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=-1.0 required=3.0 tests=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 441EEC282DA for ; Fri, 19 Apr 2019 19:39:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1293C2171F for ; Fri, 19 Apr 2019 19:39:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727995AbfDSTjT convert rfc822-to-8bit (ORCPT ); Fri, 19 Apr 2019 15:39:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47392 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726134AbfDSTjS (ORCPT ); Fri, 19 Apr 2019 15:39:18 -0400 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 2857E3082A24; Fri, 19 Apr 2019 19:39:18 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-19.bos.redhat.com [10.18.17.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id E12AA5C219; Fri, 19 Apr 2019 19:39:16 +0000 (UTC) Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190413172259.2740-1-longman@redhat.com> <20190413172259.2740-15-longman@redhat.com> <20190418135151.GB12232@hirez.programming.kicks-ass.net> <20190418144036.GE12232@hirez.programming.kicks-ass.net> <4cbd3c18-c9c0-56eb-4e01-ee355a69057a@redhat.com> <20190419102647.GP7905@worktop.programming.kicks-ass.net> <20190419120207.GO4038@hirez.programming.kicks-ass.net> <20190419130304.GV14281@hirez.programming.kicks-ass.net> <20190419131522.GW14281@hirez.programming.kicks-ass.net> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20190419131522.GW14281@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US 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 [10.5.110.45]); Fri, 19 Apr 2019 19:39:18 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/19/2019 09:15 AM, Peter Zijlstra wrote: > On Fri, Apr 19, 2019 at 03:03:04PM +0200, Peter Zijlstra wrote: >> On Fri, Apr 19, 2019 at 02:02:07PM +0200, Peter Zijlstra wrote: >>> On Fri, Apr 19, 2019 at 12:26:47PM +0200, Peter Zijlstra wrote: >>>> I thought of a horrible horrible alternative: >>> Hurm, that's broken as heck. Let me try again. >> So I can't make that scheme work, it all ends up wanting to have >> cmpxchg(). >> >> Do we have a performance comparison somewhere of xadd vs cmpxchg >> readers? I tried looking in the old threads, but I can't seem to locate >> it. >> >> We need new instructions :/ Or more clever than I can muster just now. > In particular, an (unsigned) saturation arithmetic variant of XADD would > be very nice to have at this point. I just want to clear about my current scheme. There will be 16 bits allocated for reader count. I use the MS bit for signaling that there are too many readers. So the fast path will fail and the readers will be put into the wait list. This effectively limit readers to 32k-1, but it doesn't mean the actual reader count cannot go over that. As long as the actual count is less than 64k, everything should still work perfectly. IOW, even though we have reached the limit of 32k, we need to pile on an additional 32k readers to really overflow the count and cause problem. Cheers, Longman