From: Mathieu Desnoyers Subject: Re: [PATCH 1/1] x86: fix text_poke Date: Fri, 25 Apr 2008 12:11:36 -0400 Message-ID: <20080425161136.GD9503@Krystal> References: <20080425.021301.193689806.davem@davemloft.net> <1209343883-7991-1-git-send-email-jirislaby@gmail.com> <20080425151931.GA25510@elte.hu> <20080425152650.GA894@elte.hu> <20080425155444.GB9503@Krystal> <20080425155953.GA11243@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: Linus Torvalds , Jiri Slaby , David Miller , zdenek.kabelac@gmail.com, rjw@sisk.pl, paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org, linux-ext4@vger.kernel.org, herbert@gondor.apana.org.au, penberg@cs.helsinki.fi, clameter@sgi.com, linux-kernel@vger.kernel.org, Andi Kleen , pageexec@freemail.hu, "H. Peter Anvin" , Jeremy Fitzhardinge To: Ingo Molnar Return-path: Received: from tomts40.bellnexxia.net ([209.226.175.97]:35277 "EHLO tomts40-srv.bellnexxia.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755683AbYDYQQo (ORCPT ); Fri, 25 Apr 2008 12:16:44 -0400 Content-Disposition: inline In-Reply-To: <20080425155953.GA11243@elte.hu> Sender: linux-ext4-owner@vger.kernel.org List-ID: * Ingo Molnar (mingo@elte.hu) wrote: > > * Mathieu Desnoyers wrote: > > > Woooow, just a sec here. I removed the atomicity test _because_ there > > happen to be a case where it's safe to do non-atomic instruction > > modification. If we do : > > > > 1) replace the instruction first byte by a breakpoint, execute an > > instruction bypass (see the immediate values patches for detail) > > 2) modify the instruction non-atomically > > 3) put back the original instruction first byte. > > > > That's why I removed the BUG_ONs at the beginning of the function. > > That's also why it's required to deal with page crossing. > > but the code as-is is nonsensical. It checks for: > > BUG_ON(len > sizeof(long)); > > but then deals with page crossing... > That was in the initial version, before my patch, yes. I dealt with page crossing at first, then added a more restrictive test to "play safe" (I should have removed the page-crossing code at that point), but later on noticed that there was a single case where it's valid to do non-atomic updates, and it's when the execution flow is bypassed by a breakpoint (as the immediate values are doing), so the last patch you have removes the restrictive test and lets the page-crossing code in place. > it should also rename text_poke_early() to text_poke_core(), and call > _that_ from text_poke() if core_kernel_text(). From that alone the whole > poke_text() function would look a whole lot cleaner. > hrm, I am not convinced it's safe to call vmap() very early at boot time. In the immediate values implementation, I do call text_poke very very early at boot to populate the initial values. Or maybe are you proposing something different from what I currently understand ? Mathieu > Ingo -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68