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

From: Jiri Slaby
Date: Fri Jan 27 2017 - 07:06:41 EST

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

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@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: David Daney <ddaney@xxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20161130210030.431379999@xxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
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);