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 CFC5DC282DD for ; Tue, 23 Apr 2019 19:12:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA15E218B0 for ; Tue, 23 Apr 2019 19:12:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726336AbfDWTMY (ORCPT ); Tue, 23 Apr 2019 15:12:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60590 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725957AbfDWTMY (ORCPT ); Tue, 23 Apr 2019 15:12:24 -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 CE13EC02E60C; Tue, 23 Apr 2019 19:12:23 +0000 (UTC) Received: from llong.remote.csb (ovpn-123-167.rdu2.redhat.com [10.10.123.167]) by smtp.corp.redhat.com (Postfix) with ESMTP id 836355D705; Tue, 23 Apr 2019 19:12:22 +0000 (UTC) Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative To: Linus Torvalds , Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , Linux List Kernel Mailing , the arch/x86 maintainers , Davidlohr Bueso , Tim Chen , huang ying References: <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> <57620139-92a3-4a21-56bd-5d6fff23214f@redhat.com> <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com> <20190423141714.GO11158@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <4f62d7f2-e5f6-500e-3e70-b1d1978f7140@redhat.com> Date: Tue, 23 Apr 2019 15:12:16 -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: 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 [10.5.110.32]); Tue, 23 Apr 2019 19:12:23 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/23/19 12:27 PM, Linus Torvalds wrote: > On Tue, Apr 23, 2019 at 7:17 AM Peter Zijlstra wrote: >> I'm not aware of an architecture where disabling interrupts is faster >> than disabling preemption. > I don't thin kit ever is, but I'd worry a bit about the > preempt_enable() just because it also checks if need_resched() is true > when re-enabling preemption. > > So doing preempt_enable() as part of rwsem_read_trylock() might cause > us to schedule in *exactly* the wrong place, You are right on that. However, there is a variant called preempt_enable_no_resched() that doesn't have this side effect. So I am going to use that one instead. > So if we play preemption games, I wonder if we should make them more > explicit than hiding them in that helper function, because > particularly for the slow path case, I think we'd be much better off > just avoiding the busy-loop in the slow path, rather than first > scheduling due to preempt_enable(), and then starting to look at the > slow path onlly afterwards. > > IOW, I get the feeling that the preemption-off area might be better > off being potentially much bigger, and covering the whole (or a large > portion) of the semaphore operation, rather than just the > rwsem_read_trylock() fastpath. > > Hmm? That is true in general, but doing preempt_disable/enable across function boundary is ugly and prone to further problems down the road. Cheers, Longman > Linus