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    SubjectRe: [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<>
    >> 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<>
    >> Cc: Stephen Hemminger<>
    >> Cc: Michael S. Tsirkin<>
    >> Cc: Jason Wang<>
    >> ---
    > 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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