Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752351AbaKJJik (ORCPT ); Mon, 10 Nov 2014 04:38:40 -0500 Received: from mail-wi0-f177.google.com ([209.85.212.177]:52132 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751580AbaKJJii convert rfc822-to-8bit (ORCPT ); Mon, 10 Nov 2014 04:38:38 -0500 MIME-Version: 1.0 In-Reply-To: References: <20141106203832.GB30170@kroah.com> <20141106205644.GA31435@kroah.com> Date: Mon, 10 Nov 2014 10:38:34 +0100 X-Google-Sender-Auth: 8Tmpn1PdyjETQ-E1EuSyKomyBPI Message-ID: Subject: Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path From: Christian Riesch To: =?UTF-8?B?TcOlbnMgUnVsbGfDpXJk?= Cc: Greg Kroah-Hartman , Jiri Slaby , "linux-kernel@vger.kernel.org" , Peter Hurley , stable Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Måns, On Mon, Nov 10, 2014 at 10:25 AM, Måns Rullgård wrote: > Christian Riesch writes: > >> On Thu, Nov 6, 2014 at 9:56 PM, Greg Kroah-Hartman >> wrote: >>> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote: >>>> Greg Kroah-Hartman writes: >>>> >>>> > On Thu, Nov 06, 2014 at 12:39:59PM +0100, Christian Riesch wrote: >>>> >> The current implementation of put_tty_queue() causes a race condition >>>> >> when re-arranged by the compiler. >>>> >> >>>> >> On my build with gcc 4.8.3, cross-compiling for ARM, the line >>>> >> >>>> >> *read_buf_addr(ldata, ldata->read_head++) = c; >>>> >> >>>> >> was re-arranged by the compiler to something like >>>> >> >>>> >> x = ldata->read_head >>>> >> ldata->read_head++ >>>> >> *read_buf_addr(ldata, x) = c; >>>> >> >>>> >> which causes a race condition. Invalid data is read if data is read >>>> >> before it is actually written to the read buffer. >>>> > >>>> > Really? A compiler can rearange things like that and expect things to >>>> > actually work? How is that valid? >>>> >>>> This is actually required by the C spec. There is a sequence point >>>> before a function call, after the arguments have been evaluated. Thus >>>> all side-effects, such as the post-increment, must be complete before >>>> the function is called, just like in the example. >>>> >>>> There is no "re-arranging" here. The code is simply wrong. >>> >>> Ah, ok, time to dig out the C spec... >>> >>> Anyway, because of this, no need for the wmb() calls, just rearrange the >>> logic and all should be good, right? Christian, can you test that >>> instead? >> >> I ran a test with the patch that I posted in my first email for the >> last 4 days. No communication errors occurred so the patch actually >> fixes my problem. I will run another test as suggested by Greg, just >> with rearranging the logic. > > What hardware are you running on? If it's a single-processor system, > it won't break without barriers even if they are required for SMP. Yes, single processor. Texas Instruments AM1808 SoC. Thanks, Christian -- 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/