Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756093Ab2FFLOF (ORCPT ); Wed, 6 Jun 2012 07:14:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:32382 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754492Ab2FFLOD (ORCPT ); Wed, 6 Jun 2012 07:14:03 -0400 Date: Wed, 6 Jun 2012 14:13:59 +0300 From: "Michael S. Tsirkin" To: Eric Dumazet Cc: Jason Wang , netdev@vger.kernel.org, rusty@rustcorp.com.au, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Stephen Hemminger Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches Message-ID: <20120606111357.GA15070@redhat.com> References: <1338971724.2760.3913.camel@edumazet-glaptop> <1338972341.2760.3944.camel@edumazet-glaptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1338972341.2760.3944.camel@edumazet-glaptop> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1698 Lines: 42 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 > > > > 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. 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. > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/