Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933072Ab0BYRcn (ORCPT ); Thu, 25 Feb 2010 12:32:43 -0500 Received: from mail-ww0-f46.google.com ([74.125.82.46]:51074 "EHLO mail-ww0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932947Ab0BYRcl (ORCPT ); Thu, 25 Feb 2010 12:32:41 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; b=foN1hRyWOiBuPwQjzFfO1NdUQgsg0JtnRy3c67KCW6ICf3clZGeBfnDWG/XcVl1pxV 4mjiYrNb6MQ1fnApt8agU4mW4kZXSM3aOvCF4h6+4/H5bQeXVmDfrqYzRzr15OAEuV7Z 2Xu4fUfVC8rvJXZeRFcHdySId9ZPFuRBBRYGw= MIME-Version: 1.0 In-Reply-To: <19334.44752.357207.382349@pilspetsen.it.uu.se> References: <4B8692E3.9030509@gmail.com> <19334.40337.651079.440912@pilspetsen.it.uu.se> <73c1f2161002250833n120cda05s9371e5ce13cc0aac@mail.gmail.com> <19334.44752.357207.382349@pilspetsen.it.uu.se> Date: Thu, 25 Feb 2010 12:32:37 -0500 Message-ID: <73c1f2161002250932j5167e2fan51dc11970df00f7@mail.gmail.com> Subject: Re: [PATCH] sha: prevent removal of memset as dead store in sha1_update() From: Brian Gerst To: Mikael Pettersson Cc: Roel Kluin , Herbert Xu , "David S. Miller" , linux-crypto@vger.kernel.org, Andrew Morton , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1196 Lines: 25 On Thu, Feb 25, 2010 at 12:09 PM, Mikael Pettersson wrote: > Brian Gerst wrote: >> Would barrier() (which is a simple memory clobber) after the memset work? > > I don't know. It's implemented as an asm with a "memory" clobber, > but I wouldn't bet on that forcing previous writes to a dying object > to actally be performed (it would have to have a data-dependency on > the dying object, but I don't think there is one). >From the GCC manual, section 5.37: If your assembler instructions access memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GCC to not keep memory values cached in registers across the assembler instruction and not optimize stores or loads to that memory. You will also want to add the volatile keyword if the memory affected is not listed in the inputs or outputs of the asm, as the `memory' clobber does not count as a side-effect of the asm. -- Brian Gerst -- 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/