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=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 80E95C43461 for ; Fri, 4 Sep 2020 10:37:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 60E28206D4 for ; Fri, 4 Sep 2020 10:37:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730174AbgIDKhf (ORCPT ); Fri, 4 Sep 2020 06:37:35 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:35595 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730155AbgIDKaW (ORCPT ); Fri, 4 Sep 2020 06:30:22 -0400 Received: from ip5f5af70b.dynamic.kabel-deutschland.de ([95.90.247.11] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1kE8z6-0001vn-BT; Fri, 04 Sep 2020 10:30:20 +0000 Date: Fri, 4 Sep 2020 12:30:19 +0200 From: Christian Brauner To: Josh Triplett Cc: linux-kernel@vger.kernel.org, Christian Brauner , "Peter Zijlstra (Intel)" , Ingo Molnar , Thomas Gleixner , Oleg Nesterov , "Eric W. Biederman" , Kees Cook , Sargun Dhillon , Aleksa Sarai , linux-kselftest@vger.kernel.org, Jens Axboe , linux-api@vger.kernel.org Subject: Re: [PATCH v2 0/4] Support non-blocking pidfds Message-ID: <20200904103019.6tmbfuzzmj7r5vup@wittgenstein> References: <20200902102130.147672-1-christian.brauner@ubuntu.com> <20200903235855.GD210207@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200903235855.GD210207@localhost> Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Thu, Sep 03, 2020 at 04:58:55PM -0700, Josh Triplett wrote: > On Wed, Sep 02, 2020 at 12:21:26PM +0200, Christian Brauner wrote: > > Hi, > > > > Passing a non-blocking pidfd to waitid() currently has no effect, i.e. > > is not supported. There are users which would like to use waitid() on > > pidfds that are O_NONBLOCK and mix it with pidfds that are blocking and > > both pass them to waitid(). > > The expected behavior is to have waitid() return -EAGAIN for > > non-blocking pidfds and to block for blocking pidfds without needing to > > perform any additional checks for flags set on the pidfd before passing > > it to waitid(). > > Non-blocking pidfds will return EAGAIN from waitid() when no child > > process is ready yet. Returning -EAGAIN for non-blocking pidfds makes it > > easier for event loops that handle EAGAIN specially. > > > > It also makes the API more consistent and uniform. In essence, waitid() > > is treated like a read on a non-blocking pidfd or a recvmsg() on a > > non-blocking socket. > > With the addition of support for non-blocking pidfds we support the same > > functionality that sockets do. For sockets() recvmsg() supports > > MSG_DONTWAIT for pidfds waitid() supports WNOHANG. Both flags are > > per-call options. In contrast non-blocking pidfds and non-blocking > > sockets are a setting on an open file description affecting all threads > > in the calling process as well as other processes that hold file > > descriptors referring to the same open file description. Both behaviors, > > per call and per open file description, have genuine use-cases. > > > > A concrete use-case that was brought on-list (see [1]) was Josh's async > > pidfd library. Ever since the introduction of pidfds and more advanced > > async io various programming languages such as Rust have grown support > > for async event libraries. These libraries are created to help build > > epoll-based event loops around file descriptors. A common pattern is to > > automatically make all file descriptors they manage to O_NONBLOCK. > > > > For such libraries the EAGAIN error code is treated specially. When a > > function is called that returns EAGAIN the function isn't called again > > until the event loop indicates the the file descriptor is ready. > > Supporting EAGAIN when waiting on pidfds makes such libraries just work > > with little effort. > > Thanks for the patch series, Christian! > > This will make it much easier to use pidfd in non-blocking event loops. > > Reviewed-by: Josh Triplett Thank you and thanks for your input on a bunch of other stuff as well. :) Christian