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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 BF882C4332D for ; Thu, 19 Mar 2020 00:45:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9A38120767 for ; Thu, 19 Mar 2020 00:45:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726796AbgCSApM (ORCPT ); Wed, 18 Mar 2020 20:45:12 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:59198 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726663AbgCSApM (ORCPT ); Wed, 18 Mar 2020 20:45:12 -0400 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jEjIR-0005E3-Fd; Thu, 19 Mar 2020 01:44:27 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id CFC0A103088; Thu, 19 Mar 2020 01:44:26 +0100 (CET) From: Thomas Gleixner To: Joel Fernandes Cc: LKML , Peter Zijlstra , Linus Torvalds , Ingo Molnar , Will Deacon , "Paul E . McKenney" , Steven Rostedt , Randy Dunlap , Arnd Bergmann , Sebastian Andrzej Siewior , Logan Gunthorpe , Kurt Schwemmer , Bjorn Helgaas , linux-pci@vger.kernel.org, Felipe Balbi , Greg Kroah-Hartman , linux-usb@vger.kernel.org, Kalle Valo , "David S. Miller" , linux-wireless@vger.kernel.org, netdev@vger.kernel.org, Oleg Nesterov , Davidlohr Bueso , Michael Ellerman , linuxppc-dev@lists.ozlabs.org Subject: Re: [patch V2 11/15] completion: Use simple wait queues In-Reply-To: <20200319003351.GA211584@google.com> References: <20200318204302.693307984@linutronix.de> <20200318204408.521507446@linutronix.de> <20200319003351.GA211584@google.com> Date: Thu, 19 Mar 2020 01:44:26 +0100 Message-ID: <87a74ddvh1.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org Joel, Joel Fernandes writes: > On Wed, Mar 18, 2020 at 09:43:13PM +0100, Thomas Gleixner wrote: >> The spinlock in the wait queue head cannot be replaced by a raw_spinlock >> because: >> >> - wait queues can have custom wakeup callbacks, which acquire other >> spinlock_t locks and have potentially long execution times > > Cool, makes sense. > >> - wake_up() walks an unbounded number of list entries during the wake up >> and may wake an unbounded number of waiters. > > Just to clarify here, wake_up() will really wake up just 1 waiter if all the > waiters on the queue are exclusive right? So in such scenario at least, the > "unbounded number of waiters" would not be an issue if everything waiting was > exclusive and waitqueue with wake_up() was used. Please correct me if I'm > wrong about that though. Correct. > So the main reasons to avoid waitqueue in favor of swait (as you mentioned) > would be the sleep-while-atomic issue in truly atomic context on RT, and the > fact that callbacks can take a long time. Yes. Thanks, tglx