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

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Date:	Wed, 06 Jun 2012 17:37:34 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, mst@...hat.com,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches

On 06/06/2012 04:45 PM, Eric Dumazet wrote:
> On Wed, 2012-06-06 at 10:35 +0200, Eric Dumazet wrote:
>> From: Eric Dumazet<edumazet@...gle.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@...gle.com>
>> Cc: Stephen Hemminger<shemminger@...tta.com>
>> Cc: Michael S. Tsirkin<mst@...hat.com>
>> Cc: Jason Wang<jasowang@...hat.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.

Thanks for explaing, current virtio-net is safe I think. But the patch 
is still needed as my patch would update the statistics in irq.
>
> [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.
>
>
>
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