linux-kernel - [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 16:37:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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: [PATCH 0/1] kmod: don't run async usermode helper as a child of
	kworker thread

Andrew, please drop

	revert-kmod-handle-umh_wait_proc-from-system-unbound-workqueue.patch

I sent yesterday. On a second thought we have a better solution.

On 10/14, Oleg Nesterov wrote:
>
> Hello,
>
> I noticed by accident the kworker zombies on my testing machine.
> Can't reproduce (although I think it won't be hard to make a
> test-case), but I think the reason is clear, see the changelog.
>
> We could fix this by using kthread_create() if !UMH_WAIT_PROC,
> but imo it would be better to revert this change at least for
> now.

I changed my mind. I was worried about other workqueue callbacks
which could abuse kernel_thread() and populate kworker->children
even if we change call_usermodehelper_exec_work() to not do this.

But according to git-grep nobody does this. And this is good!
Because we can do more cleanups (will try to send "soon") to
ensure that all kthreads have parent == kthreadd.

And since the worker thread is already its child, we do not need
kthread_create(), we can just use CLONE_PARENT (which should be
later used by kernel_thread() by default).

> If we really want to avoid the extra kernel_thread(), we
> can make another patch which also avoids sys_wait4() and the
> games with SIGCHLD; we can rely on wait_chldexit.

Yes, this probably makes sense too, but we can do this regardless.

Oleg.

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