Re: synchronize_qrcu_timeout()

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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