Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751871AbWIZBCP (ORCPT ); Mon, 25 Sep 2006 21:02:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751874AbWIZBCO (ORCPT ); Mon, 25 Sep 2006 21:02:14 -0400 Received: from gw.goop.org ([64.81.55.164]:56996 "EHLO mail.goop.org") by vger.kernel.org with ESMTP id S1751871AbWIZBCN (ORCPT ); Mon, 25 Sep 2006 21:02:13 -0400 Message-ID: <45187C0E.1080601@goop.org> Date: Mon, 25 Sep 2006 18:02:06 -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> In-Reply-To: <20060926004535.GA2978@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: 2560 Lines: 73 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. Likewise, the assignment to "global" can't move out of the range between the preempt_enable and the call to other_thing(). So in your case, if your equivalent of the non-preemptable block is the call to the marker function, then there's a good chance that the compiler might decide to move some other code in there. Now it might be possible to take the addresses of labels to inhibit code motion into a particular range: { __label__ before, after; asm volatile("" : : "m" (*&&before), "m" (*&&after)); // gcc can't know what we're doing with the labels before: ; // stuff after: ; } but that might be risky for several reasons: I don't know of any particular promises gcc makes in this circumstance; I suspect taking the address of a label will have a pretty severe inhibition on what optimisations gcc's is willing to use (it may prevent inlining altogether); and this looks pretty unusual, so there could be bugs. 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/