[PATCH] mutex: Fix mutex_can_spin_on_owner

[Peter Zijlstra <peterz@infradead.org>]

mutex_can_spin_on_owner() is broken in that it would allow the compiler
to load lock->owner twice, seeing a pointer first time and a MULL
pointer the second time.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 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