/* SPDX-License-Identifier: GPL-2.0 */ #ifndef _LINUX_SWAIT_H #define _LINUX_SWAIT_H #include <linux/list.h> #include <linux/stddef.h> #include <linux/spinlock.h> #include <linux/wait.h> #include <asm/current.h> /* * Simple waitqueues are semantically very different to regular wait queues * (wait.h). The most important difference is that the simple waitqueue allows * for deterministic behaviour -- IOW it has strictly bounded IRQ and lock hold * times. * * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher * priority task a chance to run. * * Secondly, we had to drop a fair number of features of the other waitqueue * code; notably: * * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue; * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right * sleeper state. * * - the !exclusive mode; because that leads to O(n) wakeups, everything is * exclusive. As such swake_up_one will only ever awake _one_ waiter. * * - custom wake callback functions; because you cannot give any guarantees * about random code. This also allows swait to be used in RT, such that * raw spinlock can be used for the swait queue head. * * As a side effect of these; the data structures are slimmer albeit more ad-hoc. * For all the above, note that simple wait queues should _only_ be used under * very specific realtime constraints -- it is best to stick with the regular * wait queues in most cases. */ struct task_struct; struct swait_queue_head { … }; struct swait_queue { … }; #define __SWAITQUEUE_INITIALIZER(name) … #define DECLARE_SWAITQUEUE(name) … #define __SWAIT_QUEUE_HEAD_INITIALIZER(name) … #define DECLARE_SWAIT_QUEUE_HEAD(name) … extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name, struct lock_class_key *key); #define init_swait_queue_head(q) … #ifdef CONFIG_LOCKDEP #define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) … #define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) … #else #define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK … #endif /** * swait_active -- locklessly test for waiters on the queue * @wq: the waitqueue to test for waiters * * returns true if the wait list is not empty * * NOTE: this function is lockless and requires care, incorrect usage _will_ * lead to sporadic and non-obvious failure. * * NOTE2: this function has the same above implications as regular waitqueues. * * Use either while holding swait_queue_head::lock or when used for wakeups * with an extra smp_mb() like: * * CPU0 - waker CPU1 - waiter * * for (;;) { * @cond = true; prepare_to_swait_exclusive(&wq_head, &wait, state); * smp_mb(); // smp_mb() from set_current_state() * if (swait_active(wq_head)) if (@cond) * wake_up(wq_head); break; * schedule(); * } * finish_swait(&wq_head, &wait); * * Because without the explicit smp_mb() it's possible for the * swait_active() load to get hoisted over the @cond store such that we'll * observe an empty wait list while the waiter might not observe @cond. * This, in turn, can trigger missing wakeups. * * Also note that this 'optimization' trades a spin_lock() for an smp_mb(), * which (when the lock is uncontended) are of roughly equal cost. */ static inline int swait_active(struct swait_queue_head *wq) { … } /** * swq_has_sleeper - check if there are any waiting processes * @wq: the waitqueue to test for waiters * * Returns true if @wq has waiting processes * * Please refer to the comment for swait_active. */ static inline bool swq_has_sleeper(struct swait_queue_head *wq) { … } extern void swake_up_one(struct swait_queue_head *q); extern void swake_up_all(struct swait_queue_head *q); extern void swake_up_locked(struct swait_queue_head *q, int wake_flags); extern void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state); extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state); extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait); extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait); /* as per ___wait_event() but for swait, therefore "exclusive == 1" */ #define ___swait_event(wq, condition, state, ret, cmd) … #define __swait_event(wq, condition) … #define swait_event_exclusive(wq, condition) … #define __swait_event_timeout(wq, condition, timeout) … #define swait_event_timeout_exclusive(wq, condition, timeout) … #define __swait_event_interruptible(wq, condition) … #define swait_event_interruptible_exclusive(wq, condition) … #define __swait_event_interruptible_timeout(wq, condition, timeout) … #define swait_event_interruptible_timeout_exclusive(wq, condition, timeout) … #define __swait_event_idle(wq, condition) … /** * swait_event_idle_exclusive - wait without system load contribution * @wq: the waitqueue to wait on * @condition: a C expression for the event to wait for * * The process is put to sleep (TASK_IDLE) until the @condition evaluates to * true. The @condition is checked each time the waitqueue @wq is woken up. * * This function is mostly used when a kthread or workqueue waits for some * condition and doesn't want to contribute to system load. Signals are * ignored. */ #define swait_event_idle_exclusive(wq, condition) … #define __swait_event_idle_timeout(wq, condition, timeout) … /** * swait_event_idle_timeout_exclusive - wait up to timeout without load contribution * @wq: the waitqueue to wait on * @condition: a C expression for the event to wait for * @timeout: timeout at which we'll give up in jiffies * * The process is put to sleep (TASK_IDLE) until the @condition evaluates to * true. The @condition is checked each time the waitqueue @wq is woken up. * * This function is mostly used when a kthread or workqueue waits for some * condition and doesn't want to contribute to system load. Signals are * ignored. * * Returns: * 0 if the @condition evaluated to %false after the @timeout elapsed, * 1 if the @condition evaluated to %true after the @timeout elapsed, * or the remaining jiffies (at least 1) if the @condition evaluated * to %true before the @timeout elapsed. */ #define swait_event_idle_timeout_exclusive(wq, condition, timeout) … #endif /* _LINUX_SWAIT_H */
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