linux-kernel - [PATCH 3.16 035/114] futex: Acknowledge a new waiter in counter before plist

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Message-ID: <lsq.1465842997.99470049@decadent.org.uk>
Date:	Mon, 13 Jun 2016 19:36:37 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, bigeasy@...utronix.de,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Davidlohr Bueso" <dave@...olabs.net>,
	"Thomas Gleixner" <tglx@...utronix.de>, dvhart@...radead.org,
	"Davidlohr Bueso" <dbueso@...e.de>
Subject: [PATCH 3.16 035/114] futex: Acknowledge a new waiter in counter
 before plist

3.16.36-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Davidlohr Bueso <dave@...olabs.net>

commit fe1bce9e2107ba3a8faffe572483b6974201a0e6 upstream.

Otherwise an incoming waker on the dest hash bucket can miss
the waiter adding itself to the plist during the lockless
check optimization (small window but still the correct way
of doing this); similarly to the decrement counterpart.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: bigeasy@...utronix.de
Cc: dvhart@...radead.org
Link: http://lkml.kernel.org/r/1461208164-29150-1-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/futex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1398,8 +1398,8 @@ void requeue_futex(struct futex_q *q, st
 	if (likely(&hb1->chain != &hb2->chain)) {
 		plist_del(&q->list, &hb1->chain);
 		hb_waiters_dec(hb1);
-		plist_add(&q->list, &hb2->chain);
 		hb_waiters_inc(hb2);
+		plist_add(&q->list, &hb2->chain);
 		q->lock_ptr = &hb2->lock;
 	}
 	get_futex_key_refs(key2);