Re: synchronize_qrcu_timeout()

[Paul E. McKenney]

On Mon, Jun 25, 2007 at 07:49:57PM +0400, Oleg Nesterov wrote:
> On 06/25, Oleg Nesterov wrote:
> >
> > On 06/25, Paul E. McKenney wrote:
> > >
> > > On Mon, Jun 25, 2007 at 02:43:32PM +0400, Oleg Nesterov wrote:
> > > > 
> > > > Sadly, you can't use srcu/qrcu because it doesn't handle timeouts.	
> > > 
> > > Interesting...  So the thought is to have a synchronize_srcu_timeout()
> > > or something similar that waited for a grace period to elapse or for
> > > a timeout to expire, whichever comes first?  It should not be too hard
> > > to arrange something, if needed.
> > 
> > Yes. As for qrcu (see http://marc.info/?t=116484476500001), I think it is easy.
> > First, we add "int interrupted" into struct qrcu_struct, then something like this
> 
> Even simpler, we don't need ->interrupted.

I have to ask...

What sorts of performance characteristics are needed here?  The reason
that I ask is because putting a straight synchronize_qrcu() into a
workqueue (or something similar) and then using a timer to provide
any needed wakeup seems a lot simpler than rearranging the innards of
synchronize_qrcu().

(Yes, I am feeling cowardly.  Why do you ask?)

						Thanx, Paul

> 	long synchronize_qrcu_timeout(struct qrcu_struct *qp, long tout)
> 	{
> 		int idx, prv;
> 
> 		smp_mb();
> 		mutex_lock(&qp->mutex);
> 
> 		idx = qp->completed & 0x1;
> 		prv = idx ^ 0x1;
> 
> 		if (unlikely(atomic_read(qp->ctr + prv))) {
> 			// the previous call has not succeed,
> 			// finish the wait
> 			__wait_event_timeout(qp->wq, !atomic_read(qp->ctr + prv), tout);
> 			if (unlikely(!tout))
> 				goto out;
> 		}
> 
> 		if (atomic_read(qp->ctr + idx) == 1)
> 			goto out;
> 
> 		atomic_inc(qp->ctr + prv);
> 		qp->completed++;
> 
> 		atomic_dec(qp->ctr + idx);
> 		__wait_event_timeout(qp->wq, !atomic_read(qp->ctr + idx), tout);
> 	out:
> 		mutex_unlock(&qp->mutex);
> 		smp_mb();
> 
> 		return tout;
> 	}
> 
> Oleg.
> 
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