linux-kernel - [PATCH 3.12 005/235] locking/rtmutex: Use READ_ONCE() in rt_mutex

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Message-Id: <15543c65c619ea88c8b3fdf97f2371ea13d4048f.1485514374.git.jslaby@suse.cz>
Date:   Fri, 27 Jan 2017 11:52:18 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        David Daney <ddaney@...iumnetworks.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 005/235] locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()

From: Thomas Gleixner <tglx@...utronix.de>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@....com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: David Daney <ddaney@...iumnetworks.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Siewior <bigeasy@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>
Link: http://lkml.kernel.org/r/20161130210030.431379999@linutronix.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/rtmutex_common.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h
index 53a66c85261b..1823c094fe96 100644
--- a/kernel/rtmutex_common.h
+++ b/kernel/rtmutex_common.h
@@ -96,8 +96,9 @@ 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);
 }
 
 /*
-- 
2.11.0