linux-kernel - [PATCH 4.4 037/163] futex: Acknowledge a new waiter in counter before plist

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Date:	Mon,  2 May 2016 17:11:05 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dbueso@...e.de>,
	Davidlohr Bueso <dave@...olabs.net>, bigeasy@...utronix.de,
	dvhart@...radead.org, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 4.4 037/163] futex: Acknowledge a new waiter in counter before plist

4.4-stable 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: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1484,8 +1484,8 @@ 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);