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 E7CF8C433E2 for ; Thu, 3 Sep 2020 23:59:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B1B02206C0 for ; Thu, 3 Sep 2020 23:59:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729486AbgICX7H (ORCPT ); Thu, 3 Sep 2020 19:59:07 -0400 Received: from relay9-d.mail.gandi.net ([217.70.183.199]:37095 "EHLO relay9-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725782AbgICX7G (ORCPT ); Thu, 3 Sep 2020 19:59:06 -0400 X-Originating-IP: 50.39.163.217 Received: from localhost (50-39-163-217.bvtn.or.frontiernet.net [50.39.163.217]) (Authenticated sender: josh@joshtriplett.org) by relay9-d.mail.gandi.net (Postfix) with ESMTPSA id 9458CFF802; Thu, 3 Sep 2020 23:58:58 +0000 (UTC) Date: Thu, 3 Sep 2020 16:58:55 -0700 From: Josh Triplett To: Christian Brauner 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: <20200903235855.GD210207@localhost> References: <20200902102130.147672-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200902102130.147672-1-christian.brauner@ubuntu.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 - Josh Triplett