kernel/sched/wait.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Generic waiting primitives.
 *
 * (C) 2004 Nadia Yvette Chambers, Oracle
 */
#include "sched.h"

void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *key)
{
    spin_lock_init(&wq_head->lock);
    lockdep_set_class_and_name(&wq_head->lock, key, name);
    INIT_LIST_HEAD(&wq_head->head);
}

EXPORT_SYMBOL(__init_waitqueue_head);

void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
    unsigned long flags;

    wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    __add_wait_queue(wq_head, wq_entry);
    spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(add_wait_queue);

void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
    unsigned long flags;

    wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    __add_wait_queue_entry_tail(wq_head, wq_entry);
    spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(add_wait_queue_exclusive);

void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
    unsigned long flags;

    spin_lock_irqsave(&wq_head->lock, flags);
    __remove_wait_queue(wq_head, wq_entry);
    spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(remove_wait_queue);

/*
 * Scan threshold to break wait queue walk.
 * This allows a waker to take a break from holding the
 * wait queue lock during the wait queue walk.
 */
#define WAITQUEUE_WALK_BREAK_CNT 64

/*
 * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
 * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
 * number) then we wake all the non-exclusive tasks and one exclusive task.
 *
 * There are circumstances in which we can try to wake a task which has already
 * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
 * zero in this (rare) case, and we handle it by continuing to scan the queue.
 */
static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode, int nr_exclusive, int wake_flags,
                            void *key, wait_queue_entry_t *bookmark)
{
    wait_queue_entry_t *curr, *next;
    int cnt = 0;

    lockdep_assert_held(&wq_head->lock);

    if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
        curr = list_next_entry(bookmark, entry);

        list_del(&bookmark->entry);
        bookmark->flags = 0;
    } else {
        curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
    }

    if (&curr->entry == &wq_head->head) {
        return nr_exclusive;
    }

    list_for_each_entry_safe_from(curr, next, &wq_head->head, entry)
    {
        unsigned flags = curr->flags;
        int ret;

        if (flags & WQ_FLAG_BOOKMARK) {
            continue;
        }

        ret = curr->func(curr, mode, wake_flags, key);
        if (ret < 0) {
            break;
        }
        if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) {
            break;
        }

        if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) && (&next->entry != &wq_head->head)) {
            bookmark->flags = WQ_FLAG_BOOKMARK;
            list_add_tail(&bookmark->entry, &next->entry);
            break;
        }
    }

    return nr_exclusive;
}

static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode, int nr_exclusive, int wake_flags,
                                  void *key)
{
    unsigned long flags;
    wait_queue_entry_t bookmark;

    bookmark.flags = 0;
    bookmark.private = NULL;
    bookmark.func = NULL;
    INIT_LIST_HEAD(&bookmark.entry);

    do {
        spin_lock_irqsave(&wq_head->lock, flags);
        nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive, wake_flags, key, &bookmark);
        spin_unlock_irqrestore(&wq_head->lock, flags);
    } while (bookmark.flags & WQ_FLAG_BOOKMARK);
}

/**
 * __wake_up - wake up threads blocked on a waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @nr_exclusive: how many wake-one or wake-many threads to wake up
 * @key: is directly passed to the wakeup function
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up(struct wait_queue_head *wq_head, unsigned int mode, int nr_exclusive, void *key)
{
    __wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
}
EXPORT_SYMBOL(__wake_up);

/*
 * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
 */
void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
{
    __wake_up_common(wq_head, mode, nr, 0, NULL, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked);

void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
{
    __wake_up_common(wq_head, mode, 1, 0, key, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key);

void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head, unsigned int mode, void *key,
                                   wait_queue_entry_t *bookmark)
{
    __wake_up_common(wq_head, mode, 1, 0, key, bookmark);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key_bookmark);

/**
 * __wake_up_sync_key - wake up threads blocked on a waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @key: opaque value to be passed to wakeup targets
 *
 * The sync wakeup differs that the waker knows that it will schedule
 * away soon, so while the target thread will be woken up, it will not
 * be migrated to another CPU - ie. the two threads are 'synchronized'
 * with each other. This can prevent needless bouncing between CPUs.
 *
 * On UP it can prevent extra preemption.
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
{
    if (unlikely(!wq_head)) {
        return;
    }

    __wake_up_common_lock(wq_head, mode, 1, WF_SYNC, key);
}
EXPORT_SYMBOL_GPL(__wake_up_sync_key);

/**
 * __wake_up_locked_sync_key - wake up a thread blocked on a locked waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @key: opaque value to be passed to wakeup targets
 *
 * The sync wakeup differs in that the waker knows that it will schedule
 * away soon, so while the target thread will be woken up, it will not
 * be migrated to another CPU - ie. the two threads are 'synchronized'
 * with each other. This can prevent needless bouncing between CPUs.
 *
 * On UP it can prevent extra preemption.
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up_locked_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
{
    __wake_up_common(wq_head, mode, 1, WF_SYNC, key, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_sync_key);

/*
 * __wake_up_sync - see __wake_up_sync_key()
 */
void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
{
    __wake_up_sync_key(wq_head, mode, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */

void __wake_up_pollfree(struct wait_queue_head *wq_head)
{
    __wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
    /* POLLFREE must have cleared the queue. */
    WARN_ON_ONCE(waitqueue_active(wq_head));
}

/*
 * Note: we use "set_current_state()" _after_ the wait-queue add,
 * because we need a memory barrier there on SMP, so that any
 * wake-function that tests for the wait-queue being active
 * will be guaranteed to see waitqueue addition _or_ subsequent
 * tests in this thread will see the wakeup having taken place.
 *
 * The spin_unlock() itself is semi-permeable and only protects
 * one way (it only protects stuff inside the critical region and
 * stops them from bleeding out - it would still allow subsequent
 * loads to move into the critical region).
 */
void prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
    unsigned long flags;

    wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    if (list_empty(&wq_entry->entry)) {
        __add_wait_queue(wq_head, wq_entry);
    }
    set_current_state(state);
    spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(prepare_to_wait);

/* Returns true if we are the first waiter in the queue, false otherwise. */
bool prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
    unsigned long flags;
    bool was_empty = false;

    wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    if (list_empty(&wq_entry->entry)) {
        was_empty = list_empty(&wq_head->head);
        __add_wait_queue_entry_tail(wq_head, wq_entry);
    }
    set_current_state(state);
    spin_unlock_irqrestore(&wq_head->lock, flags);
    return was_empty;
}
EXPORT_SYMBOL(prepare_to_wait_exclusive);

void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
{
    wq_entry->flags = flags;
    wq_entry->private = current;
    wq_entry->func = autoremove_wake_function;
    INIT_LIST_HEAD(&wq_entry->entry);
}
EXPORT_SYMBOL(init_wait_entry);

long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
    unsigned long flags;
    long ret = 0;

    spin_lock_irqsave(&wq_head->lock, flags);
    if (signal_pending_state(state, current)) {
        /*
         * Exclusive waiter must not fail if it was selected by wakeup,
         * it should "consume" the condition we were waiting for.
         *
         * The caller will recheck the condition and return success if
         * we were already woken up, we can not miss the event because
         * wakeup locks/unlocks the same wq_head->lock.
         *
         * But we need to ensure that set-condition + wakeup after that
         * can't see us, it should wake up another exclusive waiter if
         * we fail.
         */
        list_del_init(&wq_entry->entry);
        ret = -ERESTARTSYS;
    } else {
        if (list_empty(&wq_entry->entry)) {
            if (wq_entry->flags & WQ_FLAG_EXCLUSIVE) {
                __add_wait_queue_entry_tail(wq_head, wq_entry);
            } else {
                __add_wait_queue(wq_head, wq_entry);
            }
        }
        set_current_state(state);
    }
    spin_unlock_irqrestore(&wq_head->lock, flags);

    return ret;
}
EXPORT_SYMBOL(prepare_to_wait_event);

/*
 * Note! These two wait functions are entered with the
 * wait-queue lock held (and interrupts off in the _irq
 * case), so there is no race with testing the wakeup
 * condition in the caller before they add the wait
 * entry to the wake queue.
 */
int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
{
    if (likely(list_empty(&wait->entry))) {
        __add_wait_queue_entry_tail(wq, wait);
    }

    set_current_state(TASK_INTERRUPTIBLE);
    if (signal_pending(current)) {
        return -ERESTARTSYS;
    }

    spin_unlock(&wq->lock);
    schedule();
    spin_lock(&wq->lock);

    return 0;
}
EXPORT_SYMBOL(do_wait_intr);

int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
{
    if (likely(list_empty(&wait->entry))) {
        __add_wait_queue_entry_tail(wq, wait);
    }

    set_current_state(TASK_INTERRUPTIBLE);
    if (signal_pending(current)) {
        return -ERESTARTSYS;
    }

    spin_unlock_irq(&wq->lock);
    schedule();
    spin_lock_irq(&wq->lock);

    return 0;
}
EXPORT_SYMBOL(do_wait_intr_irq);

/**
 * finish_wait - clean up after waiting in a queue
 * @wq_head: waitqueue waited on
 * @wq_entry: wait descriptor
 *
 * Sets current thread back to running state and removes
 * the wait descriptor from the given waitqueue if still
 * queued.
 */
void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
    unsigned long flags;

    __set_current_state(TASK_RUNNING);
    /*
     * We can check for list emptiness outside the lock
     * IFF:
     *  - we use the "careful" check that verifies both
     *    the next and prev pointers, so that there cannot
     *    be any half-pending updates in progress on other
     *    CPU's that we haven't seen yet (and that might
     *    still change the stack area.
     * and
     *  - all other users take the lock (ie we can only
     *    have _one_ other CPU that looks at or modifies
     *    the list).
     */
    if (!list_empty_careful(&wq_entry->entry)) {
        spin_lock_irqsave(&wq_head->lock, flags);
        list_del_init(&wq_entry->entry);
        spin_unlock_irqrestore(&wq_head->lock, flags);
    }
}
EXPORT_SYMBOL(finish_wait);

__sched int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, int sync, void *key)
{
    int ret = default_wake_function(wq_entry, mode, sync, key);
    if (ret) {
        list_del_init_careful(&wq_entry->entry);
    }

    return ret;
}
EXPORT_SYMBOL(autoremove_wake_function);

static inline bool is_kthread_should_stop(void)
{
    return (current->flags & PF_KTHREAD) && kthread_should_stop();
}

/*
 * DEFINE_WAIT_FUNC(wait, woken_wake_func);
 *
 * add_wait_queue(&wq_head, &wait);
 * for (;;) {
 *     if (condition)
 *         break;
 *
 *     // in wait_woken()            // in woken_wake_function()
 *
 *     p->state = mode;                wq_entry->flags |= WQ_FLAG_WOKEN;
 *     smp_mb(); // A                try_to_wake_up():
 *     if (!(wq_entry->flags & WQ_FLAG_WOKEN))       <full barrier>
 *         schedule()                   if (p->state & mode)
 *     p->state = TASK_RUNNING;                  p->state = TASK_RUNNING;
 *     wq_entry->flags &= ~WQ_FLAG_WOKEN;    ~~~~~~~~~~~~~~~~~~
 *     smp_mb(); // B                condition = true;
 * }                        smp_mb(); // C
 * remove_wait_queue(&wq_head, &wait);        wq_entry->flags |= WQ_FLAG_WOKEN;
 */
__sched long wait_woken(struct wait_queue_entry *wq_entry, unsigned int mode, long timeout)
{
    /*
     * The below executes an smp_mb(), which matches with the full barrier
     * executed by the try_to_wake_up() in woken_wake_function() such that
     * either we see the store to wq_entry->flags in woken_wake_function()
     * or woken_wake_function() sees our store to current->state.
     */
    set_current_state(mode); /* A */
    if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop()) {
        timeout = schedule_timeout(timeout);
    }
    __set_current_state(TASK_RUNNING);

    /*
     * The below executes an smp_mb(), which matches with the smp_mb() (C)
     * in woken_wake_function() such that either we see the wait condition
     * being true or the store to wq_entry->flags in woken_wake_function()
     * follows ours in the coherence order.
     */
    smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */

    return timeout;
}
EXPORT_SYMBOL(wait_woken);

__sched int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, int sync, void *key)
{
    /* Pairs with the smp_store_mb() in wait_woken(). */
    smp_mb(); /* C */
    wq_entry->flags |= WQ_FLAG_WOKEN;

    return default_wake_function(wq_entry, mode, sync, key);
}
EXPORT_SYMBOL(woken_wake_function);