futex: Clarify mark_wake_futex memory barrier usage
Clarify the scenario described in mark_wake_futex requiring the smp_store_release(). Update the comment to explicitly refer to the plist_del now under __unqueue_futex() (previously plist_del was in the same function as the comment). Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org> Cc: Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/20170414223138.GA4222@fury Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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@ -1380,10 +1380,11 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
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wake_q_add(wake_q, p);
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wake_q_add(wake_q, p);
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__unqueue_futex(q);
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__unqueue_futex(q);
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/*
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/*
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* The waiting task can free the futex_q as soon as
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* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
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* q->lock_ptr = NULL is written, without taking any locks. A
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* is written, without taking any locks. This is possible in the event
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* memory barrier is required here to prevent the following
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* of a spurious wakeup, for example. A memory barrier is required here
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* store to lock_ptr from getting ahead of the plist_del.
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* to prevent the following store to lock_ptr from getting ahead of the
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* plist_del in __unqueue_futex().
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*/
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*/
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smp_store_release(&q->lock_ptr, NULL);
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smp_store_release(&q->lock_ptr, NULL);
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}
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}
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