Messages in this thread | | | Date | Tue, 10 Dec 2013 17:44:37 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK |
| |
On 12/09, Paul E. McKenney wrote: > > @@ -1626,7 +1626,10 @@ for each construct. These operations all imply certain barriers: > operation has completed. > > Memory operations issued before the LOCK may be completed after the LOCK > - operation has completed. > + operation has completed. An smp_mb__before_spinlock(), combined > + with a following LOCK, acts as an smp_wmb(). Note the "w", > + this is smp_wmb(), not smp_mb().
Well, but smp_mb__before_spinlock + LOCK is not wmb... But it is not the full barrier. It should guarantee that, say,
CONDITION = true; // 1
// try_to_wake_up smp_mb__before_spinlock(); spin_lock(&task->pi_lock);
if (!(p->state & state)) // 2 return;
can't race with with set_current_state() + check(CONDITION), this means that 1 and 2 above must not be reordered.
But a LOAD before before spin_lock() can leak into the critical section.
Perhaps this should be clarified somehow, or perhaps it should actually imply mb (if combined with LOCK).
Oleg
| |