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    Subject[tip:locking/core] locking/rtmutex: Use READ_ONCE() in rt_mutex_owner()
    Commit-ID:  1be5d4fa0af34fb7bafa205aeb59f5c7cc7a089d
    Gitweb: http://git.kernel.org/tip/1be5d4fa0af34fb7bafa205aeb59f5c7cc7a089d
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Wed, 30 Nov 2016 21:04:42 +0000
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Fri, 2 Dec 2016 11:13:26 +0100

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

    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>
    Cc: <stable@vger.kernel.org>
    Link: http://lkml.kernel.org/r/20161130210030.431379999@linutronix.de
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/locking/rtmutex_common.h | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
    index 4f5f83c..e317e1c 100644
    --- 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);
    }

    /*
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