Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751416AbaKFWNG (ORCPT ); Thu, 6 Nov 2014 17:13:06 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:51008 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751020AbaKFWND convert rfc822-to-8bit (ORCPT ); Thu, 6 Nov 2014 17:13:03 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Greg Kroah-Hartman Cc: Christian Riesch , Jiri Slaby , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path References: <20141106203832.GB30170@kroah.com> <20141106205644.GA31435@kroah.com> <20141106211754.GA32000@kroah.com> <20141106220249.GA952@kroah.com> Date: Thu, 06 Nov 2014 22:12:54 +0000 In-Reply-To: <20141106220249.GA952@kroah.com> (Greg Kroah-Hartman's message of "Thu, 6 Nov 2014 14:02:49 -0800") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Greg Kroah-Hartman writes: > On Thu, Nov 06, 2014 at 09:38:59PM +0000, M?ns Rullg?rd wrote: >> Greg Kroah-Hartman writes: >> >> > On Thu, Nov 06, 2014 at 09:01:36PM +0000, M?ns Rullg?rd wrote: >> >> Greg Kroah-Hartman writes: >> >> >> >> > On Thu, Nov 06, 2014 at 08:49:01PM +0000, M?ns Rullg?rd 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? >> >> >> >> Weakly ordered SMP systems probably need some kind of barrier. I didn't >> >> look at it carefully. >> > >> > It shouldn't need a barier, as it is a sequence point with the function >> > call. Well, it's an inline function, but that "shouldn't" matter here, >> > right? >> >> Sequence points say nothing about the order in which stores become >> visible to other CPUs. That's why there are barrier instructions. > > Yes, but "order" matters. > > If I write code that does: > > 100 x = ldata->read_head; > 101 &ldata->read_head[x & SOME_VALUE] = y; > 102 ldata->read_head++; > > the compiler can not reorder lines 102 and 101 just because it feels > like it, right? Or is it time to go spend some reading of the C spec > again... The compiler can't. The hardware can. All the hardware promises is that at some unspecified time in the future, both memory locations will have the correct values. Another CPU might see 'read_head' updated before it sees the corresponding data value. A wmb() between the writes forces the CPU to complete preceding stores before it begins subsequent ones. Documentation/memory-barriers.txt explains it all in detail. -- M?ns Rullg?rd mans@mansr.com -- 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/