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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,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 89A50C433E1 for ; Fri, 3 Jul 2020 21:42:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 66ED120885 for ; Fri, 3 Jul 2020 21:42:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726965AbgGCVm3 (ORCPT ); Fri, 3 Jul 2020 17:42:29 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:41788 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726379AbgGCVm3 (ORCPT ); Fri, 3 Jul 2020 17:42:29 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]) by out02.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1jrTRx-0002QR-7U; Fri, 03 Jul 2020 15:42:25 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1jrTRw-0001FP-6m; Fri, 03 Jul 2020 15:42:24 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Alexei Starovoitov Cc: linux-kernel@vger.kernel.org, David Miller , Greg Kroah-Hartman , Tetsuo Handa , Kees Cook , Andrew Morton , Alexei Starovoitov , Al Viro , bpf , linux-fsdevel , Daniel Borkmann , Jakub Kicinski , Masahiro Yamada , Gary Lin , Bruno Meneguele , LSM List , Casey Schaufler , Luis Chamberlain , Linus Torvalds , Christian Brauner References: <87y2o1swee.fsf_-_@x220.int.ebiederm.org> <20200702164140.4468-13-ebiederm@xmission.com> <20200703203021.paebx25miovmaxqt@ast-mbp.dhcp.thefacebook.com> Date: Fri, 03 Jul 2020 16:37:47 -0500 In-Reply-To: <20200703203021.paebx25miovmaxqt@ast-mbp.dhcp.thefacebook.com> (Alexei Starovoitov's message of "Fri, 3 Jul 2020 13:30:21 -0700") Message-ID: <873668s2j8.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1jrTRw-0001FP-6m;;;mid=<873668s2j8.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX19Q3He2YCYof+2pylv7RWD58IY3V+dYbb0= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alexei Starovoitov writes: > On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote: >> Create an independent helper thread_group_exited report return true >> when all threads have passed exit_notify in do_exit. AKA all of the >> threads are at least zombies and might be dead or completely gone. >> >> Create this helper by taking the logic out of pidfd_poll where >> it is already tested, and adding a missing READ_ONCE on >> the read of task->exit_state. >> >> I will be changing the user mode driver code to use this same logic >> to know when a user mode driver needs to be restarted. >> >> Place the new helper thread_group_exited in kernel/exit.c and >> EXPORT it so it can be used by modules. >> >> Signed-off-by: "Eric W. Biederman" >> --- >> include/linux/sched/signal.h | 2 ++ >> kernel/exit.c | 24 ++++++++++++++++++++++++ >> kernel/fork.c | 6 +----- >> 3 files changed, 27 insertions(+), 5 deletions(-) >> >> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h >> index 0ee5e696c5d8..1bad18a1d8ba 100644 >> --- a/include/linux/sched/signal.h >> +++ b/include/linux/sched/signal.h >> @@ -674,6 +674,8 @@ static inline int thread_group_empty(struct task_struct *p) >> #define delay_group_leader(p) \ >> (thread_group_leader(p) && !thread_group_empty(p)) >> >> +extern bool thread_group_exited(struct pid *pid); >> + >> extern struct sighand_struct *__lock_task_sighand(struct task_struct *task, >> unsigned long *flags); >> >> diff --git a/kernel/exit.c b/kernel/exit.c >> index d3294b611df1..a7f112feb0f6 100644 >> --- a/kernel/exit.c >> +++ b/kernel/exit.c >> @@ -1713,6 +1713,30 @@ COMPAT_SYSCALL_DEFINE5(waitid, >> } >> #endif >> >> +/** >> + * thread_group_exited - check that a thread group has exited >> + * @pid: tgid of thread group to be checked. >> + * >> + * Test if thread group is has exited (all threads are zombies, dead >> + * or completely gone). >> + * >> + * Return: true if the thread group has exited. false otherwise. >> + */ >> +bool thread_group_exited(struct pid *pid) >> +{ >> + struct task_struct *task; >> + bool exited; >> + >> + rcu_read_lock(); >> + task = pid_task(pid, PIDTYPE_PID); >> + exited = !task || >> + (READ_ONCE(task->exit_state) && thread_group_empty(task)); >> + rcu_read_unlock(); >> + >> + return exited; >> +} > > I'm not sure why you think READ_ONCE was missing. > It's different in wait_consider_task() where READ_ONCE is needed because > of multiple checks. Here it's done once. In practice it probably has no effect on the generated code. But READ_ONCE is about telling the compiler not to be clever. Don't use tearing loads or stores etc. When all of the other readers are using READ_ONCE I just get nervous if we have a case that doesn't. > The rest all looks good to me. Tested with and without bpf_preload patches. > Feel free to create a frozen branch with this set. Can I have your Tested-by and Acked-by? > btw I'll be offline starting tomorrow for a week. > Will catch up with threads afterwards. Eric