From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752827AbcHLVdO (ORCPT ); Fri, 12 Aug 2016 17:33:14 -0400 Received: from mail-bl2nam02on0131.outbound.protection.outlook.com ([104.47.38.131]:31068 "EHLO NAM02-BL2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751136AbcHLVdN (ORCPT ); Fri, 12 Aug 2016 17:33:13 -0400 Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=waiman.long@hpe.com; Message-ID: <57AE4083.90601@hpe.com> Date: Fri, 12 Aug 2016 17:32:51 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Dave Hansen CC: Andy Lutomirski , Thomas Gleixner , John Stultz , Ingo Molnar , "linux-kernel@vger.kernel.org" , Borislav Petkov , Jiang Liu , Randy Wright , Scott J Norton , Douglas Hatch , Prarit Bhargava , X86 ML , "H. Peter Anvin" Subject: Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention References: <1470853770-37625-1-git-send-email-Waiman.Long@hpe.com> <57ACD2DE.6080306@intel.com> <57AD0898.7030506@hpe.com> <57AD18D1.1050107@intel.com> <57AE00EE.8070904@hpe.com> <57AE0469.10503@intel.com> <57AE1606.2010304@hpe.com> <57AE3B61.5010202@hpe.com> <57AE3D8F.9080202@intel.com> In-Reply-To: <57AE3D8F.9080202@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [72.71.243.96] X-ClientProxiedBy: CY1PR13CA0005.namprd13.prod.outlook.com (10.162.30.143) To AT5PR84MB0306.NAMPRD84.PROD.OUTLOOK.COM (10.162.138.28) X-MS-Office365-Filtering-Correlation-Id: fdca51c7-a329-481c-1b66-08d3c2f83ab9 X-Microsoft-Exchange-Diagnostics: 1;AT5PR84MB0306;2:akBDLWE2++0fhakoOTakYHe4na/iU8MdBivf3aEBkU7W8BSK00y/7/WI5Ct8U1NN5pvCi1IUeGQ2ujmAbCy6dekAZId7+NTYLD6mscUrnJrHo+cleWDT4/rOAwWx9YvOsv4pITbBmokRTOCcBSDT7RrrfuswOX0rCcQvIzhdrbGvfIZaqM8v6NxTrRoxDLSv;3:LDKII3oSiTXp9h58tWd0kvZqvAKWOomaCIGDXyUHa6EUtYOubedqSyQdBt75+mkXVHsJqZ+UQ9G1hKfeQ1l4c39BS+T7QEqwjhWdi/LTPXm2GGrHjc+o87OUj+N7RJOI X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:;SRVR:AT5PR84MB0306; 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If the HPET ever returns the same value >>> twice in a row (unlikely because it's generally too slow to read, but >>> it's plausible that someone will make a fast HPET some day), then this >>> could deadlock. >> What is the deadlock scenario you are talking about? > A reader loops waiting for the HPET update by looking for the value to > change. If the HPET updater does an update, but the HPET itself hasn't > advanced, it will write the same value as was there before. The reader > will keep looping thinking there was no update. I should have addressed this problem in my new v5 patch. Cheers, Longman