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>

diff --git a/kernel/futex.c b/kernel/futex.c
index ede2f1ef851104f214e637fe3932871df9f0b203..357348a6cf6b4d71dbc24c9ad89ad2d0e63b20d2 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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);
 }