linux-kernel - [PATCH 4.2.y-ckt 68/93] futex: Acknowledge a new waiter in counter before plist

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Message-Id: <1461694653-29506-69-git-send-email-kamal@canonical.com>
Date:	Tue, 26 Apr 2016 11:17:08 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Davidlohr Bueso <dbueso@...e.de>,
	Davidlohr Bueso <dave@...olabs.net>, bigeasy@...utronix.de,
	dvhart@...radead.org, Thomas Gleixner <tglx@...utronix.de>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 4.2.y-ckt 68/93] futex: Acknowledge a new waiter in counter before plist

4.2.8-ckt9 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

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: Kamal Mostafa <kamal@...onical.com>
---
 kernel/futex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index df4203b2..02113e5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1395,8 +1395,8 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
 	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);
-- 
2.7.4