linux-kernel - [tip:locking/core] futex: Use smp_store_release() in mark_wake

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Date:   Thu, 23 Mar 2017 11:19:43 -0700
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, tglx@...utronix.de
Subject: [tip:locking/core] futex: Use smp_store_release() in
 mark_wake_futex()

Commit-ID:  1b367ece0d7e696cab1c8501bab282cc6a538b3f
Gitweb:     http://git.kernel.org/tip/1b367ece0d7e696cab1c8501bab282cc6a538b3f
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Wed, 22 Mar 2017 11:35:49 +0100
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 23 Mar 2017 19:10:06 +0100

futex: Use smp_store_release() in mark_wake_futex()

Since the futex_q can dissapear the instruction after assigning NULL,
this really should be a RELEASE barrier. That stops loads from hitting
dead memory too.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: juri.lelli@....com
Cc: bigeasy@...utronix.de
Cc: xlpang@...hat.com
Cc: rostedt@...dmis.org
Cc: mathieu.desnoyers@...icios.com
Cc: jdesfossez@...icios.com
Cc: dvhart@...radead.org
Cc: bristot@...hat.com
Link: http://lkml.kernel.org/r/20170322104151.604296452@infradead.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 kernel/futex.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 1531cc4..cc10340 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1290,8 +1290,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
 	 * memory barrier is required here to prevent the following
 	 * store to lock_ptr from getting ahead of the plist_del.
 	 */
-	smp_wmb();
-	q->lock_ptr = NULL;
+	smp_store_release(&q->lock_ptr, NULL);
 }
 
 static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,