Patch "locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()" has been added to the 4.4-stable tree

Patch "locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()" 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

    locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()

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:
     locking-rtmutex-use-read_once-in-rt_mutex_owner.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 1be5d4fa0af34fb7bafa205aeb59f5c7cc7a089d Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 30 Nov 2016 21:04:42 +0000
Subject: locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()

From: Thomas Gleixner <tglx@linutronix.de>

commit 1be5d4fa0af34fb7bafa205aeb59f5c7cc7a089d upstream.

While debugging the rtmutex unlock vs. dequeue race Will suggested to use
READ_ONCE() in rt_mutex_owner() as it might race against the
cmpxchg_release() in unlock_rt_mutex_safe().

Will: "It's a minor thing which will most likely not matter in practice"

Careful search did not unearth an actual problem in todays code, but it's
better to be safe than surprised.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20161130210030.431379999@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/locking/rtmutex_common.h |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -75,8 +75,9 @@ task_top_pi_waiter(struct task_struct *p
 
 static inline struct task_struct *rt_mutex_owner(struct rt_mutex *lock)
 {
-	return (struct task_struct *)
-		((unsigned long)lock->owner & ~RT_MUTEX_OWNER_MASKALL);
+	unsigned long owner = (unsigned long) READ_ONCE(lock->owner);
+
+	return (struct task_struct *) (owner & ~RT_MUTEX_OWNER_MASKALL);
 }
 
 /*


Patches currently in stable-queue which might be from tglx@linutronix.de are

queue-4.4/locking-rtmutex-prevent-dequeue-vs.-unlock-race.patch
queue-4.4/perf-x86-fix-full-width-counter-counter-overflow.patch
queue-4.4/locking-rtmutex-use-read_once-in-rt_mutex_owner.patch