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

futex: Replace barrier() in unqueue_me() with READ_ONCE()

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: futex-replace-barrier-in-unqueue_me-with-read_once.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 29b75eb2d56a714190a93d7be4525e617591077a Mon Sep 17 00:00:00 2001

From: Jianyu Zhan nasa4836@gmail.com Date: Mon, 7 Mar 2016 09:32:24 +0800 Subject: futex: Replace barrier() in unqueue_me() with READ_ONCE()

From: Jianyu Zhan nasa4836@gmail.com

commit 29b75eb2d56a714190a93d7be4525e617591077a upstream.

Commit e91467ecd1ef ("bug in futex unqueue_me") introduced a barrier() in unqueue_me() to prevent the compiler from rereading the lock pointer which might change after a check for NULL.

Replace the barrier() with a READ_ONCE() for the following reasons:

1) READ_ONCE() is a weaker form of barrier() that affects only the specific load operation, while barrier() is a general compiler level memory barrier. READ_ONCE() was not available at the time when the barrier was added.

2) Aside of that READ_ONCE() is descriptive and self explainatory while a barrier without comment is not clear to the casual reader.

No functional change.

[ tglx: Massaged changelog ]

Signed-off-by: Jianyu Zhan nasa4836@gmail.com Acked-by: Christian Borntraeger borntraeger@de.ibm.com Acked-by: Darren Hart dvhart@linux.intel.com Cc: dave@stgolabs.net Cc: peterz@infradead.org Cc: linux@rasmusvillemoes.dk Cc: akpm@linux-foundation.org Cc: fengguang.wu@intel.com Cc: bigeasy@linutronix.de Link: http://lkml.kernel.org/r/1457314344-5685-1-git-send-email-nasa4836@gmail.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Davidlohr Bueso dbueso@suse.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/futex.c +++ b/kernel/futex.c @@ -1939,8 +1939,12 @@ static int unqueue_me(struct futex_q *q)

/* In the common case we don't take the spinlock, which is nice. */ retry: - lock_ptr = q->lock_ptr; - barrier(); + /* + * q->lock_ptr can change between this read and the following spin_lock. + * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and + * optimizing lock_ptr out of the logic below. + */ + lock_ptr = READ_ONCE(q->lock_ptr); if (lock_ptr != NULL) { spin_lock(lock_ptr); /*

Patches currently in stable-queue which might be from nasa4836@gmail.com are

queue-4.4/futex-replace-barrier-in-unqueue_me-with-read_once.patch