linux-kernel - [tip:locking/urgent] futex: Fix barrier comment

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Message-ID: <tip-6f568ebe2afefdc33a6fb06ef20a94f8b96455f1@git.kernel.org>
Date:   Fri, 8 Feb 2019 04:04:48 -0800
From:   tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, dave@...olabs.net,
        dbueso@...e.de, hpa@...or.com, peterz@...radead.org,
        tglx@...utronix.de
Subject: [tip:locking/urgent] futex: Fix barrier comment

Commit-ID:  6f568ebe2afefdc33a6fb06ef20a94f8b96455f1
Gitweb:     https://git.kernel.org/tip/6f568ebe2afefdc33a6fb06ef20a94f8b96455f1
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Wed, 6 Feb 2019 10:56:02 -0800
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 8 Feb 2019 13:00:35 +0100

futex: Fix barrier comment

The current comment for the barrier that guarantees that waiter increment
is always before taking the hb spinlock (barrier (A)) needs to be fixed as
it is misplaced.

This is obviously referring to hb_waiters_inc, which is a full barrier.

Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lkml.kernel.org/r/20190206185602.949-1-dave@stgolabs.net

---
 kernel/futex.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index fdd312da0992..5ec2473a3497 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2221,11 +2221,11 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
 	 * decrement the counter at queue_unlock() when some error has
 	 * occurred and we don't end up adding the task to the list.
 	 */
-	hb_waiters_inc(hb);
+	hb_waiters_inc(hb); /* implies smp_mb(); (A) */
 
 	q->lock_ptr = &hb->lock;
 
-	spin_lock(&hb->lock); /* implies smp_mb(); (A) */
+	spin_lock(&hb->lock);
 	return hb;
 }