linux-kernel - synchronize_qrcu

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Date:	Mon, 25 Jun 2007 19:20:11 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>, Uli Luckas <u.luckas@...d.de>,
	linux-kernel@...r.kernel.org
Subject: synchronize_qrcu_timeout()

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

	long synchronize_qrcu_timeout(struct qrcu_struct *qp, long tout)
	{
		int idx;

		smp_mb();
		mutex_lock(&qp->mutex);

	again:
		idx = qp->completed & 0x1;

		if (unlikely(qp->interrupted)) {
			idx ^= 0x1;
			goto restart;
		}


		if (atomic_read(qp->ctr + idx) == 1)
			goto out;

		atomic_inc(qp->ctr + (idx ^ 0x1));
		qp->completed++;

		atomic_dec(qp->ctr + idx);
	restart:
		__wait_event_timeout(qp->wq, !atomic_read(qp->ctr + idx), tout);
		if (unlikely(!tout)) {
			qp->interrupted = 1;
			goto out;
		}

		if (unlikely(qp->interrupted)) {
			qp->interrupted = 0;
			goto again;
		}

	out:
		mutex_unlock(&qp->mutex);
		smp_mb();

		return tout;
	}

Of course, this needs some other changes to implement synchronize_qrcu() on top
of synchronize_qrcu_timeout(). I also think that this doesn't break

	[RFC PATCH] QRCU fastpath optimization
	http://marc.info/?l=linux-kernel&m=117125577710947

Looks like we can do something similar for srcu.

Oleg.

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