Patch "futex: Acknowledge a new waiter in counter before plist" has been added to the 4.4-stable tree

Patch "futex: Acknowledge a new waiter in counter before plist" has been added to the 4.4-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    futex: Acknowledge a new waiter in counter before plist

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     futex-acknowledge-a-new-waiter-in-counter-before-plist.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From fe1bce9e2107ba3a8faffe572483b6974201a0e6 Mon Sep 17 00:00:00 2001
From: Davidlohr Bueso <dave@stgolabs.net>
Date: Wed, 20 Apr 2016 20:09:24 -0700
Subject: futex: Acknowledge a new waiter in counter before plist

From: Davidlohr Bueso <dave@stgolabs.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@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>

---
 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);


Patches currently in stable-queue which might be from dave@stgolabs.net are

queue-4.4/futex-handle-unlock_pi-race-gracefully.patch
queue-4.4/futex-acknowledge-a-new-waiter-in-counter-before-plist.patch