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 <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. 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. > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
