On Sun, Jun 10, 2012 at 04:06:34PM +0930, Rusty Russell wrote: > 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.
One question though: do we want to lay the structure out so that the rx sync structure precedes the rx counters? -- MST _______________________________________________ Virtualization mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
