Messages in this thread Patch in this message | | | Date | Tue, 23 Jul 2013 00:46:20 -0700 | From | tip-bot for Peter Zijlstra <> | Subject | [tip:core/locking] mutex: Fix/ document access-once assumption in mutex_can_spin_on_owner() |
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Commit-ID: 1e40c2edef2537f87f94d0baf80aeaeb7d51cc23 Gitweb: http://git.kernel.org/tip/1e40c2edef2537f87f94d0baf80aeaeb7d51cc23 Author: Peter Zijlstra <peterz@infradead.org> AuthorDate: Fri, 19 Jul 2013 20:31:01 +0200 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Mon, 22 Jul 2013 10:33:39 +0200
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> --- 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
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