From 1e40c2edef2537f87f94d0baf80aeaeb7d51cc23 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 19 Jul 2013 20:31:01 +0200 Subject: [PATCH] mutex: Fix/document access-once assumption in mutex_can_spin_on_owner() mutex_can_spin_on_owner() is technically broken in that it would in theory allow the compiler to load lock->owner twice, seeing a pointer first time and a NULL pointer the second time. Linus pointed out that a compiler has to be seriously broken to not compile this correctly - but nevertheless this change is correct as it will better document the implementation. Signed-off-by: Peter Zijlstra Acked-by: Davidlohr Bueso Acked-by: Waiman Long Acked-by: Linus Torvalds Acked-by: Thomas Gleixner Acked-by: Rik van Riel Cc: Paul E. McKenney Cc: David Howells Link: http://lkml.kernel.org/r/20130719183101.GA20909@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/mutex.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/mutex.c b/kernel/mutex.c index ff05f4b..7ff48c5 100644 --- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -209,11 +209,13 @@ int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) */ static inline int mutex_can_spin_on_owner(struct mutex *lock) { + struct task_struct *owner; int retval = 1; rcu_read_lock(); - if (lock->owner) - retval = lock->owner->on_cpu; + owner = ACCESS_ONCE(lock->owner); + if (owner) + retval = owner->on_cpu; rcu_read_unlock(); /* * if lock->owner is not set, the mutex owner may have just acquired -- 2.7.4