rtmutex: Simplify remove_waiter()

[deliverable/linux.git] / kernel / locking / rtmutex.c

/*
 * 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/rt-mutex-design.txt for details.
 */
#include <linux/spinlock.h>
#include <linux/export.h>
#include <linux/sched.h>
#include <linux/sched/rt.h>
#include <linux/sched/deadline.h>
#include <linux/timer.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 void
rt_mutex_set_owner(struct rt_mutex *lock, struct task_struct *owner)
{
        unsigned long val = (unsigned long)owner;

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

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

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

static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
{
        if (!rt_mutex_has_waiters(lock))
                clear_rt_mutex_waiters(lock);
}

/*
 * We can speed up the acquire/release, if the architecture
 * supports cmpxchg and if there's no debugging state to be set up
 */
#if defined(__HAVE_ARCH_CMPXCHG) && !defined(CONFIG_DEBUG_RT_MUTEXES)
# define rt_mutex_cmpxchg(l,c,n)        (cmpxchg(&l->owner, c, n) == c)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
{
        unsigned long owner, *p = (unsigned long *) &lock->owner;

        do {
                owner = *p;
        } while (cmpxchg(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 inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
        __releases(lock->wait_lock)
{
        struct task_struct *owner = rt_mutex_owner(lock);

        clear_rt_mutex_waiters(lock);
        raw_spin_unlock(&lock->wait_lock);
        /*
         * 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(lock, owner, NULL);
}

#else
# define rt_mutex_cmpxchg(l,c,n)        (0)
static inline void mark_rt_mutex_waiters(struct rt_mutex *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 inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
        __releases(lock->wait_lock)
{
        lock->owner = NULL;
        raw_spin_unlock(&lock->wait_lock);
        return true;
}
#endif

static 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 (left->task->dl.deadline < right->task->dl.deadline);

        return 0;
}

static void
rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
{
        struct rb_node **link = &lock->waiters.rb_node;
        struct rb_node *parent = NULL;
        struct rt_mutex_waiter *entry;
        int leftmost = 1;

        while (*link) {
                parent = *link;
                entry = rb_entry(parent, struct rt_mutex_waiter, tree_entry);
                if (rt_mutex_waiter_less(waiter, entry)) {
                        link = &parent->rb_left;
                } else {
                        link = &parent->rb_right;
                        leftmost = 0;
                }
        }

        if (leftmost)
                lock->waiters_leftmost = &waiter->tree_entry;

        rb_link_node(&waiter->tree_entry, parent, link);
        rb_insert_color(&waiter->tree_entry, &lock->waiters);
}

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

        if (lock->waiters_leftmost == &waiter->tree_entry)
                lock->waiters_leftmost = rb_next(&waiter->tree_entry);

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

static void
rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
{
        struct rb_node **link = &task->pi_waiters.rb_node;
        struct rb_node *parent = NULL;
        struct rt_mutex_waiter *entry;
        int leftmost = 1;

        while (*link) {
                parent = *link;
                entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
                if (rt_mutex_waiter_less(waiter, entry)) {
                        link = &parent->rb_left;
                } else {
                        link = &parent->rb_right;
                        leftmost = 0;
                }
        }

        if (leftmost)
                task->pi_waiters_leftmost = &waiter->pi_tree_entry;

        rb_link_node(&waiter->pi_tree_entry, parent, link);
        rb_insert_color(&waiter->pi_tree_entry, &task->pi_waiters);
}

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

        if (task->pi_waiters_leftmost == &waiter->pi_tree_entry)
                task->pi_waiters_leftmost = rb_next(&waiter->pi_tree_entry);

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

/*
 * Calculate task priority from the waiter tree priority
 *
 * Return task->normal_prio when the waiter tree is empty or when
 * the waiter is not allowed to do priority boosting
 */
int rt_mutex_getprio(struct task_struct *task)
{
        if (likely(!task_has_pi_waiters(task)))
                return task->normal_prio;

        return min(task_top_pi_waiter(task)->prio,
                   task->normal_prio);
}

struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
{
        if (likely(!task_has_pi_waiters(task)))
                return NULL;

        return task_top_pi_waiter(task)->task;
}

/*
 * Called by sched_setscheduler() to check whether the priority change
 * is overruled by a possible priority boosting.
 */
int rt_mutex_check_prio(struct task_struct *task, int newprio)
{
        if (!task_has_pi_waiters(task))
                return 0;

        return task_top_pi_waiter(task)->task->prio <= newprio;
}

/*
 * Adjust the priority of a task, after its pi_waiters got modified.
 *
 * This can be both boosting and unboosting. task->pi_lock must be held.
 */
static void __rt_mutex_adjust_prio(struct task_struct *task)
{
        int prio = rt_mutex_getprio(task);

        if (task->prio != prio || dl_prio(prio))
                rt_mutex_setprio(task, prio);
}

/*
 * Adjust task priority (undo boosting). Called from the exit path of
 * rt_mutex_slowunlock() and rt_mutex_slowlock().
 *
 * (Note: We do this outside of the protection of lock->wait_lock to
 * allow the lock to be taken while or before we readjust the priority
 * of task. We do not use the spin_xx_mutex() variants here as we are
 * outside of the debug path.)
 */
static void rt_mutex_adjust_prio(struct task_struct *task)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&task->pi_lock, flags);
        __rt_mutex_adjust_prio(task);
        raw_spin_unlock_irqrestore(&task->pi_lock, flags);
}

/*
 * Max number of times we'll walk the boosting chain:
 */
int max_lock_depth = 1024;

static inline struct rt_mutex *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
 * @deadlock_detect: 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 rt_mutex_adjust_prio_chain(struct task_struct *task,
                                      int deadlock_detect,
                                      struct rt_mutex *orig_lock,
                                      struct rt_mutex *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 detect_deadlock, ret = 0, depth = 0;
        struct rt_mutex *lock;
        unsigned long flags;

        detect_deadlock = debug_rt_mutex_detect_deadlock(orig_waiter,
                                                         deadlock_detect);

        /*
         * 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_irqsave(&task->pi_lock, flags);

        /*
         * [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 (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
                        goto out_unlock_pi;
        }

        /*
         * When deadlock detection is off then we check, if further
         * priority adjustment is necessary.
         */
        if (!detect_deadlock && waiter->prio == task->prio)
                goto out_unlock_pi;

        /*
         * [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_irqrestore(&task->pi_lock, flags);
                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) {
                debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
                raw_spin_unlock(&lock->wait_lock);
                ret = -EDEADLK;
                goto out_unlock_pi;
        }

        /*
         * 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 list. */
        rt_mutex_dequeue(lock, waiter);
        waiter->prio = task->prio;
        rt_mutex_enqueue(lock, waiter);

        /* [8] Release the task */
        raw_spin_unlock_irqrestore(&task->pi_lock, flags);
        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_process(rt_mutex_top_waiter(lock)->task);
                raw_spin_unlock(&lock->wait_lock);
                return 0;
        }

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

        /* [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 list 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 prority waiter. Replace waiter in
                 * the owner tasks pi waiters list 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_irqrestore(&task->pi_lock, flags);
        raw_spin_unlock(&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_irqrestore(&task->pi_lock, flags);
 out_put_task:
        put_task_struct(task);

        return ret;
}

/*
 * Try to take an rt-mutex
 *
 * Must be called with lock->wait_lock held.
 *
 * @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 list if the
 *          callsite called task_blocked_on_lock(), otherwise NULL
 */
static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
                                struct rt_mutex_waiter *waiter)
{
        unsigned long flags;

        /*
         * 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 list. 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 list.
                 */
                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 (task->prio >= rt_mutex_top_waiter(lock)->prio)
                                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 list.
                         */
                        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_irqsave(&task->pi_lock, flags);
        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 list.
         */
        if (rt_mutex_has_waiters(lock))
                rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
        raw_spin_unlock_irqrestore(&task->pi_lock, flags);

takeit:
        /* We got the lock. */
        debug_rt_mutex_lock(lock);

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

        rt_mutex_deadlock_account_lock(lock, task);

        return 1;
}

/*
 * Task blocks on lock.
 *
 * Prepare waiter and propagate pi chain
 *
 * This must be called with lock->wait_lock held.
 */
static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
                                   struct rt_mutex_waiter *waiter,
                                   struct task_struct *task,
                                   int detect_deadlock)
{
        struct task_struct *owner = rt_mutex_owner(lock);
        struct rt_mutex_waiter *top_waiter = waiter;
        struct rt_mutex *next_lock;
        int chain_walk = 0, res;
        unsigned long flags;

        /*
         * 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_irqsave(&task->pi_lock, flags);
        __rt_mutex_adjust_prio(task);
        waiter->task = task;
        waiter->lock = lock;
        waiter->prio = task->prio;

        /* 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_irqrestore(&task->pi_lock, flags);

        if (!owner)
                return 0;

        raw_spin_lock_irqsave(&owner->pi_lock, flags);
        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 (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
                chain_walk = 1;
        }

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

        raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
        /*
         * 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(&lock->wait_lock);

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

        raw_spin_lock(&lock->wait_lock);

        return res;
}

/*
 * Wake up the next waiter on the lock.
 *
 * Remove the top waiter from the current tasks pi waiter list and
 * wake it up.
 *
 * Called with lock->wait_lock held.
 */
static void wakeup_next_waiter(struct rt_mutex *lock)
{
        struct rt_mutex_waiter *waiter;
        unsigned long flags;

        raw_spin_lock_irqsave(&current->pi_lock, flags);

        waiter = rt_mutex_top_waiter(lock);

        /*
         * Remove it from current->pi_waiters. We do not adjust a
         * possible priority boost right now. We execute wakeup in the
         * boosted mode and go back to normal after releasing
         * lock->wait_lock.
         */
        rt_mutex_dequeue_pi(current, waiter);

        /*
         * 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;

        raw_spin_unlock_irqrestore(&current->pi_lock, flags);

        /*
         * It's safe to dereference waiter as it cannot go away as
         * long as we hold lock->wait_lock. The waiter task needs to
         * acquire it in order to dequeue the waiter.
         */
        wake_up_process(waiter->task);
}

/*
 * Remove a waiter from a lock and give up
 *
 * Must be called with lock->wait_lock held and
 * have just failed to try_to_take_rt_mutex().
 */
static void remove_waiter(struct rt_mutex *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 *next_lock;
        unsigned long flags;

        raw_spin_lock_irqsave(&current->pi_lock, flags);
        rt_mutex_dequeue(lock, waiter);
        current->pi_blocked_on = NULL;
        raw_spin_unlock_irqrestore(&current->pi_lock, flags);

        /*
         * 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_irqsave(&owner->pi_lock, flags);

        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_irqrestore(&owner->pi_lock, flags);

        /*
         * 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(&lock->wait_lock);

        rt_mutex_adjust_prio_chain(owner, 0, lock, next_lock, NULL, current);

        raw_spin_lock(&lock->wait_lock);
}

/*
 * Recheck the pi chain, in case we got a priority setting
 *
 * Called from sched_setscheduler
 */
void rt_mutex_adjust_pi(struct task_struct *task)
{
        struct rt_mutex_waiter *waiter;
        struct rt_mutex *next_lock;
        unsigned long flags;

        raw_spin_lock_irqsave(&task->pi_lock, flags);

        waiter = task->pi_blocked_on;
        if (!waiter || (waiter->prio == task->prio &&
                        !dl_prio(task->prio))) {
                raw_spin_unlock_irqrestore(&task->pi_lock, flags);
                return;
        }
        next_lock = waiter->lock;
        raw_spin_unlock_irqrestore(&task->pi_lock, flags);

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

        rt_mutex_adjust_prio_chain(task, 0, NULL, next_lock, NULL, task);
}

/**
 * __rt_mutex_slowlock() - 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
 *
 * lock->wait_lock must be held by the caller.
 */
static int __sched
__rt_mutex_slowlock(struct rt_mutex *lock, 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;

                /*
                 * TASK_INTERRUPTIBLE checks for signals and
                 * timeout. Ignored otherwise.
                 */
                if (unlikely(state == TASK_INTERRUPTIBLE)) {
                        /* Signal pending? */
                        if (signal_pending(current))
                                ret = -EINTR;
                        if (timeout && !timeout->task)
                                ret = -ETIMEDOUT;
                        if (ret)
                                break;
                }

                raw_spin_unlock(&lock->wait_lock);

                debug_rt_mutex_print_deadlock(waiter);

                schedule_rt_mutex(lock);

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

        return ret;
}

static void 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 lowdly and stop the task right here.
         */
        rt_mutex_print_deadlock(w);
        while (1) {
                set_current_state(TASK_INTERRUPTIBLE);
                schedule();
        }
}

/*
 * Slow path lock function:
 */
static int __sched
rt_mutex_slowlock(struct rt_mutex *lock, int state,
                  struct hrtimer_sleeper *timeout,
                  int detect_deadlock)
{
        struct rt_mutex_waiter waiter;
        int ret = 0;

        debug_rt_mutex_init_waiter(&waiter);
        RB_CLEAR_NODE(&waiter.pi_tree_entry);
        RB_CLEAR_NODE(&waiter.tree_entry);

        raw_spin_lock(&lock->wait_lock);

        /* Try to acquire the lock again: */
        if (try_to_take_rt_mutex(lock, current, NULL)) {
                raw_spin_unlock(&lock->wait_lock);
                return 0;
        }

        set_current_state(state);

        /* Setup the timer, when timeout != NULL */
        if (unlikely(timeout)) {
                hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
                if (!hrtimer_active(&timeout->timer))
                        timeout->task = NULL;
        }

        ret = task_blocks_on_rt_mutex(lock, &waiter, current, detect_deadlock);

        if (likely(!ret))
                ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);

        set_current_state(TASK_RUNNING);

        if (unlikely(ret)) {
                remove_waiter(lock, &waiter);
                rt_mutex_handle_deadlock(ret, detect_deadlock, &waiter);
        }

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

        raw_spin_unlock(&lock->wait_lock);

        /* Remove pending timer: */
        if (unlikely(timeout))
                hrtimer_cancel(&timeout->timer);

        debug_rt_mutex_free_waiter(&waiter);

        return ret;
}

/*
 * Slow path try-lock function:
 */
static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
{
        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.
         */
        raw_spin_lock(&lock->wait_lock);

        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);

        raw_spin_unlock(&lock->wait_lock);

        return ret;
}

/*
 * Slow path to release a rt-mutex:
 */
static void __sched
rt_mutex_slowunlock(struct rt_mutex *lock)
{
        raw_spin_lock(&lock->wait_lock);

        debug_rt_mutex_unlock(lock);

        rt_mutex_deadlock_account_unlock(current);

        /*
         * 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) == true)
                        return;
                /* Relock the rtmutex and try again */
                raw_spin_lock(&lock->wait_lock);
        }

        /*
         * The wakeup next waiter path does not suffer from the above
         * race. See the comments there.
         */
        wakeup_next_waiter(lock);

        raw_spin_unlock(&lock->wait_lock);

        /* Undo pi boosting if necessary: */
        rt_mutex_adjust_prio(current);
}

/*
 * debug aware fast / slowpath lock,trylock,unlock
 *
 * The atomic acquire/release ops are compiled away, when either the
 * architecture does not support cmpxchg or when debugging is enabled.
 */
static inline int
rt_mutex_fastlock(struct rt_mutex *lock, int state,
                  int detect_deadlock,
                  int (*slowfn)(struct rt_mutex *lock, int state,
                                struct hrtimer_sleeper *timeout,
                                int detect_deadlock))
{
        if (!detect_deadlock && likely(rt_mutex_cmpxchg(lock, NULL, current))) {
                rt_mutex_deadlock_account_lock(lock, current);
                return 0;
        } else
                return slowfn(lock, state, NULL, detect_deadlock);
}

static inline int
rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
                        struct hrtimer_sleeper *timeout, int detect_deadlock,
                        int (*slowfn)(struct rt_mutex *lock, int state,
                                      struct hrtimer_sleeper *timeout,
                                      int detect_deadlock))
{
        if (!detect_deadlock && likely(rt_mutex_cmpxchg(lock, NULL, current))) {
                rt_mutex_deadlock_account_lock(lock, current);
                return 0;
        } else
                return slowfn(lock, state, timeout, detect_deadlock);
}

static inline int
rt_mutex_fasttrylock(struct rt_mutex *lock,
                     int (*slowfn)(struct rt_mutex *lock))
{
        if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
                rt_mutex_deadlock_account_lock(lock, current);
                return 1;
        }
        return slowfn(lock);
}

static inline void
rt_mutex_fastunlock(struct rt_mutex *lock,
                    void (*slowfn)(struct rt_mutex *lock))
{
        if (likely(rt_mutex_cmpxchg(lock, current, NULL)))
                rt_mutex_deadlock_account_unlock(current);
        else
                slowfn(lock);
}

/**
 * rt_mutex_lock - lock a rt_mutex
 *
 * @lock: the rt_mutex to be locked
 */
void __sched rt_mutex_lock(struct rt_mutex *lock)
{
        might_sleep();

        rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, 0, rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_lock);

/**
 * rt_mutex_lock_interruptible - lock a rt_mutex interruptible
 *
 * @lock:               the rt_mutex to be locked
 * @detect_deadlock:    deadlock detection on/off
 *
 * Returns:
 *  0           on success
 * -EINTR       when interrupted by a signal
 * -EDEADLK     when the lock would deadlock (when deadlock detection is on)
 */
int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock,
                                                 int detect_deadlock)
{
        might_sleep();

        return rt_mutex_fastlock(lock, TASK_INTERRUPTIBLE,
                                 detect_deadlock, rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);

/**
 * rt_mutex_timed_lock - lock a rt_mutex interruptible
 *                      the timeout structure is provided
 *                      by the caller
 *
 * @lock:               the rt_mutex to be locked
 * @timeout:            timeout structure or NULL (no timeout)
 * @detect_deadlock:    deadlock detection on/off
 *
 * Returns:
 *  0           on success
 * -EINTR       when interrupted by a signal
 * -ETIMEDOUT   when the timeout expired
 * -EDEADLK     when the lock would deadlock (when deadlock detection is on)
 */
int
rt_mutex_timed_lock(struct rt_mutex *lock, struct hrtimer_sleeper *timeout,
                    int detect_deadlock)
{
        might_sleep();

        return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
                                       detect_deadlock, rt_mutex_slowlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);

/**
 * rt_mutex_trylock - try to lock a rt_mutex
 *
 * @lock:       the rt_mutex to be locked
 *
 * Returns 1 on success and 0 on contention
 */
int __sched rt_mutex_trylock(struct rt_mutex *lock)
{
        return rt_mutex_fasttrylock(lock, rt_mutex_slowtrylock);
}
EXPORT_SYMBOL_GPL(rt_mutex_trylock);

/**
 * rt_mutex_unlock - unlock a rt_mutex
 *
 * @lock: the rt_mutex to be unlocked
 */
void __sched rt_mutex_unlock(struct rt_mutex *lock)
{
        rt_mutex_fastunlock(lock, rt_mutex_slowunlock);
}
EXPORT_SYMBOL_GPL(rt_mutex_unlock);

/**
 * rt_mutex_destroy - mark a mutex unusable
 * @lock: the mutex to be destroyed
 *
 * This function marks the mutex uninitialized, and any subsequent
 * use of the mutex is forbidden. The mutex must not be locked when
 * this function is called.
 */
void rt_mutex_destroy(struct rt_mutex *lock)
{
        WARN_ON(rt_mutex_is_locked(lock));
#ifdef CONFIG_DEBUG_RT_MUTEXES
        lock->magic = NULL;
#endif
}

EXPORT_SYMBOL_GPL(rt_mutex_destroy);

/**
 * __rt_mutex_init - initialize the rt lock
 *
 * @lock: the rt lock to be initialized
 *
 * Initialize the rt lock to unlocked state.
 *
 * Initializing of a locked rt lock is not allowed
 */
void __rt_mutex_init(struct rt_mutex *lock, const char *name)
{
        lock->owner = NULL;
        raw_spin_lock_init(&lock->wait_lock);
        lock->waiters = RB_ROOT;
        lock->waiters_leftmost = NULL;

        debug_rt_mutex_init(lock, name);
}
EXPORT_SYMBOL_GPL(__rt_mutex_init);

/**
 * rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a
 *                              proxy owner
 *
 * @lock:       the rt_mutex to be locked
 * @proxy_owner:the task to set as owner
 *
 * No locking. Caller has to do serializing itself
 * Special API call for PI-futex support
 */
void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
                                struct task_struct *proxy_owner)
{
        __rt_mutex_init(lock, NULL);
        debug_rt_mutex_proxy_lock(lock, proxy_owner);
        rt_mutex_set_owner(lock, proxy_owner);
        rt_mutex_deadlock_account_lock(lock, proxy_owner);
}

/**
 * rt_mutex_proxy_unlock - release a lock on behalf of owner
 *
 * @lock:       the rt_mutex to be locked
 *
 * No locking. Caller has to do serializing itself
 * Special API call for PI-futex support
 */
void rt_mutex_proxy_unlock(struct rt_mutex *lock,
                           struct task_struct *proxy_owner)
{
        debug_rt_mutex_proxy_unlock(lock);
        rt_mutex_set_owner(lock, NULL);
        rt_mutex_deadlock_account_unlock(proxy_owner);
}

/**
 * rt_mutex_start_proxy_lock() - Start lock acquisition for another task
 * @lock:               the rt_mutex to take
 * @waiter:             the pre-initialized rt_mutex_waiter
 * @task:               the task to prepare
 * @detect_deadlock:    perform deadlock detection (1) or not (0)
 *
 * Returns:
 *  0 - task blocked on lock
 *  1 - acquired the lock for task, caller should wake it up
 * <0 - error
 *
 * Special API call for FUTEX_REQUEUE_PI support.
 */
int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
                              struct rt_mutex_waiter *waiter,
                              struct task_struct *task, int detect_deadlock)
{
        int ret;

        raw_spin_lock(&lock->wait_lock);

        if (try_to_take_rt_mutex(lock, task, NULL)) {
                raw_spin_unlock(&lock->wait_lock);
                return 1;
        }

        /* We enforce deadlock detection for futexes */
        ret = task_blocks_on_rt_mutex(lock, waiter, task, 1);

        if (ret && !rt_mutex_owner(lock)) {
                /*
                 * Reset the return value. We might have
                 * returned with -EDEADLK and the owner
                 * released the lock while we were walking the
                 * pi chain.  Let the waiter sort it out.
                 */
                ret = 0;
        }

        if (unlikely(ret))
                remove_waiter(lock, waiter);

        raw_spin_unlock(&lock->wait_lock);

        debug_rt_mutex_print_deadlock(waiter);

        return ret;
}

/**
 * rt_mutex_next_owner - return the next owner of the lock
 *
 * @lock: the rt lock query
 *
 * Returns the next owner of the lock or NULL
 *
 * Caller has to serialize against other accessors to the lock
 * itself.
 *
 * Special API call for PI-futex support
 */
struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
{
        if (!rt_mutex_has_waiters(lock))
                return NULL;

        return rt_mutex_top_waiter(lock)->task;
}

/**
 * rt_mutex_finish_proxy_lock() - Complete lock acquisition
 * @lock:               the rt_mutex we were woken on
 * @to:                 the timeout, null if none. hrtimer should already have
 *                      been started.
 * @waiter:             the pre-initialized rt_mutex_waiter
 * @detect_deadlock:    perform deadlock detection (1) or not (0)
 *
 * Complete the lock acquisition started our behalf by another thread.
 *
 * Returns:
 *  0 - success
 * <0 - error, one of -EINTR, -ETIMEDOUT, or -EDEADLK
 *
 * Special API call for PI-futex requeue support
 */
int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
                               struct hrtimer_sleeper *to,
                               struct rt_mutex_waiter *waiter,
                               int detect_deadlock)
{
        int ret;

        raw_spin_lock(&lock->wait_lock);

        set_current_state(TASK_INTERRUPTIBLE);

        ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);

        set_current_state(TASK_RUNNING);

        if (unlikely(ret))
                remove_waiter(lock, waiter);

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

        raw_spin_unlock(&lock->wait_lock);

        return ret;
}