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Date: Thu, 15 Oct 2015 17:18:19 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>,
        Christoph Lameter <cl@linux.com>, Tejun Heo <tj@kernel.org>,
        Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] Revert "kmod: handle UMH_WAIT_PROC from system
	unbound workqueue"
Message-ID: <20151015151819.GA22187@redhat.com>
References: <20151014185153.GA8117@redhat.com> <20151014185209.GB8117@redhat.com> <20151015133737.GC12822@lerouge>
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On 10/15, Frederic Weisbecker wrote:
>
> On Wed, Oct 14, 2015 at 08:52:09PM +0200, Oleg Nesterov wrote:
> > This reverts commit bb304a5c6fc63d8506cd9741a3a5f35b73605625.
> >
> > Because this patch leads to kthread zombies.
> >
> > 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 missed that indeed.

Heh me too ;)

> But then when we create the async thread with
> UMH_NO_WAIT, who reaps it? It's created by the workqueue which never
> exits.

It is auto-reaped because SIGCHILD is ignored. And this is why
bb304a5c6fc6 is wrong; it can die while UMH_WAIT_PROC case waits
for the new child.

> And on others cases, who buries the sync thread?

The same.

Please see V2 I sent. I'll try to send more cleanups soon to make
this all more explicit.

Oleg.