From: Rusty Russell <rusty@rustcorp.com.au>
To: Eric Dumazet <eric.dumazet@gmail.com>, Jason Wang <jasowang@redhat.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, mst@redhat.com,
        Stephen Hemminger <shemminger@vyatta.com>
Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches
In-Reply-To: <1338972341.2760.3944.camel@edumazet-glaptop>
References: <1338971724.2760.3913.camel@edumazet-glaptop> <1338972341.2760.3944.camel@edumazet-glaptop>
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Date: Sun, 10 Jun 2012 16:06:34 +0930
Message-ID: <87ipezhdvx.fsf@rustcorp.com.au>
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On Wed, 06 Jun 2012 10:45:41 +0200, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2012-06-06 at 10:35 +0200, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > commit 3fa2a1df909 (virtio-net: per cpu 64 bit stats (v2)) added a race
> > on 32bit arches.
> > 
> > We must use separate syncp for rx and tx path as they can be run at the
> > same time on different cpus. Thus one sequence increment can be lost and
> > readers spin forever.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Stephen Hemminger <shemminger@vyatta.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Jason Wang <jasowang@redhat.com>
> > ---
> 
> Just to make clear : even using percpu stats/syncp, we have no guarantee
> that write_seqcount_begin() is done with one instruction. [1]
> 
> It is OK on x86 if "incl" instruction is generated by the compiler, but
> on a RISC cpu, the "load memory,%reg ; inc %reg ; store %reg,memory" can
> be interrupted.
> 
> So if you are 100% sure all paths are safe against preemption/BH, then
> this patch is not needed, but a big comment in the code would avoid
> adding possible races in the future.

Too fragile; let's keep them separate as per this patch.

Acked-by: Rusty Russell <rusty@rustcorp.com.au>

Thanks,
Rusty.
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