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 EFFD1C43461 for ; Fri, 4 Sep 2020 10:29:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CED7920791 for ; Fri, 4 Sep 2020 10:29:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730139AbgIDK3Q (ORCPT ); Fri, 4 Sep 2020 06:29:16 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:35547 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726171AbgIDK3O (ORCPT ); Fri, 4 Sep 2020 06:29:14 -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 1kE8xv-0001oE-SE; Fri, 04 Sep 2020 10:29:07 +0000 Date: Fri, 4 Sep 2020 12:29:07 +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, Jann Horn Subject: Re: [PATCH v2 2/4] exit: support non-blocking pidfds Message-ID: <20200904102907.2t6w6k7ud5i6ak2o@wittgenstein> References: <20200902102130.147672-1-christian.brauner@ubuntu.com> <20200902102130.147672-3-christian.brauner@ubuntu.com> <20200903235659.GC210207@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200903235659.GC210207@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 04:56:59PM -0700, Josh Triplett wrote: > 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 I think that WNOHANG compatibility change might be a good idea. So I've changed this to: ret = do_wait(&wo); if (!ret && !(options & WNOHANG) && (f_flags & O_NONBLOCK)) ret = -EAGAIN; > > 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. Good point! I've changed the order now. Thanks! Christian