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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 285C4C43441 for ; Thu, 29 Nov 2018 12:34:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D90FC205C9 for ; Thu, 29 Nov 2018 12:34:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D90FC205C9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728161AbeK2XjZ (ORCPT ); Thu, 29 Nov 2018 18:39:25 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56530 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726683AbeK2XjZ (ORCPT ); Thu, 29 Nov 2018 18:39:25 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 673C7307D931; Thu, 29 Nov 2018 12:34:12 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.12]) by smtp.corp.redhat.com (Postfix) with SMTP id 6662868377; Thu, 29 Nov 2018 12:34:10 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Thu, 29 Nov 2018 13:34:12 +0100 (CET) Date: Thu, 29 Nov 2018 13:34:09 +0100 From: Oleg Nesterov To: "Eric W. Biederman" Cc: =?iso-8859-1?Q?J=FCrg?= Billeter , Andrew Morton , Thomas Gleixner , Kees Cook , Andy Lutomirski , linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] prctl: add PR_{GET,SET}_KILL_DESCENDANTS_ON_EXIT Message-ID: <20181129123409.GA10645@redhat.com> References: <20181127225408.7553-1-j@bitron.ch> <20181127225408.7553-2-j@bitron.ch> <20181128144230.GG30395@redhat.com> <87r2f5gr9g.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87r2f5gr9g.fsf@xmission.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Thu, 29 Nov 2018 12:34:12 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/28, Eric W. Biederman wrote: > > Oleg Nesterov writes: > > > On 11/27, Jürg Billeter wrote: > >> > >> @@ -704,6 +713,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead) > >> struct task_struct *p, *n; > >> LIST_HEAD(dead); > >> > >> + if (group_dead && tsk->signal->kill_descendants_on_exit) > >> + walk_process_tree(tsk, kill_descendant_visitor, NULL); > > > > Well, this is not exactly right, at least this is suboptimal in that > > other sub-threads can too call walk_process_tree(kill_descendant_visitor) > > later for no reason. > > Oleg I think I am missing something. No, it is stupid me who can't read, > Reading kernel/exit.c I see "group_dead = atomic_dec_and_test(&tsk->signal->live)". > Which seems like enough to ensure exactly one task/thread calls walk_process_tree. Of course you right, sorry for confusion. To me it would be more clean to call walk_process_tree(kill_descendant_visitor) unconditionally in find_new_reaper() right before "if (has_child_subreaper)", but then we will need to shift read_lock(tasklist) from walk_process_tree(). So I think the patch is mostly fine, the only problem I can see is that PR_SET_KILL_DESCENDANTS_ON_EXIT can race with PR_SET_CHILD_SUBREAPER, they both need to update the bits in the same word. Oleg.