Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750919AbWIZFD0 (ORCPT ); Tue, 26 Sep 2006 01:03:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750923AbWIZFD0 (ORCPT ); Tue, 26 Sep 2006 01:03:26 -0400 Received: from gw.goop.org ([64.81.55.164]:44724 "EHLO mail.goop.org") by vger.kernel.org with ESMTP id S1750919AbWIZFDZ (ORCPT ); Tue, 26 Sep 2006 01:03:25 -0400 Message-ID: <4518B4A0.6070509@goop.org> Date: Mon, 25 Sep 2006 22:03:28 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 1.5.0.7 (X11/20060913) MIME-Version: 1.0 To: Mathieu Desnoyers CC: Martin Bligh , "Frank Ch. Eigler" , Masami Hiramatsu , prasanna@in.ibm.com, Andrew Morton , Ingo Molnar , Paul Mundt , linux-kernel , Jes Sorensen , Tom Zanussi , Richard J Moore , Michel Dagenais , Christoph Hellwig , Greg Kroah-Hartman , Thomas Gleixner , William Cohen , ltt-dev@shafik.org, systemtap@sources.redhat.com, Alan Cox , Karim Yaghmour , Pavel Machek , Joe Perches , "Randy.Dunlap" , "Jose R. Santos" Subject: Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17 References: <20060925233349.GA2352@Krystal> <20060925235617.GA3147@Krystal> <45187146.8040302@goop.org> <20060926002551.GA18276@Krystal> <20060926004535.GA2978@Krystal> <45187C0E.1080601@goop.org> <20060926025924.GA27366@Krystal> In-Reply-To: <20060926025924.GA27366@Krystal> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3429 Lines: 85 Mathieu Desnoyers wrote: >> Mathieu Desnoyers wrote: >> >>> To protect code from being preempted, the macros preempt_disable and >>> preempt_enable must normally be used. Logically, this macro must make sure >>> gcc >>> doesn't interleave preemptible code and non-preemptible code. >>> >>> >> No, it only needs to prevent globally visible side-effects from being >> moved into/out of preemptable blocks. In practice that means memory >> updates (including the implicit ones that calls to external functions >> are assumed to make). >> >> >>> Which makes me think that if I put barriers around my asm, call, asm trio, >>> no >>> other code will be interleaved. Is it right ? >>> >>> >> No global side effects, but code with local side effects could be moved >> around without changing the meaning of preempt. >> >> For example: >> >> int foo; >> extern int global; >> >> foo = some_function(); >> >> foo += 42; >> >> preempt_disable(); >> // stuff >> preempt_enable(); >> >> global = foo; >> foo += other_thing(); >> >> Assume here that some_function and other_function are extern, and so gcc >> has no insight into their behaviour and therefore conservatively assumes >> they have global side-effects. >> >> The memory barriers in preempt_disable/enable will prevent gcc from >> moving any of the function calls into the non-preemptable region. But >> because "foo" is local and isn't visible to any other code, there's no >> reason why the "foo += 42" couldn't move into the preempt region. >> > > I am not sure about this last statement. The same reference : > http://developer.apple.com/documentation/DeveloperTools/gcc-4.0.1/gcc/Extended-Asm.html > (This is pretty old, and this is an area which changes quite a lot. You should refer to something more recent; http://www.cims.nyu.edu/cgi-systems/info2html?/usr/local/info(gcc)Top for example, though in this case the quoted text looks the same.) > I am just wondering how gcc can assume that I will not modify variables on the > stack from within a function with a memory clobber. If I would like to do some > nasty things in my assembly code, like accessing directly to a local variable by > using an offset from the stack pointer, I would expect gcc not to relocate this > local variable around my asm volatile memory clobbered statement, as it falls > under the category "access memory in an unpredictable fashion". > That not really what it means. gcc is free to put local variables in memory or register, and unless you pass the local to your asm as a parameter, your code has no way of knowing how to find the current location of the local. You could trash your stack frame from within the asm if you like, but I don't think gcc is under any obligation to behave in a deterministic way if you do. "Unpredictable" in this case means that the memory modified isn't easily specified as a normal asm parameter. For example, if you have an asm which does a memset(), the modified memory's size is a runtime variable rather than a compile-time constant. Or perhaps your asm follows a linked list and modifies memory as it traverses the list. J - 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/