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=-8.6 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=ham 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 5372EC282E0 for ; Fri, 19 Apr 2019 22:08:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F3980217F9 for ; Fri, 19 Apr 2019 22:08:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="kODWHtTT" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725860AbfDSWI2 (ORCPT ); Fri, 19 Apr 2019 18:08:28 -0400 Received: from mail-oi1-f195.google.com ([209.85.167.195]:42886 "EHLO mail-oi1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725817AbfDSWI1 (ORCPT ); Fri, 19 Apr 2019 18:08:27 -0400 Received: by mail-oi1-f195.google.com with SMTP id w139so4850559oie.9 for ; Fri, 19 Apr 2019 15:08:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=fMiIUBFTvUHEyKHfCZq+VAab6/92ykh9ZHPcA4LjBC4=; b=kODWHtTTwTp1om+UFdXl6FMtlQq/BKBoa+LizmTc41xMnghRS/rg7bquqmvIvVEU1c RP55ry20HWwOTnr1UaCIQFKsTr6o2NTvDWbksxzVzcqA+fP/VUb1SEkBGO4qHl0XBeo2 JLX86fFYEM5+ilQTyLK6GIDq3RBKoAUzGFWpXhqWee1PWf2LesvzClGnspTWWub3EaLW dyzwW4/kusobqOkDiiHOa4oKWtoSJS8axOIwF+ZRPOow/jFTWNFOPuV+PZS1MbzJDWwA K7EgyPNN2y4q6dx1FeJYLKKbjXKGyAhkBqHbwdbb53x8CVMpaynwlIEEVqc1SQ4uUozt ZoOw== 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=fMiIUBFTvUHEyKHfCZq+VAab6/92ykh9ZHPcA4LjBC4=; b=oMxTVTj+x0NuaJ4zK85ZDCkS42stLSPREsGd5Y2Ul9Kg5KNpydgSqzmpqzCIag728B R3wVy5wfUbk0eFIThO3O+U6sX7h63V7Zk63Pjr0t5O5Q2P/bwyvxntrBekO2PjtgZjQt oh6d92GqdamR1lyaR9DnSpcbkrQlsAwViKdA1H4Dd5GoeTKMjUg7ST2ujwduxx+Ivfvw v6B8jkMK+8nNCn0maWgjU5wZOK2dcEVaUs1QRNvTnMbdlVPimrt3xz6YRJaz+RhCh92o ritao6570XG1TLRvyLqnxbl3Bu3zJiNWFYsL03PoePMJYhVX4/AAqRhbNNC9iaIaeZiq w3LA== X-Gm-Message-State: APjAAAXqmg62wXbt4EtJ8LDBfcx9vGOA/nnL4L4DJpPUP6hH6CSRU0z/ Jhe1lkRLM66aGwjSmQ1DrAZqZN055WX+Fxys0oLLnA== X-Google-Smtp-Source: APXvYqzSi1L86l7z8I8dIrVj1JjKX9WCSYtGMmauqzp1Vu03G0gGAZeC71iwlSN1G4hnr6HpVtr0VukN5q8MjPkFxnI= X-Received: by 2002:aca:f086:: with SMTP id o128mr3045361oih.101.1555711706423; Fri, 19 Apr 2019 15:08:26 -0700 (PDT) MIME-Version: 1.0 References: <20190417130940.GC32622@redhat.com> <20190419190247.GB251571@google.com> <20190419191858.iwcvqm6fihbkaata@brauner.io> <20190419194902.GE251571@google.com> <20190419212002.GB44851@google.com> <20190419214506.GF251571@google.com> In-Reply-To: <20190419214506.GF251571@google.com> From: Daniel Colascione Date: Fri, 19 Apr 2019 15:08:14 -0700 Message-ID: Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd To: Joel Fernandes Cc: Christian Brauner , 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 2:45 PM Joel Fernandes wrote: > > On Fri, Apr 19, 2019 at 02:24:09PM -0700, Daniel Colascione wrote: > > On Fri, Apr 19, 2019 at 2:20 PM Joel Fernandes wrote: > > > > > > On Fri, Apr 19, 2019 at 10:57:11PM +0200, Christian Brauner wrote: > > > > 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(). > > > > > > According to Linus, POLLHUP usually indicates that something is readable: > > > > I don't think Linus said that POLLHUP means readable. He did say that > > it usually doesn't make sense to set POLLHUP without POLLIN, but > > that's not the same as POLLHUP indicating readability. > > Ok, fair enough. > > > > https://lkml.org/lkml/2019/4/18/1181 > > > "So generally a HUP condition should mean that POLLIN and POLLOUT also > > > get set. Not because there's any actual _data_ to be read, but simply > > > because the read will not block." > > > > > > I feel the future state changes such as for NOTIFY_EXEC can easily be > > > implemented on top of this patch. > > > > > > Just for the exit notification purposes, the states are: > > > if process has exit_state == 0, block. > > > if process is zombie/dead but not reaped, then return POLLIN > > > if process is reaped, then return POLLIN | POLLHUP > > > > Setting POLLHUP when the process is reaped is harmless, but I don't > > think it's useful. I can't think of a reason that anyone would care. > > We can also outright remove it. Oleg seemed to not mind it, in fact he said > it may be useful to indicate the reap status so at least I am inclined to > leave it in. > > > You can't block and wait on reaping, so you could only busy-wait, and > > you can look for ESRCH on any proc file today to detect reaping. I'd > > proc file reading is racy though. We shouldn't even talk about that since the > point of pidfd is to avoid such raw "pid" related races. It's not racy if you have a procfs dirfd handle open anyway. But since we moved to the two-kinds-of-file-descriptor model, there's no way to reliably open the procfs directory FD for a process that's exited but that's not yet been reaped. The fdinfo mechanism doesn't solve this problem. But adding a reap flag to poll output doesn't close the race either. I don't see any actual benefit to knowing via poll whether a process was reaped. If you can identify such a use case, great. Otherwise, I want to keep the poll interface simple and not specify semantics for POLLHUP.