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

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Message-ID: <20111220193055.GA26392@redhat.com>
Date:	Tue, 20 Dec 2011 21:30:55 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Amit Shah <amit.shah@...hat.com>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] virtio_net: fix refill related races

On Tue, Dec 20, 2011 at 11:09:46AM -0800, Tejun Heo wrote:
> Hello, Michael.
> 
> On Tue, Dec 20, 2011 at 09:09:08PM +0200, Michael S. Tsirkin wrote:
> > 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?
> 
> No, it's not.  Normal workqueues only guarantee non-reentrance on
> local CPU.  If you want to guarantee that only one instance of a given
> item is executing across all CPUs, you need to use the nrt workqueue.
> 
> Thanks.

Hmm, in that case it looks like a nasty race could get
triggered, with try_fill_recv run on multiple CPUs in parallel,
corrupting the linked list within the vq.

Using the mutex as my patch did will fix that naturally, as well.

Rusty, am I missing something?

> -- 
> tejun
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