sched/wake_q: Document wake_q_add() (e6018c0f) · Commits · 戴 / test

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>

include/linux/sched/wake_q.h

+5 −1

Original line number	Diff line number	Diff line
		@@ -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>

kernel/sched/core.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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;

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