Re: sem_lock() vs qspinlocks

[Waiman Long]

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