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sched/wake_q: Document wake_q_add()
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>
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@ -24,9 +24,13 @@
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* called near the end of a function. Otherwise, the list can be
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* re-initialized for later re-use by wake_q_init().
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*
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* Note that this can cause spurious wakeups. schedule() callers
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* NOTE that this can cause spurious wakeups. schedule() callers
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* must ensure the call is done inside a loop, confirming that the
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* wakeup condition has in fact occurred.
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*
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* NOTE that there is no guarantee the wakeup will happen any later than the
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* wake_q_add() location. Therefore task must be ready to be woken at the
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* location of the wake_q_add().
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*/
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#include <linux/sched.h>
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@ -396,6 +396,18 @@ static bool set_nr_if_polling(struct task_struct *p)
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#endif
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#endif
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/**
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* wake_q_add() - queue a wakeup for 'later' waking.
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* @head: the wake_q_head to add @task to
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* @task: the task to queue for 'later' wakeup
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*
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* Queue a task for later wakeup, most likely by the wake_up_q() call in the
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* same context, _HOWEVER_ this is not guaranteed, the wakeup can come
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* instantly.
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*
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* This function must be used as-if it were wake_up_process(); IOW the task
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* must be ready to be woken at this location.
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*/
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void wake_q_add(struct wake_q_head *head, struct task_struct *task)
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{
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struct wake_q_node *node = &task->wake_q;
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