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mutex: Fix/document access-once assumption in mutex_can_spin_on_owner()

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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>
This commit is contained in:
Peter Zijlstra 2013-07-19 20:31:01 +02:00 committed by Ingo Molnar
parent c4be9cb4f1
commit 1e40c2edef
1 changed files with 4 additions and 2 deletions
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6
kernel/mutex.c
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@ -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) static inline int mutex_can_spin_on_owner(struct mutex *lock)
{ {
struct task_struct *owner;
int retval = 1; int retval = 1;
rcu_read_lock(); rcu_read_lock();
if (lock->owner) owner = ACCESS_ONCE(lock->owner);
retval = lock->owner->on_cpu; if (owner)
retval = owner->on_cpu;
rcu_read_unlock(); rcu_read_unlock();
/* /*
* if lock->owner is not set, the mutex owner may have just acquired * if lock->owner is not set, the mutex owner may have just acquired