linux-kernel - Re: [PATCH 0/1] kmod: don't run async usermode helper as a child of kworker thread

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Date:	Thu, 15 Oct 2015 18:32:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] kmod: don't run async usermode helper as a child
	of kworker thread

On 10/15, Frederic Weisbecker wrote:
>
> 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:

OK, I'll try to update the changelog and send v2...

> "When the parent masks SIGCHLD, a child autoreaps itself, this is
> what we expect from !UMH_WAIT_PROC children.
                      ^^^^^^^^^^^^^^^^^^^^^^^

Not really. This is what we _usually_ expect from kernel_thread().

> > @@ -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.
>  +		 */

Well, OK...

But I would like to keep "we do not want to pollute current->children"
because this the goal of the next cleanups.

Plus I don't really like "parent that always ignore SIGCHLD". To remind,
we can also remove kernel_sigaction() and sys_wait4() from
call_usermodehelper_exec_sync(). Plus I have other changes in mind,
kernel_thread() should not rely on SIGCHLD at all. The auto-reapable
kernel threads should run with ->exit_signal == 0.

Finally, this comment should go into kernel_thread() eventually.

Oleg.

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