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sched/wake_q: Document wake_q_add()

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The only guarantee provided by wake_q_add() is that a wakeup will
happen after it, it does _NOT_ guarantee the wakeup will be delayed
until the matching wake_up_q().

If wake_q_add() fails the cmpxchg() a concurrent wakeup is pending and
that can happen at any time after the cmpxchg(). This means we should
not rely on the wakeup happening at wake_q_up(), but should be ready
for wake_q_add() to issue the wakeup.

The delay; if provided (most likely); should only result in more efficient
behaviour.

Reported-by: Yongji Xie <elohimes@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2018-12-17 10:14:53 +01:00 committed by Ingo Molnar
parent 6dc080eeb2
commit e6018c0f5c
2 changed files with 17 additions and 1 deletions
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6
include/linux/sched/wake_q.h
View File

@ -24,9 +24,13 @@
* called near the end of a function. Otherwise, the list can be
* re-initialized for later re-use by wake_q_init().
*
* Note that this can cause spurious wakeups. schedule() callers
* NOTE that this can cause spurious wakeups. schedule() callers
* must ensure the call is done inside a loop, confirming that the
* wakeup condition has in fact occurred.
*
* NOTE that there is no guarantee the wakeup will happen any later than the
* wake_q_add() location. Therefore task must be ready to be woken at the
* location of the wake_q_add().
*/
#include <linux/sched.h>

12
kernel/sched/core.c
View File

@ -396,6 +396,18 @@ static bool set_nr_if_polling(struct task_struct *p)
#endif
#endif
/**
* wake_q_add() - queue a wakeup for 'later' waking.
* @head: the wake_q_head to add @task to
* @task: the task to queue for 'later' wakeup
*
* Queue a task for later wakeup, most likely by the wake_up_q() call in the
* same context, _HOWEVER_ this is not guaranteed, the wakeup can come
* instantly.
*
* This function must be used as-if it were wake_up_process(); IOW the task
* must be ready to be woken at this location.
*/
void wake_q_add(struct wake_q_head *head, struct task_struct *task)
{
struct wake_q_node *node = &task->wake_q;