(null)
  *   	€l–Ь4э( ©і( 	ЕЂfаB¦-яАѓ}З^_†I|ҐЏ+ДГ %О    	/* 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 {
	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_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