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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,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 55C72C282E0 for ; Fri, 19 Apr 2019 20:57:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 112C02171F for ; Fri, 19 Apr 2019 20:57:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=brauner.io header.i=@brauner.io header.b="VG4YzUHD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726960AbfDSU5Z (ORCPT ); Fri, 19 Apr 2019 16:57:25 -0400 Received: from mail-lj1-f196.google.com ([209.85.208.196]:44743 "EHLO mail-lj1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726120AbfDSU5Z (ORCPT ); Fri, 19 Apr 2019 16:57:25 -0400 Received: by mail-lj1-f196.google.com with SMTP id h16so5532690ljg.11 for ; Fri, 19 Apr 2019 13:57:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=brauner.io; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Mc8m+moEJITkvv7tcKWJ/u36imYJnwxGy3M0tYcmGgc=; b=VG4YzUHDQUjJy4KWZ0IAXKLszdM8zM5abCV++aJeoxlGp3DWX2Og9XU3vgvJtsy4ua gwGrew7zv91gmUoYmvGUB7yRiL8k8dm1afksgP18dtzWVfM1SZJ5B+iRMGRO5gHnDcev 5E6dQOSRBR71Zy9eP+ivXyiaI18lC20sdIoC1MD7Auzcww0w2vHCwgGDsLPshUWBpFcK ZGxgqb5KEdHvHaPeFH/w+JKw6GMwtQML4E/ysKYGj8efG35cafNGGlaRDNsFvEDOhio9 G0qWMShEy6MJK0KOrCNHUEtrhYwIT11RLqXgGanOGVxOs/Vom558NIWdQiCJlu8NJlwt u1Jg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Mc8m+moEJITkvv7tcKWJ/u36imYJnwxGy3M0tYcmGgc=; b=IUKgmivIA0mJ6IxMjBMgjQvLTj6HQS2BQfWknrh1M9yjjeLO2PT+1NYJloEbGIbHRq l93jmO2TF73RFYIyhcTq79hWsLaXtQVOWGycdsdf4dRfgYe/5G27ZNeT/oskmUTEUuQq UKPpqsvNOdx+pHzkrCW6X4zHRLxLbUC6oXujrRI6EshrWCbj5EuoTjZ0xrP+Q5pq5n0A NFNfxd2A3DVvHHgpUWdgqZQVd3Jk4+oXuITmy3gEU7CvX+wkeWDUE0oXr+Qdql+tK6PB rGL2SqWuZ54cHZvnFr8RWigkUcGcft8drsBgh5gq0/pazIannEbJu18B4YldjcdptRFj v5vQ== X-Gm-Message-State: APjAAAVJRasGrl8HxQ8Drz5WbbEpfyb5Fjs4cfl6SRyq3qAHsdkVSvXL LPgjPyEnakvHbexB9CG709UsOrdvki/7/jnRFnzRZw== X-Google-Smtp-Source: APXvYqxO0HDBEvOnE5QlJr7xKgsh+21XyLDRcGR6nWVwXE5w2UlDGWuISvOhhpj5K4c0tmQyL7rMftCj3gKOlgrSVF8= X-Received: by 2002:a2e:8550:: with SMTP id u16mr3326389ljj.11.1555707442235; Fri, 19 Apr 2019 13:57:22 -0700 (PDT) MIME-Version: 1.0 References: <20190411175043.31207-1-joel@joelfernandes.org> <20190416120430.GA15437@redhat.com> <20190416192051.GA184889@google.com> <20190417130940.GC32622@redhat.com> <20190419190247.GB251571@google.com> <20190419191858.iwcvqm6fihbkaata@brauner.io> <20190419194902.GE251571@google.com> In-Reply-To: From: Christian Brauner Date: Fri, 19 Apr 2019 22:57:11 +0200 Message-ID: Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd To: Daniel Colascione Cc: Joel Fernandes , Jann Horn , Oleg Nesterov , Florian Weimer , kernel list , Andy Lutomirski , Steven Rostedt , Suren Baghdasaryan , Linus Torvalds , Alexey Dobriyan , Al Viro , Andrei Vagin , Andrew Morton , Arnd Bergmann , "Eric W. Biederman" , Kees Cook , linux-fsdevel , "open list:KERNEL SELFTEST FRAMEWORK" , Michal Hocko , Nadav Amit , Serge Hallyn , Shuah Khan , Stephen Rothwell , Taehee Yoo , Tejun Heo , Thomas Gleixner , kernel-team , Tycho Andersen Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Apr 19, 2019 at 10:34 PM Daniel Colascione wrote: > > On Fri, Apr 19, 2019 at 12:49 PM Joel Fernandes wrote: > > > > On Fri, Apr 19, 2019 at 09:18:59PM +0200, Christian Brauner wrote: > > > On Fri, Apr 19, 2019 at 03:02:47PM -0400, Joel Fernandes wrote: > > > > On Thu, Apr 18, 2019 at 07:26:44PM +0200, Christian Brauner wrote: > > > > > On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn wrote: > > > > > >On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov wrote: > > > > > >> On 04/16, Joel Fernandes wrote: > > > > > >> > On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote: > > > > > >> > > > > > > > >> > > Could you explain when it should return POLLIN? When the whole > > > > > >process exits? > > > > > >> > > > > > > >> > It returns POLLIN when the task is dead or doesn't exist anymore, > > > > > >or when it > > > > > >> > is in a zombie state and there's no other thread in the thread > > > > > >group. > > > > > >> > > > > > >> IOW, when the whole thread group exits, so it can't be used to > > > > > >monitor sub-threads. > > > > > >> > > > > > >> just in case... speaking of this patch it doesn't modify > > > > > >proc_tid_base_operations, > > > > > >> so you can't poll("/proc/sub-thread-tid") anyway, but iiuc you are > > > > > >going to use > > > > > >> the anonymous file returned by CLONE_PIDFD ? > > > > > > > > > > > >I don't think procfs works that way. /proc/sub-thread-tid has > > > > > >proc_tgid_base_operations despite not being a thread group leader. > > > > > >(Yes, that's kinda weird.) AFAICS the WARN_ON_ONCE() in this code can > > > > > >be hit trivially, and then the code will misbehave. > > > > > > > > > > > >@Joel: I think you'll have to either rewrite this to explicitly bail > > > > > >out if you're dealing with a thread group leader, or make the code > > > > > >work for threads, too. > > > > > > > > > > The latter case probably being preferred if this API is supposed to be > > > > > useable for thread management in userspace. > > > > > > > > At the moment, we are not planning to use this for sub-thread management. I > > > > am reworking this patch to only work on clone(2) pidfds which makes the above > > > > > > Indeed and agreed. > > > > > > > discussion about /proc a bit unnecessary I think. Per the latest CLONE_PIDFD > > > > patches, CLONE_THREAD with pidfd is not supported. > > > > > > Yes. We have no one asking for it right now and we can easily add this > > > later. > > > > > > Admittedly I haven't gotten around to reviewing the patches here yet > > > completely. But one thing about using POLLIN. FreeBSD is using POLLHUP > > > on process exit which I think is nice as well. How about returning > > > POLLIN | POLLHUP on process exit? > > > We already do things like this. For example, when you proxy between > > > ttys. If the process that you're reading data from has exited and closed > > > it's end you still can't usually simply exit because it might have still > > > buffered data that you want to read. The way one can deal with this > > > from userspace is that you can observe a (POLLHUP | POLLIN) event and > > > you keep on reading until you only observe a POLLHUP without a POLLIN > > > event at which point you know you have read > > > all data. > > > I like the semantics for pidfds as well as it would indicate: > > > - POLLHUP -> process has exited > > > - POLLIN -> information can be read > > > > Actually I think a bit different about this, in my opinion the pidfd should > > always be readable (we would store the exit status somewhere in the future > > which would be readable, even after task_struct is dead). So I was thinking > > we always return EPOLLIN. If process has not exited, then it blocks. > > ITYM that a pidfd polls as readable *once a task exits* and stays > readable forever. Before a task exit, a poll on a pidfd should *not* > yield POLLIN and reading that pidfd should *not* complete immediately. > There's no way that, having observed POLLIN on a pidfd, you should > ever then *not* see POLLIN on that pidfd in the future --- it's a > one-way transition from not-ready-to-get-exit-status to > ready-to-get-exit-status. What do you consider interesting state transitions? A listener on a pidfd in epoll_wait() might be interested if the process execs for example. That's a very valid use-case for e.g. systemd. We can't use EPOLLIN for that too otherwise you'd need to to waitid(_WNOHANG) to check whether an exit status can be read which is not nice and then you multiplex different meanings on the same bit. I would prefer if the exit status can only be read from the parent which is clean and the least complicated semantics, i.e. Linus waitid() idea. EPOLLIN on a pidfd could very well mean that data can be read via a read() on the pidfd *other* than the exit status. The read could e.g. give you a lean struct that indicates the type of state transition: NOTIFY_EXIT, NOTIFY_EXEC, etc.. This way we are not bound to a specific poll event indicating a specific state. Though there's a case to be made that EPOLLHUP could indicate process exit and EPOLLIN a state change + read().