Patch "futex: Replace barrier() in unqueue_me() with READ_ONCE()" has been added to the 4.4-stable tree

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;a=summary

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 <[email protected]> know about it.

From: Jianyu Zhan <[email protected]>
Date: Mon, 7 Mar 2016 09:32:24 +0800
Subject: futex: Replace barrier() in unqueue_me() with READ_ONCE()

From: Jianyu Zhan <[email protected]>

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 ]

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

@@ -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 [email protected] are

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