netdev - Re: [PATCH V4 4/8]vhost: vhost TX zero-copy support

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Date:	Wed, 4 May 2011 18:56:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 4/8]vhost: vhost TX zero-copy support

On Wed, May 04, 2011 at 08:18:04AM -0700, Shirley Ma wrote:
> > > +void vhost_zerocopy_callback(struct sk_buff *skb)
> > > +{
> > > +     int idx = skb_shinfo(skb)->ubuf.desc;
> > > +     struct vhost_virtqueue *vq = skb_shinfo(skb)->ubuf.arg;
> > > +
> > > +     /* set len = 1 to mark this desc buffers done DMA */
> > > +     vq->heads[idx].len = 1;
> > > +}
> > 
> > So any kind of callback like that, that goes into the skb,
> > will be racy wrt module unloading because module can go away
> > after you mark dma done and before this function returns.
> > Solution is to have a core function that does the
> > final signalling (e.g. sock_wfree is in core).
> > Would be nice to fix, even though this race is
> > completely theoretical, I don't believe it will
> > trigger in practice.
> 
> I run lots of stress tests, and never hit this. 
> 
> But I can try to fix it.

Yes, it's a theoretical thing. Nice to have but not a must.

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