From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756887AbZETVBr (ORCPT ); Wed, 20 May 2009 17:01:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754944AbZETVBk (ORCPT ); Wed, 20 May 2009 17:01:40 -0400 Received: from terminus.zytor.com ([198.137.202.10]:56623 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754848AbZETVBk (ORCPT ); Wed, 20 May 2009 17:01:40 -0400 Message-ID: <4A146E78.6000600@zytor.com> Date: Wed, 20 May 2009 13:56:24 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Roland McGrath CC: Oleg Nesterov , Ingo Molnar , Christoph Hellwig , Hiroshi Shimamoto , Vitaly Mayatskikh , Andrew Morton , linux-kernel@vger.kernel.org, Thomas Gleixner Subject: Re: Q: put_user_try & co (Was: [PATCH 1/5] Split wait_noreap_copyout()) References: <1242036759-4025-1-git-send-email-v.mayatskih@gmail.com> <1242036759-4025-2-git-send-email-v.mayatskih@gmail.com> <20090511120418.GA3859@infradead.org> <20090511121708.GD13954@elte.hu> <20090520190312.GA32333@redhat.com> <20090520201127.48B54FC38D@magilla.sf.frob.com> In-Reply-To: <20090520201127.48B54FC38D@magilla.sf.frob.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Roland McGrath wrote: > > You mean &&__efault_label here (it's a funny syntax, but that's how it is). > &&label is a GCC extension that I'm not sure the kernel has used before. > > I think it can be touchy to have an asm jump into compiled code that way. > e.g., perhaps the compiler produced: > > mov reg, 40(sp) > mov $123, reg > #APP > ... inside of your asm ... > #NO_APP > mov 40(sp), reg > > or some such thing. If you jump away from inside the asm, you won't ever > do "mov 40(sp), reg". But the compiler might think that reg has its > original value at the __efault_label: code location. > > Perhaps more important than any particular compiler-confusion scenario we > can come up with is simply that this would be an obscure corner of code > generation in the compiler that the kernel has not evoked before. There > might be bugs or oddities in various compilers of various vintages, that > we don't know about because they never came up before. > Yes, it seems like a bad idea to me. -hpa