/* 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>
/*
* BROKEN wait-queues.
*
* These "simple" wait-queues are broken garbage, and should never be
* used. The comments below claim that they are "similar" to regular
* wait-queues, but the semantics are actually completely different, and
* every single user we have ever had has been buggy (or pointless).
*
* A "swake_up()" only wakes up _one_ waiter, which is not at all what
* "wake_up()" does, and has led to problems. In other cases, it has
* been fine, because there's only ever one waiter (kvm), but in that
* case gthe whole "simple" wait-queue is just pointless to begin with,
* since there is no "queue". Use "wake_up_process()" with a direct
* pointer instead.
*
* 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);
}
/**
* 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);
}
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 */
