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authorPeter Zijlstra <peterz@infradead.org>2013-07-19 20:31:01 +0200
committerIngo Molnar <mingo@kernel.org>2013-07-22 10:33:39 +0200
commit1e40c2edef2537f87f94d0baf80aeaeb7d51cc23 (patch)
tree3692ad447e3242ab2042edfd09d24107d5166652 /kernel/mutex.c
parentc4be9cb4f19cbd534a6c4c334cd48d8bb483e17a (diff)
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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>
Diffstat (limited to 'kernel/mutex.c')
-rw-r--r--kernel/mutex.c6
1 files 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
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