Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755350Ab2FFIpt (ORCPT ); Wed, 6 Jun 2012 04:45:49 -0400 Received: from mail-ey0-f174.google.com ([209.85.215.174]:61299 "EHLO mail-ey0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751064Ab2FFIpq (ORCPT ); Wed, 6 Jun 2012 04:45:46 -0400 Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches From: Eric Dumazet To: Jason Wang Cc: netdev@vger.kernel.org, rusty@rustcorp.com.au, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, mst@redhat.com, Stephen Hemminger In-Reply-To: <1338971724.2760.3913.camel@edumazet-glaptop> References: <1338971724.2760.3913.camel@edumazet-glaptop> Content-Type: text/plain; charset="UTF-8" Date: Wed, 06 Jun 2012 10:45:41 +0200 Message-ID: <1338972341.2760.3944.camel@edumazet-glaptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1468 Lines: 38 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. [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/