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Date: Tue, 13 Dec 2016 09:15:53 -0800 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Will Deacon <will.deacon@....com>, Thomas Gleixner <tglx@...utronix.de>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, David Daney <ddaney@...iumnetworks.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Mark Rutland <mark.rutland@....com>, Sebastian Siewior <bigeasy@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...nel.org> Subject: [PATCH 4.4 07/16] locking/rtmutex: Use READ_ONCE() in rt_mutex_owner() 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Gleixner <tglx@...utronix.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@....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: Greg Kroah-Hartman <gregkh@...uxfoundation.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); } /*
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