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 9928EC10F11 for ; Wed, 24 Apr 2019 16:49:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 64231218FE for ; Wed, 24 Apr 2019 16:49:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732756AbfDXQtI (ORCPT ); Wed, 24 Apr 2019 12:49:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55106 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732574AbfDXQtG (ORCPT ); Wed, 24 Apr 2019 12:49:06 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 89640307D861; Wed, 24 Apr 2019 16:49:06 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8FAD660C9C; Wed, 24 Apr 2019 16:49:05 +0000 (UTC) Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative To: Peter Zijlstra Cc: Linus Torvalds , Ingo Molnar , Will Deacon , Thomas Gleixner , Linux List Kernel Mailing , the arch/x86 maintainers , Davidlohr Bueso , Tim Chen , huang ying References: <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> <57620139-92a3-4a21-56bd-5d6fff23214f@redhat.com> <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com> <20190423141714.GO11158@hirez.programming.kicks-ass.net> <4f62d7f2-e5f6-500e-3e70-b1d1978f7140@redhat.com> <20190424070959.GE4038@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <51589ac0-3e1f-040e-02bf-b6de77cbda1d@redhat.com> Date: Wed, 24 Apr 2019 12:49:05 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190424070959.GE4038@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 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Wed, 24 Apr 2019 16:49:06 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/24/19 3:09 AM, Peter Zijlstra wrote: > On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote: >> That is true in general, but doing preempt_disable/enable across >> function boundary is ugly and prone to further problems down the road. > We do worse things in this code, and the thing Linus proposes is > actually quite simple, something like so: > > --- > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -912,7 +904,7 @@ rwsem_down_read_slowpath(struct rw_semap > raw_spin_unlock_irq(&sem->wait_lock); > break; > } > - schedule(); > + schedule_preempt_disabled(); > lockevent_inc(rwsem_sleep_reader); > } > > @@ -1121,6 +1113,7 @@ static struct rw_semaphore *rwsem_downgr > */ > inline void __down_read(struct rw_semaphore *sem) > { > + preempt_disable(); > if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, > &sem->count) & RWSEM_READ_FAILED_MASK)) { > rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE); > @@ -1129,10 +1122,12 @@ inline void __down_read(struct rw_semaph > } else { > rwsem_set_reader_owned(sem); > } > + preempt_enable(); > } > > static inline int __down_read_killable(struct rw_semaphore *sem) > { > + preempt_disable(); > if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, > &sem->count) & RWSEM_READ_FAILED_MASK)) { > if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE))) > @@ -1142,6 +1137,7 @@ static inline int __down_read_killable(s > } else { > rwsem_set_reader_owned(sem); > } > + preempt_enable(); > return 0; > } > Making that change will help the slowpath to has less preemption points. For an uncontended rwsem, this offers no real benefit. Adding preempt_disable() is more complicated than I originally thought. Maybe we are too paranoid about the possibility of a large number of preemptions happening just at the right moment. If p is the probably of a preemption in the middle of the inc-check-dec sequence, which I have already moved as close to each other as possible. We are talking a probability of p^32768. Since p will be really small, the compound probability will be infinitesimally small. So I would like to not do preemption now for the current patchset. We can restart the discussion later on if there is a real concern that it may actually happen. Please let me know if you still want to add preempt_disable() for the read lock. Cheers, Longman