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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 56F19C3403F for ; Tue, 18 Feb 2020 14:56:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 300AB2176D for ; Tue, 18 Feb 2020 14:56:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="FkOGCI4l" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726795AbgBRO4z (ORCPT ); Tue, 18 Feb 2020 09:56:55 -0500 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:53874 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726540AbgBRO4y (ORCPT ); Tue, 18 Feb 2020 09:56:54 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1582037813; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Q31XUITBYUDgelay51ZGFeoRFTX+LU6SfnNcMWjnStI=; b=FkOGCI4lYVqt7Fdlth0mvz15Sirk/ofYfjSXlD3sWzKb1wCla7tUA887/SNNlG+L9BSeI9 hu4rjvrRIExnekbIkk4E3vy+8sca5Cvw3Ph7p6kQlW4BHAhnyCA0dVmv3ZGFqPeB12L7sY 1d3m81uS7H9EZgTC+T+olddm6PZ+FRU= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-316-ljhHeChzO8uqm4XMH2e9Ug-1; Tue, 18 Feb 2020 09:56:49 -0500 X-MC-Unique: ljhHeChzO8uqm4XMH2e9Ug-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8CFF51005516; Tue, 18 Feb 2020 14:56:48 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.70]) by smtp.corp.redhat.com (Postfix) with SMTP id 2597E60BE1; Tue, 18 Feb 2020 14:56:46 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 18 Feb 2020 15:56:48 +0100 (CET) Date: Tue, 18 Feb 2020 15:56:46 +0100 From: Oleg Nesterov To: Peter Zijlstra Cc: Jens Axboe , Carter Li =?utf-8?B?5p2O6YCa5rSy?= , Pavel Begunkov , io-uring Subject: Re: [ISSUE] The time cost of IOSQE_IO_LINK Message-ID: <20200218145645.GB3466@redhat.com> References: <5995f84e-8a6c-e774-6bb5-5b9b87a9cd3c@kernel.dk> <7c4c3996-4886-eb58-cdee-fe0951907ab5@kernel.dk> <20200217120920.GQ14914@hirez.programming.kicks-ass.net> <53de3581-b902-89ba-3f53-fd46b052df40@kernel.dk> <43c066d1-a892-6a02-82e7-7be850d9454d@kernel.dk> <20200217174610.GU14897@hirez.programming.kicks-ass.net> <592cf069-41ee-0bc1-1f83-e058e5dd53ff@kernel.dk> <20200218131310.GZ14914@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218131310.GZ14914@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 02/18, Peter Zijlstra wrote: > > But this has me wondering about task_work_run(), as it is it will > unconditionally take pi_lock, because spin_unlock_wait() was removed ;) task_work_run() doesn't really need to take pi_lock at all. > would not something like this make sense? I think yes, but see below. > --- a/kernel/task_work.c > +++ b/kernel/task_work.c > @@ -93,16 +93,20 @@ void task_work_run(void) > struct callback_head *work, *head, *next; > > for (;;) { > + work = READ_ONCE(task->task_work); > + if (!work) > + break This is wrong if PF_EXITING is set, in this case we must set task->task_works = work_exited. > + > /* > * work->func() can do task_work_add(), do not set > * work_exited unless the list is empty. > */ > raw_spin_lock_irq(&task->pi_lock); > do { > - work = READ_ONCE(task->task_works); > - head = !work && (task->flags & PF_EXITING) ? > - &work_exited : NULL; > - } while (cmpxchg(&task->task_works, work, head) != work); > + head = NULL; > + if (unlikely(!work && (task->flags & PF_EXITING))) > + head = &work_exited; > + } while (!try_cmpxchg(&task->task_works, &work, head)); > raw_spin_unlock_irq(&task->pi_lock); > > if (!work) otherwise I think this is correct, but how about the patch below? Then this code can be changed to use try_cmpxchg(). Oleg. --- a/kernel/task_work.c +++ b/kernel/task_work.c @@ -97,17 +97,24 @@ void task_work_run(void) * work->func() can do task_work_add(), do not set * work_exited unless the list is empty. */ - raw_spin_lock_irq(&task->pi_lock); do { + head = NULL; work = READ_ONCE(task->task_works); - head = !work && (task->flags & PF_EXITING) ? - &work_exited : NULL; + if (!work) { + if (task->flags & PF_EXITING) + head = &work_exited; + else + break; + } } while (cmpxchg(&task->task_works, work, head) != work); - raw_spin_unlock_irq(&task->pi_lock); if (!work) break; + // Synchronize with task_work_cancel() + raw_spin_lock_irq(&task->pi_lock); + raw_spin_unlock_irq(&task->pi_lock); + do { next = work->next; work->func(work);