From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753527AbbJOPxB (ORCPT ); Thu, 15 Oct 2015 11:53:01 -0400 Received: from mail-lb0-f180.google.com ([209.85.217.180]:33171 "EHLO mail-lb0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751098AbbJOPw7 (ORCPT ); Thu, 15 Oct 2015 11:52:59 -0400 Date: Thu, 15 Oct 2015 17:52:56 +0200 From: Frederic Weisbecker To: Oleg Nesterov Cc: Andrew Morton , Rik van Riel , Christoph Lameter , Tejun Heo , Rusty Russell , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/1] kmod: don't run async usermode helper as a child of kworker thread Message-ID: <20151015155255.GE12822@lerouge> References: <20151014185153.GA8117@redhat.com> <20151015143739.GA20060@redhat.com> <20151015143757.GB20060@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151015143757.GB20060@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 15, 2015 at 04:37:57PM +0200, Oleg Nesterov wrote: > call_usermodehelper_exec_sync() does fork() + wait() with "unignored" > SIGCHLD. What we have missed is that this worker thread can have other > children previously forked by call_usermodehelper_exec_work() without > UMH_WAIT_PROC. If such a child exits in between it becomes a zombie and > nobody can reap it (unless/until this worker thread exits too). I think we should elaborate a tiny bit the last sentence here: "When the parent masks SIGCHLD, a child autoreaps itself, this is what we expect from !UMH_WAIT_PROC children. Now if such a child exits during this unlucky window where the parent worker enabled SIGCHLD to wait for a sibling, the autoreap will fail and the child then becomes a zombie because nobody can reap it (unless/until this worker thread exits too)." > > Change the !UMH_WAIT_PROC case to use CLONE_PARENT. > > Note: this is only first step. All PF_KTHREAD tasks, even created by > kernel_thread() should have ->parent == kthreadd by default. > > Signed-off-by: Oleg Nesterov > --- > kernel/kmod.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/kernel/kmod.c b/kernel/kmod.c > index da98d05..e7185a2 100644 > --- a/kernel/kmod.c > +++ b/kernel/kmod.c > @@ -327,9 +327,13 @@ static void call_usermodehelper_exec_work(struct work_struct *work) > call_usermodehelper_exec_sync(sub_info); > } else { > pid_t pid; > - > + /* > + * Use CLONE_PARENT to reparent it to kthreadd; we do not > + * want to pollute current->children, in particular because > + * call_usermodehelper_exec_sync() assumes it is empty. > + */ IMHO, that too should get some more details. Maybe: + /* + * Use CLONE_PARENT to reparent it to kthreadd. We need a parent + * that always ignore SIGCHLD such that the child always autoreaps + * as expected. + */ Thanks! > pid = kernel_thread(call_usermodehelper_exec_async, sub_info, > - SIGCHLD); > + CLONE_PARENT | SIGCHLD); > if (pid < 0) { > sub_info->retval = pid; > umh_complete(sub_info); > -- > 2.4.3 > >