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[PATCH 3.12 005/235] locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()

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From: Thomas Gleixner <tglx@linutronix.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@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: Jiri Slaby <jslaby@suse.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

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  [PATCH 3.12 004/235] locking/rtmutex: Prevent dequeue vs. unlock race Jiri Slaby <jslaby@suse.cz> - 2017-01-27 13:10 +0100
  [PATCH 3.12 030/235] ext4: add sanity checking to count_overhead() Jiri Slaby <jslaby@suse.cz> - 2017-01-27 13:10 +0100
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  [PATCH 3.12 005/235] locking/rtmutex: Use READ_ONCE() in rt_mutex_owner() Jiri Slaby <jslaby@suse.cz> - 2017-01-27 13:10 +0100

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