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=-12.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 64F6FC43461 for ; Thu, 3 Sep 2020 23:57:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 265D32083B for ; Thu, 3 Sep 2020 23:57:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728271AbgICX5O (ORCPT ); Thu, 3 Sep 2020 19:57:14 -0400 Received: from relay8-d.mail.gandi.net ([217.70.183.201]:36013 "EHLO relay8-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725782AbgICX5O (ORCPT ); Thu, 3 Sep 2020 19:57:14 -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 relay8-d.mail.gandi.net (Postfix) with ESMTPSA id B11B61BF203; Thu, 3 Sep 2020 23:57:03 +0000 (UTC) Date: Thu, 3 Sep 2020 16:56:59 -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, Jann Horn Subject: Re: [PATCH v2 2/4] exit: support non-blocking pidfds Message-ID: <20200903235659.GC210207@localhost> References: <20200902102130.147672-1-christian.brauner@ubuntu.com> <20200902102130.147672-3-christian.brauner@ubuntu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200902102130.147672-3-christian.brauner@ubuntu.com> Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Wed, Sep 02, 2020 at 12:21:28PM +0200, Christian Brauner wrote: > 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. > > The implementation should be straightforward, we simply raise the WNOHANG flag > when a non-blocking pidfd is passed and when do_wait() returns without finding > an eligible task and the pidfd is non-blocking we set EAGAIN. If no child > process exists non-blocking pidfd users will continue to see ECHILD but if > child processes exist but have not yet exited users will see EAGAIN. > > A concrete use-case that was brought on-list 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. > > Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/ > Link: https://github.com/joshtriplett/async-pidfd > Cc: Kees Cook > Cc: Sargun Dhillon > Cc: Jann Horn > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: Oleg Nesterov > Cc: "Peter Zijlstra (Intel)" > Suggested-by: Josh Triplett > Signed-off-by: Christian Brauner With or without the discussed change to WNOHANG behavior for compatibility: Reviewed-by: Josh Triplett Also, I think you should flip the order of patches 1 and 2, so that there isn't a one-patch window in kernel history where you can create an O_NONBLOCK pidfd with pidfd_open but it has no effect. I'd expect userspace to use pidfd_open accepting or EINVAL-ing the flag as an indication of whether it'll work.