From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756914AbbCMT5P (ORCPT ); Fri, 13 Mar 2015 15:57:15 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:49772 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752035AbbCMT5N (ORCPT ); Fri, 13 Mar 2015 15:57:13 -0400 Date: Fri, 13 Mar 2015 12:57:07 -0700 From: josh@joshtriplett.org To: Oleg Nesterov Cc: Al Viro , Andrew Morton , Andy Lutomirski , Ingo Molnar , Kees Cook , "Paul E. McKenney" , "H. Peter Anvin" , Rik van Riel , Thomas Gleixner , Thiago Macieira , Michael Kerrisk , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd Message-ID: <20150313195707.GA10487@cloud> References: <9c39c576e1d9a9912b4aec54d833a73a84d2f592.1426180120.git.josh@joshtriplett.org> <20150313162113.GA25966@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150313162113.GA25966@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 13, 2015 at 05:21:13PM +0100, Oleg Nesterov wrote: > Josh, > > I'll certainly try to read this series, but not before next week. Thanks for looking at it. > but a couple of nits right now. > > On 03/12, Josh Triplett wrote: > > > > When passed CLONE_FD, clone4 will return a file descriptor rather than a > > PID. When the child process exits, it gets automatically reaped, > > And even I have no idea what you are actually doing, this doesn't look > right, see below. > > > +static unsigned int clonefd_poll(struct file *file, poll_table *wait) > > +{ > > + struct task_struct *p = file->private_data; > > + poll_wait(file, &p->clonefd_wqh, wait); > > + return p->exit_state == EXIT_DEAD ? (POLLIN | POLLRDNORM) : 0; > > +} > > + > > +static ssize_t clonefd_read(struct file *file, char __user *buf, size_t count, loff_t *ppos) > > +{ > > + struct task_struct *p = file->private_data; > > + int ret = 0; > > + > > + /* EOF after first read */ > > + if (*ppos) > > + return 0; > > + > > + if (file->f_flags & O_NONBLOCK) > > + ret = -EAGAIN; > > + else > > + ret = wait_event_interruptible(p->clonefd_wqh, p->exit_state == EXIT_DEAD); > > + > > + if (p->exit_state == EXIT_DEAD) { > > Again, I simply do not know what this code does at all. But I bet the usage > of EXIT_DEAD is wrong ;) > > OK, OK, I can be wrong. But I simply do not see what protects this task_struct > if it is EXIT_DEAD (in fact even if it is EXIT_ZOMBIE). If by "what protects" you mean "what keeps it alive", the file descriptor holds a reference to the task_struct by calling get_task_struct when created and put_task_struct when released. This wait_event_interruptible pairs with the wake_up_all called from clonefd_do_notify, which exit_notify calls *after* setting the task to TASK_DEAD. Apart from that, what about what the code is doing isn't clear? > > @@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead) > > if (group_dead) > > kill_orphaned_pgrp(tsk->group_leader, NULL); > > > > - if (unlikely(tsk->ptrace)) { > > + if (tsk->autoreap) { > > + autoreap = true; > > Debuggers won't be happy. A ptraced task should not autoreap itself. A process launching a new process with CLONE_FD is explicitly requesting that the process be automatically reaped without any other process having to wait on it. The task needs to not become a zombie, because otherwise, it'll show up in waitpid(-1, ...) calls in the parent process, which would break the ability to use this to completely encapsulate process management within a library and not interfere with the parent's process handling via SIGCHLD and wait{pid,3,4}. - Josh Triplett