-Mutexes: simple blocking mutual exclusion locks with PI support
 *
 * started by Ingo Molnar and Thomas Gleixner.
 *
 *  Copyright (C) 2004-2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *  Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
 *  Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt
 *  Copyright (C) 2006 Esben Nielsen
 *
 *  See Documentation/locking/rt-mutex-design.rst for details.
 */
#include <linux/sched.h>
#include <linux/sched/debug.h>
#include <linux/sched/deadline.h>
#include <linux/sched/signal.h>
#include <linux/sched/rt.h>
#include <linux/sched/wake_q.h>

#include "rtmutex_common.h"

/*
 * lock->owner state tracking:
 *
 * lock->owner holds the task_struct pointer of the owner. Bit 0
 * is used to keep track of the "lock has waiters" state.
 *
 * owner        bit0
 * NULL         0       lock is free (fast acquire possible)
 * NULL         1       lock is free and has waiters and the top waiter
 *                              is going to take the lock*
 * taskpointer  0       lock is held (fast release possible)
 * taskpointer  1       lock is held and has waiters**
 *
 * The fast atomic compare exchange based acquire and release is only
 * possible when bit 0 of lock->owner is 0.
 *
 * (*) It also can be a transitional state when grabbing the lock
 * with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
 * we need to set the bit0 before looking at the lock, and the owner may be
 * NULL in this small time, hence this can be a transitional state.
 *
 * (**) There is a small time when bit 0 is set but there are no
 * waiters. This can happen when grabbing the lock in the slow path.
 * To prevent a cmpxchg of the owner releasing the lock, we need to
 * set this bit before looking at the lock.
 */

static __always_inline void
rt_mutex_set_owner(struct rt_mutex_base *lock, struct task_struct *owner)
{
        unsigned long val = (unsigned long)owner;

        if (rt_mutex_has_waiters(lock))
                val |= RT_MUTEX_HAS_WAITERS;

        WRITE_ONCE(lock->owner, (struct task_struct *)val);
}

static __always_inline void clear_rt_mutex_waiters(struct rt_mutex_base *lock)
{
        lock->owner = (struct task_struct *)
                        ((unsigned long)lock->owner & ~RT_MUTEX_HAS_WAITERS);
}

static __always_inline void fixup_rt_mutex_waiters(struct rt_mutex_base *lock)
{
        unsigned long owner, *p = (unsigned long *) &lock->owner;

        if (rt_mutex_has_waiters(lock))
                return;

        /*
         * The rbtree has no waiters enqueued, now make sure that the
         * lock->owner still has the waiters bit set, otherwise the
         * following can happen:
         *
         * CPU 0        CPU 1           CPU2
         * l->owner=T1
         *              rt_mutex_lock(l)
         *              lock(l->lock)
         *              l->owner = T1 | HAS_WAITERS;
         *              enqueue(T2)
         *              boost()
         *                unlock(l->lock)
         *              block()
         *
         *                              rt_mutex_lock(l)
         *                              lock(l->lock)
         *                              l->owner = T1 | HAS_WAITERS;
         *                              enqueue(T3)
         *                              boost()
         *                                unlock(l->lock)
         *                              block()
         *              signal(->T2)    signal(->T3)
         *              lock(l->lock)
         *              dequeue(T2)
         *              deboost()
         *                unlock(l->lock)
         *                              lock(l->lock)
         *                              dequeue(T3)
         *                               ==> wait list is empty
         *                              deboost()
         *                               unlock(l->lock)
         *              lock(l->lock)
         *              fixup_rt_mutex_waiters()
         *                if (wait_list_empty(l) {
         *                  l->owner = owner
         *                  owner = l->owner & ~HAS_WAITERS;
         *                    ==> l->owner = T1
         *                }
         *                              lock(l->lock)
         * rt_mutex_unlock(l)           fixup_rt_mutex_waiters()
         *                                if (wait_list_empty(l) {
         *                                  owner = l->owner & ~HAS_WAITERS;
         * cmpxchg(l->owner, T1, NULL)
         *  ===> Success (l->owner = NULL)
         *
         *                                  l->owner = owner
         *                                    ==> l->owner = T1
         *                                }
         *
         * With the check for the waiter bit in place T3 on CPU2 will not
         * overwrite. All tasks fiddling with the waiters bit are
         * serialized by l->lock, so nothing else can modify the waiters
         * bit. If the bit is set then nothing can change l->owner either
         * so the simple RMW is safe. The cmpxchg() will simply fail if it
         * happens in the middle of the RMW because the waiters bit is
         * still set.
         */
        owner = READ_ONCE(*p);
        if (owner & RT_MUTEX_HAS_WAITERS)
                WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
}

/*
 * We can speed up the acquire/release, if there's no debugging state to be
 * set up.
 */
#ifndef CONFIG_DEBUG_RT_MUTEXES
static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,
                                                     struct task_struct *old,
                                                     struct task_struct *new)
{
        return try_cmpxchg_acquire(&lock->owner, &old, new);
}

static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
                                                     struct task_struct *old,
                                                     struct task_struct *new)
{
        return try_cmpxchg_release(&lock->owner, &old, new);
}

/*
 * Callers must hold the ->wait_lock -- which is the whole purpose as we force
 * all future threads that attempt to [Rmw] the lock to the slowpath. As such
 * relaxed semantics suffice.
 */
static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
{
        unsigned long owner, *p = (unsigned long *) &lock->owner;

        do {
                owner = *p;
        } while (cmpxchg_relaxed(p, owner,
                                 owner | RT_MUTEX_HAS_WAITERS) != owner);
}

/*
 * Safe fastpath aware unlock:
 * 1) Clear the waiters bit
 * 2) Drop lock->wait_lock
 * 3) Try to unlock the lock with cmpxchg
 */
static __always_inline bool unlock_rt_mutex_safe(struct rt_mutex_base *lock,
                                                 unsigned long flags)
        __releases(lock->wait_lock)
{
        struct task_struct *owner = rt_mutex_owner(lock);

        clear_rt_mutex_waiters(lock);
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
        /*
         * If a new waiter comes in between the unlock and the cmpxchg
         * we have two situations:
         *
         * unlock(wait_lock);
         *                                      lock(wait_lock);
         * cmpxchg(p, owner, 0) == owner
         *                                      mark_rt_mutex_waiters(lock);
         *                                      acquire(lock);
         * or:
         *
         * unlock(wait_lock);
         *                                      lock(wait_lock);
         *                                      mark_rt_mutex_waiters(lock);
         *
         * cmpxchg(p, owner, 0) != owner
         *                                      enqueue_waiter();
         *                                      unlock(wait_lock);
         * lock(wait_lock);
         * wake waiter();
         * unlock(wait_lock);
         *                                      lock(wait_lock);
         *                                      acquire(lock);
         */
        return rt_mutex_cmpxchg_release(lock, owner, NULL);
}

#else
static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,
                                                     struct task_struct *old,
                                                     struct task_struct *new)
{
        return false;

}

static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
                                                     struct task_struct *old,
                                                     struct task_struct *new)
{
        return false;
}

static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
{
        lock->owner = (struct task_struct *)
                        ((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
}

/*
 * Simple slow path only version: lock->owner is protected by lock->wait_lock.
 */
static __always_inline bool unlock_rt_mutex_safe(struct rt_mutex_base *lock,
                                                 unsigned long flags)
        __releases(lock->wait_lock)
{
        lock->owner = NULL;
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
        return true;
}
#endif

static __always_inline int __waiter_prio(struct task_struct *task)
{
        int prio = task->prio;

        if (!rt_prio(prio))
                return DEFAULT_PRIO;

        return prio;
}

static __always_inline void
waiter_update_prio(struct rt_mutex_waiter *waiter, struct task_struct *task)
{
        waiter->prio = __waiter_prio(task);
        waiter->deadline = task->dl.deadline;
}

/*
 * Only use with rt_mutex_waiter_{less,equal}()
 */
#define task_to_waiter(p)       \
        &(struct rt_mutex_waiter){ .prio = __waiter_prio(p), .deadline = (p)->dl.deadline }

static __always_inline int rt_mutex_waiter_less(struct rt_mutex_waiter *left,
                                                struct rt_mutex_waiter *right)
{
        if (left->prio < right->prio)
                return 1;

        /*
         * If both waiters have dl_prio(), we check the deadlines of the
         * associated tasks.
         * If left waiter has a dl_prio(), and we didn't return 1 above,
         * then right waiter has a dl_prio() too.
         */
        if (dl_prio(left->prio))
                return dl_time_before(left->deadline, right->deadline);

        return 0;
}

static __always_inline int rt_mutex_waiter_equal(struct rt_mutex_waiter *left,
                                                 struct rt_mutex_waiter *right)
{
        if (left->prio != right->prio)
                return 0;

        /*
         * If both waiters have dl_prio(), we check the deadlines of the
         * associated tasks.
         * If left waiter has a dl_prio(), and we didn't return 0 above,
         * then right waiter has a dl_prio() too.
         */
        if (dl_prio(left->prio))
                return left->deadline == right->deadline;

        return 1;
}

#define __node_2_waiter(node) \
        rb_entry((node), struct rt_mutex_waiter, tree_entry)

static __always_inline bool __waiter_less(struct rb_node *a, const struct rb_node *b)
{
        return rt_mutex_waiter_less(__node_2_waiter(a), __node_2_waiter(b));
}

static __always_inline void
rt_mutex_enqueue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter)
{
        rb_add_cached(&waiter->tree_entry, &lock->waiters, __waiter_less);
}

static __always_inline void
rt_mutex_dequeue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter)
{
        if (RB_EMPTY_NODE(&waiter->tree_entry))
                return;

        rb_erase_cached(&waiter->tree_entry, &lock->waiters);
        RB_CLEAR_NODE(&waiter->tree_entry);
}

#define __node_2_pi_waiter(node) \
        rb_entry((node), struct rt_mutex_waiter, pi_tree_entry)

static __always_inline bool
__pi_waiter_less(struct rb_node *a, const struct rb_node *b)
{
        return rt_mutex_waiter_less(__node_2_pi_waiter(a), __node_2_pi_waiter(b));
}

static __always_inline void
rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
{
        rb_add_cached(&waiter->pi_tree_entry, &task->pi_waiters, __pi_waiter_less);
}

static __always_inline void
rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
{
        if (RB_EMPTY_NODE(&waiter->pi_tree_entry))
                return;

        rb_erase_cached(&waiter->pi_tree_entry, &task->pi_waiters);
        RB_CLEAR_NODE(&waiter->pi_tree_entry);
}

static __always_inline void rt_mutex_adjust_prio(struct task_struct *p)
{
        struct task_struct *pi_task = NULL;

        lockdep_assert_held(&p->pi_lock);

        if (task_has_pi_waiters(p))
                pi_task = task_top_pi_waiter(p)->task;

        rt_mutex_setprio(p, pi_task);
}

/* RT mutex specific wake_q wrappers */
static __always_inline void rt_mutex_wake_q_add(struct rt_wake_q_head *wqh,
                                                struct rt_mutex_waiter *w)
{
        if (IS_ENABLED(CONFIG_PREEMPT_RT) && w->wake_state != TASK_NORMAL) {
                if (IS_ENABLED(CONFIG_PROVE_LOCKING))
                        WARN_ON_ONCE(wqh->rtlock_task);
                get_task_struct(w->task);
                wqh->rtlock_task = w->task;
        } else {
                wake_q_add(&wqh->head, w->task);
        }
}

static __always_inline void rt_mutex_wake_up_q(struct rt_wake_q_head *wqh)
{
        if (IS_ENABLED(CONFIG_PREEMPT_RT) && wqh->rtlock_task) {
                wake_up_state(wqh->rtlock_task, TASK_RTLOCK_WAIT);
                put_task_struct(wqh->rtlock_task);
                wqh->rtlock_task = NULL;
        }

        if (!wake_q_empty(&wqh->head))
                wake_up_q(&wqh->head);

        /* Pairs with preempt_disable() in mark_wakeup_next_waiter() */
        preempt_enable();
}

/*
 * Deadlock detection is conditional:
 *
 * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
 * if the detect argument is == RT_MUTEX_FULL_CHAINWALK.
 *
 * If CONFIG_DEBUG_RT_MUTEXES=y, deadlock detection is always
 * conducted independent of the detect argument.
 *
 * If the waiter argument is NULL this indicates the deboost path and
 * deadlock detection is disabled independent of the detect argument
 * and the config settings.
 */
static __always_inline bool
rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,
                              enum rtmutex_chainwalk chwalk)
{
        if (IS_ENABLED(CONFIG_DEBUG_RT_MUTEXES))
                return waiter != NULL;
        return chwalk == RT_MUTEX_FULL_CHAINWALK;
}

static __always_inline struct rt_mutex_base *task_blocked_on_lock(struct task_struct *p)
{
        return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
}

/*
 * Adjust the priority chain. Also used for deadlock detection.
 * Decreases task's usage by one - may thus free the task.
 *
 * @task:       the task owning the mutex (owner) for which a chain walk is
 *              probably needed
 * @chwalk:     do we have to carry out deadlock detection?
 * @orig_lock:  the mutex (can be NULL if we are walking the chain to recheck
 *              things for a task that has just got its priority adjusted, and
 *              is waiting on a mutex)
 * @next_lock:  the mutex on which the owner of @orig_lock was blocked before
 *              we dropped its pi_lock. Is never dereferenced, only used for
 *              comparison to detect lock chain changes.
 * @orig_waiter: rt_mutex_waiter struct for the task that has just donated
 *              its priority to the mutex owner (can be NULL in the case
 *              depicted above or if the top waiter is gone away and we are
 *              actually deboosting the owner)
 * @top_task:   the current top waiter
 *
 * Returns 0 or -EDEADLK.
 *
 * Chain walk basics and protection scope
 *
 * [R] refcount on task
 * [P] task->pi_lock held
 * [L] rtmutex->wait_lock held
 *
 * Step Description                             Protected by
 *      function arguments:
 *      @task                                   [R]
 *      @orig_lock if != NULL                   @top_task is blocked on it
 *      @next_lock                              Unprotected. Cannot be
 *                                              dereferenced. Only used for
 *                                              comparison.
 *      @orig_waiter if != NULL                 @top_task is blocked on it
 *      @top_task                               current, or in case of proxy
 *                                              locking protected by calling
 *                                              code
 *      again:
 *        loop_sanity_check();
 *      retry:
 * [1]    lock(task->pi_lock);                  [R] acquire [P]
 * [2]    waiter = task->pi_blocked_on;         [P]
 * [3]    check_exit_conditions_1();            [P]
 * [4]    lock = waiter->lock;                  [P]
 * [5]    if (!try_lock(lock->wait_lock)) {     [P] try to acquire [L]
 *          unlock(task->pi_lock);              release [P]
 *          goto retry;
 *        }
 * [6]    check_exit_conditions_2();            [P] + [L]
 * [7]    requeue_lock_waiter(lock, waiter);    [P] + [L]
 * [8]    unlock(task->pi_lock);                release [P]
 *        put_task_struct(task);                release [R]
 * [9]    check_exit_conditions_3();            [L]
 * [10]   task = owner(lock);                   [L]
 *        get_task_struct(task);                [L] acquire [R]
 *        lock(task->pi_lock);                  [L] acquire [P]
 * [11]   requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
 * [12]   check_exit_conditions_4();            [P] + [L]
 * [13]   unlock(task->pi_lock);                release [P]
 *        unlock(lock->wait_lock);              release [L]
 *        goto again;
 */
static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
                                              enum rtmutex_chainwalk chwalk,
                                              struct rt_mutex_base *orig_lock,
                                              struct rt_mutex_base *next_lock,
                                              struct rt_mutex_waiter *orig_waiter,
                                              struct task_struct *top_task)
{
        struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
        struct rt_mutex_waiter *prerequeue_top_waiter;
        int ret = 0, depth = 0;
        struct rt_mutex_base *lock;
        bool detect_deadlock;
        bool requeue = true;

        detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);

        /*
         * The (de)boosting is a step by step approach with a lot of
         * pitfalls. We want this to be preemptible and we want hold a
         * maximum of two locks per step. So we have to check
         * carefully whether things change under us.
         */
 again:
        /*
         * We limit the lock chain length for each invocation.
         */
        if (++depth > max_lock_depth) {
                static int prev_max;

                /*
                 * Print this only once. If the admin changes the limit,
                 * print a new message when reaching the limit again.
                 */
                if (prev_max != max_lock_depth) {
                        prev_max = max_lock_depth;
                        printk(KERN_WARNING "Maximum lock depth %d reached "
                               "task: %s (%d)\n", max_lock_depth,
                               top_task->comm, task_pid_nr(top_task));
                }
                put_task_struct(task);

                return -EDEADLK;
        }

        /*
         * We are fully preemptible here and only hold the refcount on
         * @task. So everything can have changed under us since the
         * caller or our own code below (goto retry/again) dropped all
         * locks.
         */
 retry:
        /*
         * [1] Task cannot go away as we did a get_task() before !
         */
        raw_spin_lock_irq(&task->pi_lock);

        /*
         * [2] Get the waiter on which @task is blocked on.
         */
        waiter = task->pi_blocked_on;

        /*
         * [3] check_exit_conditions_1() protected by task->pi_lock.
         */

        /*
         * Check whether the end of the boosting chain has been
         * reached or the state of the chain has changed while we
         * dropped the locks.
         */
        if (!waiter)
                goto out_unlock_pi;

        /*
         * Check the orig_waiter state. After we dropped the locks,
         * the previous owner of the lock might have released the lock.
         */
        if (orig_waiter && !rt_mutex_owner(orig_lock))
                goto out_unlock_pi;

        /*
         * We dropped all locks after taking a refcount on @task, so
         * the task might have moved on in the lock chain or even left
         * the chain completely and blocks now on an unrelated lock or
         * on @orig_lock.
         *
         * We stored the lock on which @task was blocked in @next_lock,
         * so we can detect the chain change.
         */
        if (next_lock != waiter->lock)
                goto out_unlock_pi;

        /*
         * Drop out, when the task has no waiters. Note,
         * top_waiter can be NULL, when we are in the deboosting
         * mode!
         */
        if (top_waiter) {
                if (!task_has_pi_waiters(task))
                        goto out_unlock_pi;
                /*
                 * If deadlock detection is off, we stop here if we
                 * are not the top pi waiter of the task. If deadlock
                 * detection is enabled we continue, but stop the
                 * requeueing in the chain walk.
                 */
                if (top_waiter != task_top_pi_waiter(task)) {
                        if (!detect_deadlock)
                                goto out_unlock_pi;
                        else
                                requeue = false;
                }
        }

        /*
         * If the waiter priority is the same as the task priority
         * then there is no further priority adjustment necessary.  If
         * deadlock detection is off, we stop the chain walk. If its
         * enabled we continue, but stop the requeueing in the chain
         * walk.
         */
        if (rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
                if (!detect_deadlock)
                        goto out_unlock_pi;
                else
                        requeue = false;
        }

        /*
         * [4] Get the next lock
         */
        lock = waiter->lock;
        /*
         * [5] We need to trylock here as we are holding task->pi_lock,
         * which is the reverse lock order versus the other rtmutex
         * operations.
         */
        if (!raw_spin_trylock(&lock->wait_lock)) {
                raw_spin_unlock_irq(&task->pi_lock);
                cpu_relax();
                goto retry;
        }

        /*
         * [6] check_exit_conditions_2() protected by task->pi_lock and
         * lock->wait_lock.
         *
         * Deadlock detection. If the lock is the same as the original
         * lock which caused us to walk the lock chain or if the
         * current lock is owned by the task which initiated the chain
         * walk, we detected a deadlock.
         */
        if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
                raw_spin_unlock(&lock->wait_lock);
                ret = -EDEADLK;
                goto out_unlock_pi;
        }

        /*
         * If we just follow the lock chain for deadlock detection, no
         * need to do all the requeue operations. To avoid a truckload
         * of conditionals around the various places below, just do the
         * minimum chain walk checks.
         */
        if (!requeue) {
                /*
                 * No requeue[7] here. Just release @task [8]
                 */
                raw_spin_unlock(&task->pi_lock);
                put_task_struct(task);

                /*
                 * [9] check_exit_conditions_3 protected by lock->wait_lock.
                 * If there is no owner of the lock, end of chain.
                 */
                if (!rt_mutex_owner(lock)) {
                        raw_spin_unlock_irq(&lock->wait_lock);
                        return 0;
                }

                /* [10] Grab the next task, i.e. owner of @lock */
                task = get_task_struct(rt_mutex_owner(lock));
                raw_spin_lock(&task->pi_lock);

                /*
                 * No requeue [11] here. We just do deadlock detection.
                 *
                 * [12] Store whether owner is blocked
                 * itself. Decision is made after dropping the locks
                 */
                next_lock = task_blocked_on_lock(task);
                /*
                 * Get the top waiter for the next iteration
                 */
                top_waiter = rt_mutex_top_waiter(lock);

                /* [13] Drop locks */
                raw_spin_unlock(&task->pi_lock);
                raw_spin_unlock_irq(&lock->wait_lock);

                /* If owner is not blocked, end of chain. */
                if (!next_lock)
                        goto out_put_task;
                goto again;
        }

        /*
         * Store the current top waiter before doing the requeue
         * operation on @lock. We need it for the boost/deboost
         * decision below.
         */
        prerequeue_top_waiter = rt_mutex_top_waiter(lock);

        /* [7] Requeue the waiter in the lock waiter tree. */
        rt_mutex_dequeue(lock, waiter);

        /*
         * Update the waiter prio fields now that we're dequeued.
         *
         * These values can have changed through either:
         *
         *   sys_sched_set_scheduler() / sys_sched_setattr()
         *
         * or
         *
         *   DL CBS enforcement advancing the effective deadline.
         *
         * Even though pi_waiters also uses these fields, and that tree is only
         * updated in [11], we can do this here, since we hold [L], which
         * serializes all pi_waiters access and rb_erase() does not care about
         * the values of the node being removed.
         */
        waiter_update_prio(waiter, task);

        rt_mutex_enqueue(lock, waiter);

        /* [8] Release the task */
        raw_spin_unlock(&task->pi_lock);
        put_task_struct(task);

        /*
         * [9] check_exit_conditions_3 protected by lock->wait_lock.
         *
         * We must abort the chain walk if there is no lock owner even
         * in the dead lock detection case, as we have nothing to
         * follow here. This is the end of the chain we are walking.
         */
        if (!rt_mutex_owner(lock)) {
                /*
                 * If the requeue [7] above changed the top waiter,
                 * then we need to wake the new top waiter up to try
                 * to get the lock.
                 */
                if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
                        wake_up_state(waiter->task, waiter->wake_state);
                raw_spin_unlock_irq(&lock->wait_lock);
                return 0;
        }

        /* [10] Grab the next task, i.e. the owner of @lock */
        task = get_task_struct(rt_mutex_owner(lock));
        raw_spin_lock(&task->pi_lock);

        /* [11] requeue the pi waiters if necessary */
        if (waiter == rt_mutex_top_waiter(lock)) {
                /*
                 * The waiter became the new top (highest priority)
                 * waiter on the lock. Replace the previous top waiter
                 * in the owner tasks pi waiters tree with this waiter
                 * and adjust the priority of the owner.
                 */
                rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
                rt_mutex_enqueue_pi(task, waiter);
                rt_mutex_adjust_prio(task);

        } else if (prerequeue_top_waiter == waiter) {
                /*
                 * The waiter was the top waiter on the lock, but is
                 * no longer the top priority waiter. Replace waiter in
                 * the owner tasks pi waiters tree with the new top
                 * (highest priority) waiter and adjust the priority
                 * of the owner.
                 * The new top waiter is stored in @waiter so that
                 * @waiter == @top_waiter evaluates to true below and
                 * we continue to deboost the rest of the chain.
                 */
                rt_mutex_dequeue_pi(task, waiter);
                waiter = rt_mutex_top_waiter(lock);
                rt_mutex_enqueue_pi(task, waiter);
                rt_mutex_adjust_prio(task);
        } else {
                /*
                 * Nothing changed. No need to do any priority
                 * adjustment.
                 */
        }

        /*
         * [12] check_exit_conditions_4() protected by task->pi_lock
         * and lock->wait_lock. The actual decisions are made after we
         * dropped the locks.
         *
         * Check whether the task which owns the current lock is pi
         * blocked itself. If yes we store a pointer to the lock for
         * the lock chain change detection above. After we dropped
         * task->pi_lock next_lock cannot be dereferenced anymore.
         */
        next_lock = task_blocked_on_lock(task);
        /*
         * Store the top waiter of @lock for the end of chain walk
         * decision below.
         */
        top_waiter = rt_mutex_top_waiter(lock);

        /* [13] Drop the locks */
        raw_spin_unlock(&task->pi_lock);
        raw_spin_unlock_irq(&lock->wait_lock);

        /*
         * Make the actual exit decisions [12], based on the stored
         * values.
         *
         * We reached the end of the lock chain. Stop right here. No
         * point to go back just to figure that out.
         */
        if (!next_lock)
                goto out_put_task;

        /*
         * If the current waiter is not the top waiter on the lock,
         * then we can stop the chain walk here if we are not in full
         * deadlock detection mode.
         */
        if (!detect_deadlock && waiter != top_waiter)
                goto out_put_task;

        goto again;

 out_unlock_pi:
        raw_spin_unlock_irq(&task->pi_lock);
 out_put_task:
        put_task_struct(task);

        return ret;
}

/*
 * Try to take an rt-mutex
 *
 * Must be called with lock->wait_lock held and interrupts disabled
 *
 * @lock:   The lock to be acquired.
 * @task:   The task which wants to acquire the lock
 * @waiter: The waiter that is queued to the lock's wait tree if the
 *          callsite called task_blocked_on_lock(), otherwise NULL
 */
static int __sched
try_to_take_rt_mutex(struct rt_mutex_base *lock, struct task_struct *task,
                     struct rt_mutex_waiter *waiter)
{
        lockdep_assert_held(&lock->wait_lock);

        /*
         * Before testing whether we can acquire @lock, we set the
         * RT_MUTEX_HAS_WAITERS bit in @lock->owner. This forces all
         * other tasks which try to modify @lock into the slow path
         * and they serialize on @lock->wait_lock.
         *
         * The RT_MUTEX_HAS_WAITERS bit can have a transitional state
         * as explained at the top of this file if and only if:
         *
         * - There is a lock owner. The caller must fixup the
         *   transient state if it does a trylock or leaves the lock
         *   function due to a signal or timeout.
         *
         * - @task acquires the lock and there are no other
         *   waiters. This is undone in rt_mutex_set_owner(@task) at
         *   the end of this function.
         */
        mark_rt_mutex_waiters(lock);

        /*
         * If @lock has an owner, give up.
         */
        if (rt_mutex_owner(lock))
                return 0;

        /*
         * If @waiter != NULL, @task has already enqueued the waiter
         * into @lock waiter tree. If @waiter == NULL then this is a
         * trylock attempt.
         */
        if (waiter) {
                /*
                 * If waiter is not the highest priority waiter of
                 * @lock, give up.
                 */
                if (waiter != rt_mutex_top_waiter(lock))
                        return 0;

                /*
                 * We can acquire the lock. Remove the waiter from the
                 * lock waiters tree.
                 */
                rt_mutex_dequeue(lock, waiter);

        } else {
                /*
                 * If the lock has waiters already we check whether @task is
                 * eligible to take over the lock.
                 *
                 * If there are no other waiters, @task can acquire
                 * the lock.  @task->pi_blocked_on is NULL, so it does
                 * not need to be dequeued.
                 */
                if (rt_mutex_has_waiters(lock)) {
                        /*
                         * If @task->prio is greater than or equal to
                         * the top waiter priority (kernel view),
                         * @task lost.
                         */
                        if (!rt_mutex_waiter_less(task_to_waiter(task),
                                                  rt_mutex_top_waiter(lock)))
                                return 0;

                        /*
                         * The current top waiter stays enqueued. We
                         * don't have to change anything in the lock
                         * waiters order.
                         */
                } else {
                        /*
                         * No waiters. Take the lock without the
                         * pi_lock dance.@task->pi_blocked_on is NULL
                         * and we have no waiters to enqueue in @task
                         * pi waiters tree.
                         */
                        goto takeit;
                }
        }

        /*
         * Clear @task->pi_blocked_on. Requires protection by
         * @task->pi_lock. Redundant operation for the @waiter == NULL
         * case, but conditionals are more expensive than a redundant
         * store.
         */
        raw_spin_lock(&task->pi_lock);
        task->pi_blocked_on = NULL;
        /*
         * Finish the lock acquisition. @task is the new owner. If
         * other waiters exist we have to insert the highest priority
         * waiter into @task->pi_waiters tree.
         */
        if (rt_mutex_has_waiters(lock))
                rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
        raw_spin_unlock(&task->pi_lock);

takeit:
        /*
         * This either preserves the RT_MUTEX_HAS_WAITERS bit if there
         * are still waiters or clears it.
         */
        rt_mutex_set_owner(lock, task);

        return 1;
}

/*
 * Task blocks on lock.
 *
 * Prepare waiter and propagate pi chain
 *
 * This must be called with lock->wait_lock held and interrupts disabled
 */
static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
                                           struct rt_mutex_waiter *waiter,
                                           struct task_struct *task,
                                           enum rtmutex_chainwalk chwalk)
{
        struct task_struct *owner = rt_mutex_owner(lock);
        struct rt_mutex_waiter *top_waiter = waiter;
        struct rt_mutex_base *next_lock;
        int chain_walk = 0, res;

        lockdep_assert_held(&lock->wait_lock);

        /*
         * Early deadlock detection. We really don't want the task to
         * enqueue on itself just to untangle the mess later. It's not
         * only an optimization. We drop the locks, so another waiter
         * can come in before the chain walk detects the deadlock. So
         * the other will detect the deadlock and return -EDEADLOCK,
         * which is wrong, as the other waiter is not in a deadlock
         * situation.
         */
        if (owner == task)
                return -EDEADLK;

        raw_spin_lock(&task->pi_lock);
        waiter->task = task;
        waiter->lock = lock;
        waiter_update_prio(waiter, task);

        /* Get the top priority waiter on the lock */
        if (rt_mutex_has_waiters(lock))
                top_waiter = rt_mutex_top_waiter(lock);
        rt_mutex_enqueue(lock, waiter);

        task->pi_blocked_on = waiter;

        raw_spin_unlock(&task->pi_lock);

        if (!owner)
                return 0;

        raw_spin_lock(&owner->pi_lock);
        if (waiter == rt_mutex_top_waiter(lock)) {
                rt_mutex_dequeue_pi(owner, top_waiter);
                rt_mutex_enqueue_pi(owner, waiter);

                rt_mutex_adjust_prio(owner);
                if (owner->pi_blocked_on)
                        chain_walk = 1;
        } else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
                chain_walk = 1;
        }

        /* Store the lock on which owner is blocked or NULL */
        next_lock = task_blocked_on_lock(owner);

        raw_spin_unlock(&owner->pi_lock);
        /*
         * Even if full deadlock detection is on, if the owner is not
         * blocked itself, we can avoid finding this out in the chain
         * walk.
         */
        if (!chain_walk || !next_lock)
                return 0;

        /*
         * The owner can't disappear while holding a lock,
         * so the owner struct is protected by wait_lock.
         * Gets dropped in rt_mutex_adjust_prio_chain()!
         */
        get_task_struct(owner);

        raw_spin_unlock_irq(&lock->wait_lock);

        res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
                                         next_lock, waiter, task);

        raw_spin_lock_irq(&lock->wait_lock);

        return res;
}

/*
 * Remove the top waiter from the current tasks pi waiter tree and
 * queue it up.
 *
 * Called with lock->wait_lock held and interrupts disabled.
 */
static void __sched mark_wakeup_next_waiter(struct rt_wake_q_head *wqh,
                                            struct rt_mutex_base *lock)
{
        struct rt_mutex_waiter *waiter;

        raw_spin_lock(&current->pi_lock);

        waiter = rt_mutex_top_waiter(lock);

        /*
         * Remove it from current->pi_waiters and deboost.
         *
         * We must in fact deboost here in order to ensure we call
         * rt_mutex_setprio() to update p->pi_top_task before the
         * task unblocks.
         */
        rt_mutex_dequeue_pi(current, waiter);
        rt_mutex_adjust_prio(current);

        /*
         * As we are waking up the top waiter, and the waiter stays
         * queued on the lock until it gets the lock, this lock
         * obviously has waiters. Just set the bit here and this has
         * the added benefit of forcing all new tasks into the
         * slow path making sure no task of lower priority than
         * the top waiter can steal this lock.
         */
        lock->owner = (void *) RT_MUTEX_HAS_WAITERS;

        /*
         * We deboosted before waking the top waiter task such that we don't
         * run two tasks with the 'same' priority (and ensure the
         * p->pi_top_task pointer points to a blocked task). This however can
         * lead to priority inversion if we would get preempted after the
         * deboost but before waking our donor task, hence the preempt_disable()
         * before unlock.
         *
         * Pairs with preempt_enable() in rt_mutex_wake_up_q();
         */
        preempt_disable();
        rt_mutex_wake_q_add(wqh, waiter);
        raw_spin_unlock(&current->pi_lock);
}

static int __sched __rt_mutex_slowtrylock(struct rt_mutex_base *lock)
{
        int ret = try_to_take_rt_mutex(lock, current, NULL);

        /*
         * try_to_take_rt_mutex() sets the lock waiters bit
         * unconditionally. Clean this up.
         */
        fixup_rt_mutex_waiters(lock);

        return ret;
}

/*
 * Slow path try-lock function:
 */
static int __sched rt_mutex_slowtrylock(struct rt_mutex_base *lock)
{
        unsigned long flags;
        int ret;

        /*
         * If the lock already has an owner we fail to get the lock.
         * This can be done without taking the @lock->wait_lock as
         * it is only being read, and this is a trylock anyway.
         */
        if (rt_mutex_owner(lock))
                return 0;

        /*
         * The mutex has currently no owner. Lock the wait lock and try to
         * acquire the lock. We use irqsave here to support early boot calls.
         */
        raw_spin_lock_irqsave(&lock->wait_lock, flags);

        ret = __rt_mutex_slowtrylock(lock);

        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

        return ret;
}

static __always_inline int __rt_mutex_trylock(struct rt_mutex_base *lock)
{
        if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
                return 1;

        return rt_mutex_slowtrylock(lock);
}

/*
 * Slow path to release a rt-mutex.
 */
static void __sched rt_mutex_slowunlock(struct rt_mutex_base *lock)
{
        DEFINE_RT_WAKE_Q(wqh);
        unsigned long flags;

        /* irqsave required to support early boot calls */
        raw_spin_lock_irqsave(&lock->wait_lock, flags);

        debug_rt_mutex_unlock(lock);

        /*
         * We must be careful here if the fast path is enabled. If we
         * have no waiters queued we cannot set owner to NULL here
         * because of:
         *
         * foo->lock->owner = NULL;
         *                      rtmutex_lock(foo->lock);   <- fast path
         *                      free = atomic_dec_and_test(foo->refcnt);
         *                      rtmutex_unlock(foo->lock); <- fast path
         *                      if (free)
         *                              kfree(foo);
         * raw_spin_unlock(foo->lock->wait_lock);
         *
         * So for the fastpath enabled kernel:
         *
         * Nothing can set the waiters bit as long as we hold
         * lock->wait_lock. So we do the following sequence:
         *
         *      owner = rt_mutex_owner(lock);
         *      clear_rt_mutex_waiters(lock);
         *      raw_spin_unlock(&lock->wait_lock);
         *      if (cmpxchg(&lock->owner, owner, 0) == owner)
         *              return;
         *      goto retry;
         *
         * The fastpath disabled variant is simple as all access to
         * lock->owner is serialized by lock->wait_lock:
         *
         *      lock->owner = NULL;
         *      raw_spin_unlock(&lock->wait_lock);
         */
        while (!rt_mutex_has_waiters(lock)) {
                /* Drops lock->wait_lock ! */
                if (unlock_rt_mutex_safe(lock, flags) == true)
                        return;
                /* Relock the rtmutex and try again */
                raw_spin_lock_irqsave(&lock->wait_lock, flags);
        }

        /*
         * The wakeup next waiter path does not suffer from the above
         * race. See the comments there.
         *
         * Queue the next waiter for wakeup once we release the wait_lock.
         */
        mark_wakeup_next_waiter(&wqh, lock);
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

        rt_mutex_wake_up_q(&wqh);
}

static __always_inline void __rt_mutex_unlock(struct rt_mutex_base *lock)
{
        if (likely(rt_mutex_cmpxchg_release(lock, current, NULL)))
                return;

        rt_mutex_slowunlock(lock);
}

#ifdef RT_MUTEX_BUILD_MUTEX
/*
 * Functions required for:
 *      - rtmutex, futex on all kernels
 *      - mutex and rwsem substitutions on RT kernels
 */

/*
 * Remove a waiter from a lock and give up
 *
 * Must be called with lock->wait_lock held and interrupts disabled. It must
 * have just failed to try_to_take_rt_mutex().
 */
static void __sched remove_waiter(struct rt_mutex_base *lock,
                                  struct rt_mutex_waiter *waiter)
{
        bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
        struct task_struct *owner = rt_mutex_owner(lock);
        struct rt_mutex_base *next_lock;

        lockdep_assert_held(&lock->wait_lock);

        raw_spin_lock(&current->pi_lock);
        rt_mutex_dequeue(lock, waiter);
        current->pi_blocked_on = NULL;
        raw_spin_unlock(&current->pi_lock);

        /*
         * Only update priority if the waiter was the highest priority
         * waiter of the lock and there is an owner to update.
         */
        if (!owner || !is_top_waiter)
                return;

        raw_spin_lock(&owner->pi_lock);

        rt_mutex_dequeue_pi(owner, waiter);

        if (rt_mutex_has_waiters(lock))
                rt_mutex_enqueue_pi(owner, rt_mutex_top_waiter(lock));

        rt_mutex_adjust_prio(owner);

        /* Store the lock on which owner is blocked or NULL */
        next_lock = task_blocked_on_lock(owner);

        raw_spin_unlock(&owner->pi_lock);

        /*
         * Don't walk the chain, if the owner task is not blocked
         * itself.
         */
        if (!next_lock)
                return;

        /* gets dropped in rt_mutex_adjust_prio_chain()! */
        get_task_struct(owner);

        raw_spin_unlock_irq(&lock->wait_lock);

        rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
                                   next_lock, NULL, current);

        raw_spin_lock_irq(&lock->wait_lock);
}

/**
 * rt_mutex_slowlock_block() - Perform the wait-wake-try-to-take loop
 * @lock:                the rt_mutex to take
 * @state:               the state the task should block in (TASK_INTERRUPTIBLE
 *                       or TASK_UNINTERRUPTIBLE)
 * @timeout:             the pre-initialized and started timer, or NULL for none
 * @waiter:              the pre-initialized rt_mutex_waiter
 *
 * Must be called with lock->wait_lock held and interrupts disabled
 */
static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock,
                                           unsigned int state,
                                           struct hrtimer_sleeper *timeout,
                                           struct rt_mutex_waiter *waiter)
{
        int ret = 0;

        for (;;) {
                /* Try to acquire the lock: */
                if (try_to_take_rt_mutex(lock, current, waiter))
                        break;

                if (timeout && !timeout->task) {
                        ret = -ETIMEDOUT;
                        break;
                }
                if (signal_pending_state(state, current)) {
                        ret = -EINTR;
                        break;
                }

                raw_spin_unlock_irq(&lock->wait_lock);

                schedule();

                raw_spin_lock_irq(&lock->wait_lock);
                set_current_state(state);
        }

        __set_current_state(TASK_RUNNING);
        return ret;
}

static void __sched rt_mutex_handle_deadlock(int res, int detect_deadlock,
                                             struct rt_mutex_waiter *w)
{
        /*
         * If the result is not -EDEADLOCK or the caller requested
         * deadlock detection, nothing to do here.
         */
        if (res != -EDEADLOCK || detect_deadlock)
                return;

        /*
         * Yell loudly and stop the task right here.
         */
        WARN(1, "rtmutex deadlock detected\n");
        while (1) {
                set_current_state(TASK_INTERRUPTIBLE);
                schedule();
        }
}

/**
 * __rt_mutex_slowlock - Locking slowpath invoked with lock::wait_lock held
 * @lock:       The rtmutex to block lock
 * @state:      The task state for sleeping
 * @chwalk:     Indicator whether full or partial chainwalk is requested
 * @waiter:     Initializer waiter for blocking
 */
static int __sched __rt_mutex_slowlock(struct rt_mutex_base *lock,
                                       unsigned int state,
                                       enum rtmutex_chainwalk chwalk,
                                       struct rt_mutex_waiter *waiter)
{
        int ret;

        lockdep_assert_held(&lock->wait_lock);

        /* Try to acquire the lock again: */
        if (try_to_take_rt_mutex(lock, current, NULL))
                return 0;

        set_current_state(state);

        ret = task_blocks_on_rt_mutex(lock, waiter, current, chwalk);

        if (likely(!ret))
                ret = rt_mutex_slowlock_block(lock, state, NULL, waiter);

        if (unlikely(ret)) {
                __set_current_state(TASK_RUNNING);
                remove_waiter(lock, waiter);
                rt_mutex_handle_deadlock(ret, chwalk, waiter);
        }

        /*
         * try_to_take_rt_mutex() sets the waiter bit
         * unconditionally. We might have to fix that up.
         */
        fixup_rt_mutex_waiters(lock);
        return ret;
}

static inline int __rt_mutex_slowlock_locked(struct rt_mutex_base *lock,
                                             unsigned int state)
{
        struct rt_mutex_waiter waiter;
        int ret;

        rt_mutex_init_waiter(&waiter);

        ret = __rt_mutex_slowlock(lock, state, RT_MUTEX_MIN_CHAINWALK, &waiter);

        debug_rt_mutex_free_waiter(&waiter);
        return ret;
}

/*
 * rt_mutex_slowlock - Locking slowpath invoked when fast path fails
 * @lock:       The rtmutex to block lock
 * @state:      The task state for sleeping
 */
static int __sched rt_mutex_slowlock(struct rt_mutex_base *lock,
                                     unsigned int state)
{
        unsigned long flags;
        int ret;

        /*
         * Technically we could use raw_spin_[un]lock_irq() here, but this can
         * be called in early boot if the cmpxchg() fast path is disabled
         * (debug, no architecture support). In this case we will acquire the
         * rtmutex with lock->wait_lock held. But we cannot unconditionally
         * enable interrupts in that early boot case. So we need to use the
         * irqsave/restore variants.
         */
        raw_spin_lock_irqsave(&lock->wait_lock, flags);
        ret = __rt_mutex_slowlock_locked(lock, state);
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);

        return ret;
}

static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
                                           unsigned int state)
{
        if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
                return 0;

        return rt_mutex_slowlock(lock, state);
}
#endif /* RT_MUTEX_BUILD_MUTEX */

#ifdef RT_MUTEX_BUILD_SPINLOCKS
/*
 * Functions required for spin/rw_lock substitution on RT kernels
 */

/**
 * rtlock_slowlock_locked - Slow path lock acquisition for RT locks
 * @lock:       The underlying RT mutex
 */
static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock)
{
        struct rt_mutex_waiter waiter;

        lockdep_assert_held(&lock->wait_lock);

        if (try_to_take_rt_mutex(lock, current, NULL))
                return;

        rt_mutex_init_rtlock_waiter(&waiter);

        /* Save current state and set state to TASK_RTLOCK_WAIT */
        current_save_and_set_rtlock_wait_state();

        task_blocks_on_rt_mutex(lock, &waiter, current, RT_MUTEX_MIN_CHAINWALK);

        for (;;) {
                /* Try to acquire the lock again */
                if (try_to_take_rt_mutex(lock, current, &waiter))
                        break;

                raw_spin_unlock_irq(&lock->wait_lock);

                schedule_rtlock();

                raw_spin_lock_irq(&lock->wait_lock);
                set_current_state(TASK_RTLOCK_WAIT);
        }

        /* Restore the task state */
        current_restore_rtlock_saved_state();

        /*
         * try_to_take_rt_mutex() sets the waiter bit unconditionally.
         * We might have to fix that up:
         */
        fixup_rt_mutex_waiters(lock);
        debug_rt_mutex_free_waiter(&waiter);
}

static __always_inline void __sched rtlock_slowlock(struct rt_mutex_base *lock)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&lock->wait_lock, flags);
        rtlock_slowlock_locked(lock);
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
}

#endif /* RT_MUTEX_BUILD_SPINLOCKS */