From 0626284ab1bd9d4b0fd48e7edf84e2f675fab76f Mon Sep 17 00:00:00 2001 From: "Darren Hart (VMware)" Date: Fri, 14 Apr 2017 15:31:38 -0700 Subject: [PATCH 028/352] futex: Clarify mark_wake_futex memory barrier usage Upstream commit 38fcd06e9b7f6855db1f3ebac5e18b8fdb467ffd 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) Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/20170414223138.GA4222@fury Signed-off-by: Thomas Gleixner --- kernel/futex.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index 8f5a876..239d5ea 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1379,10 +1379,11 @@ 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); } -- 2.7.4