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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: 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>
This commit is contained in:
Darren Hart (VMware) 2017-04-14 15:31:38 -07:00 committed by Thomas Gleixner
parent 94ffac5d84
commit 38fcd06e9b
1 changed files with 5 additions and 4 deletions
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9
kernel/futex.c
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@ -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);
}