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) <[email protected]> Cc: Peter Zijlstra <[email protected]> Link: http://lkml.kernel.org/r/20170414223138.GA4222@fury Signed-off-by: Thomas Gleixner <[email protected]>
strager · Apr 15, 2017 · 38fcd06 · 38fcd06
1 parent 94ffac5
commit 38fcd06
Showing 1 changed file with 5 additions and 4 deletions.
diff --git 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);
 }