Re: [PATCH 1/1] Revert "kmod: handle UMH_WAIT_PROC from system unbound workqueue"

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Oct 15, 2015 at 05:18:19PM +0200, Oleg Nesterov wrote:
> 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.

Oooh, that's subtle!

> 
> > 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.

Ok.

> Oleg.
>