netdev - Re: [PATCH RFC] virtio_net: fix refill related races

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Message-ID: <20111220190908.GC25689@redhat.com>
Date:	Tue, 20 Dec 2011 21:09:08 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Amit Shah <amit.shah@...hat.com>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH RFC] virtio_net: fix refill related races

On Tue, Dec 13, 2011 at 01:05:11PM +1030, Rusty Russell wrote:
> On Mon, 12 Dec 2011 13:54:06 +0200, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > On Mon, Dec 12, 2011 at 09:25:07AM +1030, Rusty Russell wrote:
> > > Orthogonally, the refill-stop code is still buggy, as you noted.
> > 
> > Sorry I don't understand how it's still buggy.
> 
> Both places where we call:
> 
> 	cancel_delayed_work_sync(&vi->refill);
> 
> Do not actually guarantee that vi->refill isn't running, because it
> can requeue itself.  A 'bool no_more_refill' field seems like the
> simplest fix for this, but I don't think it's sufficient.
> 
> Tejun, is this correct?  What's the correct way to synchronously stop a
> delayed_work which can "schedule_delayed_work(&vi->refill, HZ/2);" on
> itself?
> 
> Thanks,
> Rusty.

Another question, wanted to make sure:
virtnet_poll does schedule_delayed_work(&vi->refill, 0);
separately refill work itself also does
schedule_delayed_work(&vi->refill, HZ/2);
If two such events happen twice, on different CPUs, we are still guaranteed
the work will only run once, right?

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