On 05/20/2016 07:58 AM, Peter Zijlstra wrote:
On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
As such, the following restores the behavior of the ticket locks and 'fixes'
(or hides?) the bug in sems. Naturally incorrect approach:
@@ -290,7 +290,8 @@ static void sem_wait_array(struct sem_array *sma)
for (i = 0; i< sma->sem_nsems; i++) {
sem = sma->sem_base + i;
- spin_unlock_wait(&sem->lock);
+ while (atomic_read(&sem->lock))
+ cpu_relax();
}
ipc_smp_acquire__after_spin_is_unlocked();
}
The actual bug is clear_pending_set_locked() not having acquire
semantics. And the above 'fixes' things because it will observe the old
pending bit or the locked bit, so it doesn't matter if the store
flipping them is delayed.
The clear_pending_set_locked() is not the only place where the lock is
set. If there are more than one waiter, the queuing patch will be used
instead. The set_locked(), which is also an unordered store, will then
be used to set the lock.
Cheers,
Longman
