netdev - Re: virtio_net: Fix napi poll list corruption

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Message-Id: <20141222.161049.278043772767314122.davem@davemloft.net>
Date:	Mon, 22 Dec 2014 16:10:49 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	david.vrabel@...rix.com, netdev@...r.kernel.org,
	xen-devel@...ts.xenproject.org, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, edumazet@...gle.com
Subject: Re: virtio_net: Fix napi poll list corruption

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Sat, 20 Dec 2014 11:23:27 +1100

> The commit d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less
> interrupt masking in NAPI) breaks virtio_net in an insidious way.
> 
> It is now required that if the entire budget is consumed when poll
> returns, the napi poll_list must remain empty.  However, like some
> other drivers virtio_net tries to do a last-ditch check and if
> there is more work it will call napi_schedule and then immediately
> process some of this new work.  Should the entire budget be consumed
> while processing such new work then we will violate the new caller
> contract.
> 
> This patch fixes this by not touching any work when we reschedule
> in virtio_net.
> 
> The worst part of this bug is that the list corruption causes other
> napi users to be moved off-list.  In my case I was chasing a stall
> in IPsec (IPsec uses netif_rx) and I only belatedly realised that it
> was virtio_net which caused the stall even though the virtio_net
> poll was still functioning perfectly after IPsec stalled.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Applied, thanks Herbert.
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