Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 10 Jun 2012 10:03:27 +0300

On Sun, Jun 10, 2012 at 04:06:34PM +0930, Rusty Russell wrote:
> On Wed, 06 Jun 2012 10:45:41 +0200, Eric Dumazet <[email-masked]> wrote:
> > On Wed, 2012-06-06 at 10:35 +0200, Eric Dumazet wrote:
> > > From: Eric Dumazet <[email-masked]>
> > > 
> > > 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 <[email-masked]>
> > > Cc: Stephen Hemminger <[email-masked]>
> > > Cc: Michael S. Tsirkin <[email-masked]>
> > > Cc: Jason Wang <[email-masked]>
> > > ---
> > 
> > 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 <[email-masked]>
> 
> Thanks,
> Rusty.

One question though: do we want to lay the structure
out so that the rx sync structure precedes the rx counters?

-- 
MST