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

[Eric Dumazet]

On Wed, 2012-06-06 at 10:35 +0200, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Stephen Hemminger <shemminger@xxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> ---

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.

[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.



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/