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 BBBC3C18E5B for ; Mon, 16 Mar 2020 19:34:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9320920674 for ; Mon, 16 Mar 2020 19:34:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732455AbgCPTet (ORCPT ); Mon, 16 Mar 2020 15:34:49 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:52812 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732366AbgCPTet (ORCPT ); Mon, 16 Mar 2020 15:34:49 -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 1jDvVT-0004MQ-5x; Mon, 16 Mar 2020 20:34:35 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 84ABC1013B2; Mon, 16 Mar 2020 20:34:34 +0100 (CET) From: Thomas Gleixner To: Logan Gunthorpe , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , "Paul E . McKenney" , Joel Fernandes , Steven Rostedt , Linus Torvalds , Kurt Schwemmer , Bjorn Helgaas , linux-pci@vger.kernel.org Subject: Re: [PATCH 3/9] pci/switchtec: Don't abuse completion wait queue for poll In-Reply-To: <4d3a997d-ced4-3dbe-d766-0b1e9fc35b29@deltatee.com> References: <20200313174701.148376-1-bigeasy@linutronix.de> <20200313174701.148376-4-bigeasy@linutronix.de> <4d3a997d-ced4-3dbe-d766-0b1e9fc35b29@deltatee.com> Date: Mon, 16 Mar 2020 20:34:34 +0100 Message-ID: <87v9n4ccvp.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-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Logan Gunthorpe writes: > On 2020-03-13 11:46 a.m., Sebastian Andrzej Siewior wrote: >> 1) It cannot work with EPOLLEXCLUSIVE > > Why? You don't explain this. man epoll_ctt(2) EPOLLEXCLUSIVE (since Linux 4.5) Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor, fd. When a wakeup event occurs and multiple epoll file descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or more of the epoll file descriptors will receive an event with epoll_wait(2). As this uses complete_all() there is no distinction possible, because complete_all() wakes up everything. > And I don't see how this patch would change anything to do with the > call to poll_wait(). All you've done is open-code the completion. wake_up_interruptible(x) resolves to: __wake_up(x, TASK_INTERRUPTIBLE, 1, NULL) which wakes exactly 1 exclusive waiter. Also the other way round is just working because the waker side uses complete_all(). Why? Because completion internally defaults to exclusive mode and complete() wakes exactly one exlusive waiter. There is a conceptual difference and while it works for that particular purpose to some extent it's not suitable as a general wait notification construct. Thanks, tglx