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Commit 38fcd06e authored by Darren Hart (VMware)'s avatar Darren Hart (VMware) Committed by Thomas Gleixner
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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: default avatarDarren Hart (VMware) <dvhart@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170414223138.GA4222@fury


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 94ffac5d

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);

}