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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS 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 E7677C43387 for ; Fri, 11 Jan 2019 16:47:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B100F20872 for ; Fri, 11 Jan 2019 16:47:52 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="1zeoLaw7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388213AbfAKQrv (ORCPT ); Fri, 11 Jan 2019 11:47:51 -0500 Received: from aserp2130.oracle.com ([141.146.126.79]:40044 "EHLO aserp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728266AbfAKQrv (ORCPT ); Fri, 11 Jan 2019 11:47:51 -0500 Received: from pps.filterd (aserp2130.oracle.com [127.0.0.1]) by aserp2130.oracle.com (8.16.0.22/8.16.0.22) with SMTP id x0BGXuBU070269; Fri, 11 Jan 2019 16:46:45 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : references : cc : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2018-07-02; bh=XszuSG1fZq1SXekI14e2rK2zFUxdJHScohOeqKdhnfs=; b=1zeoLaw7c6/MLbA/kJDN2XFmA7I43N2NbFhQZ9NerxKsi754M5kMNkr1PDqIG5WRdWzu XDL2Tn5OGaI4RQDbldoCIN+BIYYoa6IFaCacqNdicNWshBrh3S+1pI2vtpYkuzyRPy66 6+G9gCqM32JF5cUgpYE/IGZA4OHCXwh6iE9gHMhbNM83qJVNkvqFhRuSS57S8qNWwPss 3nmaUaDB2Yykb1KRG0k9Lqwck7ggpxSqhTYBNS/3DDEhOD44YzXMJFy6QW5jbBtfA/vk m9hnYN+dMhQaaTNdHZfI+B0XNVSbBMwdDYLLEpVRetVW1yXTMOw5d2TqhaZzTuoBK8/l 5A== Received: from aserv0021.oracle.com (aserv0021.oracle.com [141.146.126.233]) by aserp2130.oracle.com with ESMTP id 2ptj3ee6m7-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 11 Jan 2019 16:46:45 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id x0BGkhCD010795 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 11 Jan 2019 16:46:43 GMT Received: from abhmp0011.oracle.com (abhmp0011.oracle.com [141.146.116.17]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x0BGkfw0030830; Fri, 11 Jan 2019 16:46:41 GMT Received: from [10.166.106.34] (/10.166.106.34) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 11 Jan 2019 08:46:40 -0800 Subject: Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible To: Josh Poimboeuf 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> <20190111152809.ejutcmqrx4ud3fli@treble> 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 From: Alexandre Chartre Organization: Oracle Corporation Message-ID: Date: Fri, 11 Jan 2019 17:46:36 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 MIME-Version: 1.0 In-Reply-To: <20190111152809.ejutcmqrx4ud3fli@treble> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9133 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=731 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901110136 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/11/2019 04:28 PM, Josh Poimboeuf wrote: > 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 > If the function1 call is active then it won't be changed, you will change function2. However, I presume you can still have a race but if the function is changed twice before calling function1: CPU1 CPU2 static key is false, doesn't jump task gets preempted before calling function1 -- first function change -- patch "call function2" change static key to true -- second function change -- start patching "call function1" task resumes, sees inconsistent call instruction So right, that's a problem. alex.