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Release 4.14 include/linux/swait.h

Directory: include/linux
/* 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 <asm/current.h>

/*
 * Simple wait queues
 *
 * While these are very similar 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 this requires preserving the list order
 *    and this is hard.
 *
 *  - 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 {
	
raw_spinlock_t		lock;
	
struct list_head	task_list;
};


struct swait_queue {
	
struct task_struct	*task;
	
struct list_head	task_list;
};


#define __SWAITQUEUE_INITIALIZER(name) {                               \
        .task           = current,                                      \
        .task_list      = LIST_HEAD_INIT((name).task_list),             \
}


#define DECLARE_SWAITQUEUE(name)					\
	struct swait_queue name = __SWAITQUEUE_INITIALIZER(name)


#define __SWAIT_QUEUE_HEAD_INITIALIZER(name) {                               \
        .lock           = __RAW_SPIN_LOCK_UNLOCKED(name.lock),          \
        .task_list      = LIST_HEAD_INIT((name).task_list),             \
}


#define DECLARE_SWAIT_QUEUE_HEAD(name)					\
	struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(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)				\
	do {                                                    \
                static struct lock_class_key __key;             \
                __init_swait_queue_head((q), #q, &__key);       \
        } while (0)

#ifdef CONFIG_LOCKDEP

# define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name)			\
	({ init_swait_queue_head(&name); name; })

# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name)			\
	struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name)
#else

# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name)			\
	DECLARE_SWAIT_QUEUE_HEAD(name)
#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(&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) { return !list_empty(&wq->task_list); }

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Peter Zijlstra2090.91%150.00%
Davidlohr Bueso A29.09%150.00%
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/** * 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) { /* * We need to be sure we are in sync with the list_add() * modifications to the wait queue (task_list). * * This memory barrier should be paired with one on the * waiting side. */ smp_mb(); return swait_active(wq); }

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extern void swake_up(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); extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait); extern void prepare_to_swait(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 == 0" */ #define ___swait_event(wq, condition, state, ret, cmd) \ ({ \ struct swait_queue __wait; \ long __ret = ret; \ \ INIT_LIST_HEAD(&__wait.task_list); \ for (;;) { \ long __int = prepare_to_swait_event(&wq, &__wait, state);\ \ if (condition) \ break; \ \ if (___wait_is_interruptible(state) && __int) { \ __ret = __int; \ break; \ } \ \ cmd; \ } \ finish_swait(&wq, &__wait); \ __ret; \ }) #define __swait_event(wq, condition) \ (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \ schedule()) #define swait_event(wq, condition) \ do { \ if (condition) \ break; \ __swait_event(wq, condition); \ } while (0) #define __swait_event_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_UNINTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_timeout(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_timeout(wq, condition, timeout); \ __ret; \ }) #define __swait_event_interruptible(wq, condition) \ ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \ schedule()) #define swait_event_interruptible(wq, condition) \ ({ \ int __ret = 0; \ if (!(condition)) \ __ret = __swait_event_interruptible(wq, condition); \ __ret; \ }) #define __swait_event_interruptible_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_INTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_interruptible_timeout(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_interruptible_timeout(wq, \ condition, timeout); \ __ret; \ }) #define __swait_event_idle(wq, condition) \ (void)___swait_event(wq, condition, TASK_IDLE, 0, schedule()) /** * swait_event_idle - 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(wq, condition) \ do { \ if (condition) \ break; \ __swait_event_idle(wq, condition); \ } while (0) #define __swait_event_idle_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_IDLE, timeout, \ __ret = schedule_timeout(__ret)) /** * swait_event_idle_timeout - 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(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_idle_timeout(wq, \ condition, timeout); \ __ret; \ }) #endif /* _LINUX_SWAIT_H */

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Peter Zijlstra37583.52%120.00%
Luis R. Rodriguez4610.24%120.00%
Davidlohr Bueso A276.01%240.00%
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Total449100.00%5100.00%
Directory: include/linux
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