On Wed, 06 Jun 2012 10:45:41 +0200, Eric Dumazet <[email protected]> wrote: > On Wed, 2012-06-06 at 10:35 +0200, Eric Dumazet wrote: > > From: Eric Dumazet <[email protected]> > > > > 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 <[email protected]> > > Cc: Stephen Hemminger <[email protected]> > > Cc: Michael S. Tsirkin <[email protected]> > > Cc: Jason Wang <[email protected]> > > --- > > 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.
Too fragile; let's keep them separate as per this patch. Acked-by: Rusty Russell <[email protected]> Thanks, Rusty. _______________________________________________ Virtualization mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
