Messages in this thread | | | Date | Fri, 20 May 2016 22:53:00 +0200 | From | Peter Zijlstra <> | Subject | Re: sem_lock() vs qspinlocks |
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On Fri, May 20, 2016 at 04:44:19PM -0400, Waiman Long wrote: > 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.
Ah yes. I didn't get that far. One case was enough :-)
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