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 3B971C282DA for ; Wed, 17 Apr 2019 18:50:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EF21B20663 for ; Wed, 17 Apr 2019 18:50:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732041AbfDQSuT (ORCPT ); Wed, 17 Apr 2019 14:50:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60164 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727340AbfDQSuT (ORCPT ); Wed, 17 Apr 2019 14:50:19 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4C31D4628B; Wed, 17 Apr 2019 18:50:18 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-47.bos.redhat.com [10.18.17.47]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2FEBE5D707; Wed, 17 Apr 2019 18:50:17 +0000 (UTC) Subject: Re: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner() return owner state 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> <20190413172259.2740-9-longman@redhat.com> <20190417124101.GE4038@hirez.programming.kicks-ass.net> <20190417130016.GU14281@hirez.programming.kicks-ass.net> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFgsZGsBEAC3l/RVYISY3M0SznCZOv8aWc/bsAgif1H8h0WPDrHnwt1jfFTB26EzhRea XQKAJiZbjnTotxXq1JVaWxJcNJL7crruYeFdv7WUJqJzFgHnNM/upZuGsDIJHyqBHWK5X9ZO jRyfqV/i3Ll7VIZobcRLbTfEJgyLTAHn2Ipcpt8mRg2cck2sC9+RMi45Epweu7pKjfrF8JUY r71uif2ThpN8vGpn+FKbERFt4hW2dV/3awVckxxHXNrQYIB3I/G6mUdEZ9yrVrAfLw5M3fVU CRnC6fbroC6/ztD40lyTQWbCqGERVEwHFYYoxrcGa8AzMXN9CN7bleHmKZrGxDFWbg4877zX 0YaLRypme4K0ULbnNVRQcSZ9UalTvAzjpyWnlnXCLnFjzhV7qsjozloLTkZjyHimSc3yllH7 VvP/lGHnqUk7xDymgRHNNn0wWPuOpR97J/r7V1mSMZlni/FVTQTRu87aQRYu3nKhcNJ47TGY evz/U0ltaZEU41t7WGBnC7RlxYtdXziEn5fC8b1JfqiP0OJVQfdIMVIbEw1turVouTovUA39 Qqa6Pd1oYTw+Bdm1tkx7di73qB3x4pJoC8ZRfEmPqSpmu42sijWSBUgYJwsziTW2SBi4hRjU h/Tm0NuU1/R1bgv/EzoXjgOM4ZlSu6Pv7ICpELdWSrvkXJIuIwARAQABzR9Mb25nbWFuIExv bmcgPGxsb25nQHJlZGhhdC5jb20+wsF/BBMBAgApBQJYLGRrAhsjBQkJZgGABwsJCAcDAgEG FQgCCQoLBBYCAwECHgECF4AACgkQbjBXZE7vHeYwBA//ZYxi4I/4KVrqc6oodVfwPnOVxvyY oKZGPXZXAa3swtPGmRFc8kGyIMZpVTqGJYGD9ZDezxpWIkVQDnKM9zw/qGarUVKzElGHcuFN ddtwX64yxDhA+3Og8MTy8+8ZucM4oNsbM9Dx171bFnHjWSka8o6qhK5siBAf9WXcPNogUk4S fMNYKxexcUayv750GK5E8RouG0DrjtIMYVJwu+p3X1bRHHDoieVfE1i380YydPd7mXa7FrRl 7unTlrxUyJSiBc83HgKCdFC8+ggmRVisbs+1clMsK++ehz08dmGlbQD8Fv2VK5KR2+QXYLU0 rRQjXk/gJ8wcMasuUcywnj8dqqO3kIS1EfshrfR/xCNSREcv2fwHvfJjprpoE9tiL1qP7Jrq 4tUYazErOEQJcE8Qm3fioh40w8YrGGYEGNA4do/jaHXm1iB9rShXE2jnmy3ttdAh3M8W2OMK 4B/Rlr+Awr2NlVdvEF7iL70kO+aZeOu20Lq6mx4Kvq/WyjZg8g+vYGCExZ7sd8xpncBSl7b3 99AIyT55HaJjrs5F3Rl8dAklaDyzXviwcxs+gSYvRCr6AMzevmfWbAILN9i1ZkfbnqVdpaag QmWlmPuKzqKhJP+OMYSgYnpd/vu5FBbc+eXpuhydKqtUVOWjtp5hAERNnSpD87i1TilshFQm TFxHDzbOwU0EWCxkawEQALAcdzzKsZbcdSi1kgjfce9AMjyxkkZxcGc6Rhwvt78d66qIFK9D Y9wfcZBpuFY/AcKEqjTo4FZ5LCa7/dXNwOXOdB1Jfp54OFUqiYUJFymFKInHQYlmoES9EJEU yy+2ipzy5yGbLh3ZqAXyZCTmUKBU7oz/waN7ynEP0S0DqdWgJnpEiFjFN4/ovf9uveUnjzB6 lzd0BDckLU4dL7aqe2ROIHyG3zaBMuPo66pN3njEr7IcyAL6aK/IyRrwLXoxLMQW7YQmFPSw drATP3WO0x8UGaXlGMVcaeUBMJlqTyN4Swr2BbqBcEGAMPjFCm6MjAPv68h5hEoB9zvIg+fq M1/Gs4D8H8kUjOEOYtmVQ5RZQschPJle95BzNwE3Y48ZH5zewgU7ByVJKSgJ9HDhwX8Ryuia 79r86qZeFjXOUXZjjWdFDKl5vaiRbNWCpuSG1R1Tm8o/rd2NZ6l8LgcK9UcpWorrPknbE/pm MUeZ2d3ss5G5Vbb0bYVFRtYQiCCfHAQHO6uNtA9IztkuMpMRQDUiDoApHwYUY5Dqasu4ZDJk bZ8lC6qc2NXauOWMDw43z9He7k6LnYm/evcD+0+YebxNsorEiWDgIW8Q/E+h6RMS9kW3Rv1N qd2nFfiC8+p9I/KLcbV33tMhF1+dOgyiL4bcYeR351pnyXBPA66ldNWvABEBAAHCwWUEGAEC AA8FAlgsZGsCGwwFCQlmAYAACgkQbjBXZE7vHeYxSQ/+PnnPrOkKHDHQew8Pq9w2RAOO8gMg 9Ty4L54CsTf21Mqc6GXj6LN3WbQta7CVA0bKeq0+WnmsZ9jkTNh8lJp0/RnZkSUsDT9Tza9r GB0svZnBJMFJgSMfmwa3cBttCh+vqDV3ZIVSG54nPmGfUQMFPlDHccjWIvTvyY3a9SLeamaR jOGye8MQAlAD40fTWK2no6L1b8abGtziTkNh68zfu3wjQkXk4kA4zHroE61PpS3oMD4AyI9L 7A4Zv0Cvs2MhYQ4Qbbmafr+NOhzuunm5CoaRi+762+c508TqgRqH8W1htZCzab0pXHRfywtv 0P+BMT7vN2uMBdhr8c0b/hoGqBTenOmFt71tAyyGcPgI3f7DUxy+cv3GzenWjrvf3uFpxYx4 yFQkUcu06wa61nCdxXU/BWFItryAGGdh2fFXnIYP8NZfdA+zmpymJXDQeMsAEHS0BLTVQ3+M 7W5Ak8p9V+bFMtteBgoM23bskH6mgOAw6Cj/USW4cAJ8b++9zE0/4Bv4iaY5bcsL+h7TqQBH Lk1eByJeVooUa/mqa2UdVJalc8B9NrAnLiyRsg72Nurwzvknv7anSgIkL+doXDaG21DgCYTD wGA5uquIgb8p3/ENgYpDPrsZ72CxVC2NEJjJwwnRBStjJOGQX4lV1uhN1XsZjBbRHdKF2W9g weim8xU= Organization: Red Hat Message-ID: Date: Wed, 17 Apr 2019 14:50:16 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20190417130016.GU14281@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 17 Apr 2019 18:50:18 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/17/2019 09:00 AM, Peter Zijlstra wrote: > On Wed, Apr 17, 2019 at 02:41:01PM +0200, Peter Zijlstra wrote: >> On Sat, Apr 13, 2019 at 01:22:51PM -0400, Waiman Long wrote: >>> In the special case that there is no active lock and the handoff bit >>> is set, optimistic spinning has to be stopped. >>> @@ -500,9 +521,19 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) >>> >>> /* >>> * If there is a new owner or the owner is not set, we continue >>> - * spinning. >>> + * spinning except when here is no active locks and the handoff bit >>> + * is set. In this case, we have to stop spinning. >>> */ >>> - return is_rwsem_owner_spinnable(READ_ONCE(sem->owner)); >>> + owner = READ_ONCE(sem->owner); >>> + if (!is_rwsem_owner_spinnable(owner)) >>> + return OWNER_NONSPINNABLE; >>> + if (owner && !is_rwsem_owner_reader(owner)) >>> + return OWNER_WRITER; >>> + >>> + count = atomic_long_read(&sem->count); >>> + if (RWSEM_COUNT_HANDOFF(count) && !RWSEM_COUNT_LOCKED(count)) >>> + return OWNER_NONSPINNABLE; >>> + return !owner ? OWNER_NULL : OWNER_READER; >>> } >> So this fixes a straight up bug in the last patch (and thus should be >> done before so the bug never exists), and creates unreadable code while >> at it. >> >> Also, I think only checking HANDOFF after the loop is wrong; the moment >> HANDOFF happens you have to terminate the loop, irrespective of what >> @owner does. >> >> Does something like so work? >> >> --- > enum owner_state { > OWNER_NULL = 1 << 0, > OWNER_WRITER = 1 << 1, > OWNER_READER = 1 << 2, > OWNER_NONSPINNABLE = 1 << 3, > }; > #define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER) > > static inline enum owner_state rwsem_owner_state(unsigned long owner) > { > if (!owner) > return OWNER_NULL; > > if (owner & RWSEM_ANONYMOUSLY_OWNED) > return OWNER_NONSPINNABLE; > > if (owner & RWSEM_READER_OWNER) > return OWNER_READER; > > return OWNER_WRITER; > } > > static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem) > { > struct task_struct *tmp, *owner = READ_ONCE(sem->owner); > enum owner_state state = rwsem_owner_state((unsigned long)owner); > > if (state != OWNER_WRITER) > return state; > > rcu_read_lock(); > for (;;) { > if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) { > state = OWNER_NONSPINNABLE; > break; > } > > tmp = READ_ONCE(sem->owner); > if (tmp != owner) { > state = rwsem_owner_state((unsigned long)tmp); > break; > } > > /* > * Ensure we emit the owner->on_cpu, dereference _after_ > * checking sem->owner still matches owner, if that fails, > * owner might point to free()d memory, if it still matches, > * the rcu_read_lock() ensures the memory stays valid. > */ > barrier(); > > if (need_resched() || !owner_on_cpu(owner)) { > state = OWNER_NONSPINNABLE; > break; > } > > cpu_relax(); > } > rcu_read_unlock(); > > return state; > } That code looks good to me. Thanks for the rewrite. Cheers, Longman