From: Davidlohr Bueso <dave@stgolabs.net>
Date: Thu, 21 Apr 2016 03:09:24 +0000 (-0700)
Subject: futex: Acknowledge a new waiter in counter before plist
X-Git-Tag: Ubuntu-snapdragon-4.4.0-1050.54~3787
X-Git-Url: https://git.proxmox.com/?a=commitdiff_plain;h=f922cf6174a667e99c8eeb784f8d2aeb38e46f23;p=mirror_ubuntu-artful-kernel.git

futex: Acknowledge a new waiter in counter before plist

BugLink: http://bugs.launchpad.net/bugs/1578798

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@infradead.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: bigeasy@linutronix.de
Cc: dvhart@infradead.org
Link: http://lkml.kernel.org/r/1461208164-29150-1-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---

diff --git a/kernel/futex.c b/kernel/futex.c
index eaa3a8dfd345..9d8163afd87c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1484,8 +1484,8 @@ 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);