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 <peterz@infradead.org>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Acked-by: Waiman Long <Waiman.Long@hp.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/r/20130719183101.GA20909@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/mutex.c b/kernel/mutex.c
index ff05f4bd86eb..7ff48c55a98b 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