From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2F305C43387 for ; Fri, 11 Jan 2019 15:28:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0272621848 for ; Fri, 11 Jan 2019 15:28:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387414AbfAKP2S (ORCPT ); Fri, 11 Jan 2019 10:28:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37033 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728958AbfAKP2S (ORCPT ); Fri, 11 Jan 2019 10:28:18 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 457B94E93F; Fri, 11 Jan 2019 15:28:17 +0000 (UTC) Received: from treble (ovpn-122-231.rdu2.redhat.com [10.10.122.231]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5C3BA5C689; Fri, 11 Jan 2019 15:28:11 +0000 (UTC) Date: Fri, 11 Jan 2019 09:28:09 -0600 From: Josh Poimboeuf To: Alexandre Chartre Cc: Sean Christopherson , Steven Rostedt , Nadav Amit , X86 ML , LKML , Ard Biesheuvel , Andy Lutomirski , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Linus Torvalds , Masami Hiramatsu , Jason Baron , Jiri Kosina , David Laight , Borislav Petkov , Julia Cartwright , Jessica Yu , "H. Peter Anvin" , Rasmus Villemoes , Edward Cree , Daniel Bristot de Oliveira Subject: Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible Message-ID: <20190111152809.ejutcmqrx4ud3fli@treble> References: <279b8003f7f0a6831d090ab822d37bc958f974de.1547073843.git.jpoimboe@redhat.com> <8138A1EE-359D-4CD2-8E96-5BF00313AB3B@vmware.com> <20190110172004.wuh45xoafynfm2df@treble> <20190110123243.3b9e0856@gandalf.local.home> <20190110174257.GE16556@linux.intel.com> <20190110125757.1c8d2870@gandalf.local.home> <20190110180428.GG16556@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Fri, 11 Jan 2019 15:28:17 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote: > To avoid any issue with live patching the call instruction, what about > toggling between two call instructions: one would be the currently active > call, while the other would currently be inactive but to be used after a > change is made. You can safely patch the inactive call and then toggle > the call flow (using a jump label) between the active and inactive calls. > > So instead of having a single call instruction: > > call function > > You would have: > > STATIC_JUMP_IF_TRUE label, key > call function1 > jmp done > label: > call function2 > done: > > If the key is set so that function1 is currently called then you can > safely update the call instruction for function2. Once this is done, > just flip the key to make the function2 call active. On a next update, > you would, of course, have to switch and update the call for function1. What about the following race? CPU1 CPU2 static key is false, doesn't jump task gets preempted before calling function1 change static key to true start patching "call function1" task resumes, sees inconsistent call instruction -- Josh