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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Jun 2012 14:13:59 +0300

On Wed, Jun 06, 2012 at 10:45:41AM +0200, Eric Dumazet 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.

We currently do all stats either on napi callback or from
start_xmit callback.
This makes them safe, yes?

> [1] If done with one instruction, we still have a race, since a reader
> might see an even sequence and conclude no writer is inside the critical
> section. So read values could be wrong.
> 
>