futex: Clarify mark_wake_futex memory barrier usage (38fcd06e) · Commits · 戴 / test

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>

kernel/futex.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -1380,10 +1380,11 @@ static void mark_wake_futex(struct wake_q_head wake_q, struct futex_q q)
		wake_q_add(wake_q, p);
		__unqueue_futex(q);
		/*
		* The waiting task can free the futex_q as soon as
		* q->lock_ptr = NULL is written, without taking any locks. A
		* memory barrier is required here to prevent the following
		* store to lock_ptr from getting ahead of the plist_del.
		* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
		* is written, without taking any locks. This is possible in the event
		* of a spurious wakeup, for example. A memory barrier is required here
		* to prevent the following store to lock_ptr from getting ahead of the
		* plist_del in __unqueue_futex().
		*/
		smp_store_release(&q->lock_ptr, NULL);
		}

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