linux-kernel - [tip:locking/urgent] futex: Acknowledge a new waiter in counter before plist

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Message-ID: <tip-fe1bce9e2107ba3a8faffe572483b6974201a0e6@git.kernel.org>
Date:	Thu, 21 Apr 2016 03:06:38 -0700
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	hpa@...or.com, dbueso@...e.de, linux-kernel@...r.kernel.org,
	mingo@...nel.org, dave@...olabs.net, peterz@...radead.org,
	tglx@...utronix.de
Subject: [tip:locking/urgent] futex: Acknowledge a new waiter in counter
 before plist

Commit-ID:  fe1bce9e2107ba3a8faffe572483b6974201a0e6
Gitweb:     http://git.kernel.org/tip/fe1bce9e2107ba3a8faffe572483b6974201a0e6
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Wed, 20 Apr 2016 20:09:24 -0700
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 21 Apr 2016 11:06:09 +0200

futex: Acknowledge a new waiter in counter before plist

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
Cc: stable@...nel.org
Link: http://lkml.kernel.org/r/1461208164-29150-1-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

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

diff --git a/kernel/futex.c b/kernel/futex.c
index fd204e1..c20f06f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1535,8 +1535,8 @@ 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);