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@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.
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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