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Date: Wed, 18 Apr 2012 02:29:56 +0000
From: "Serge E. Hallyn" <serge@hallyn.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Daniel Lezcano <daniel.lezcano@free.fr>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] pidns: reboot_pid_ns: use SEND_SIG_FORCED instead of
 force_sig()
Message-ID: <20120418022956.GA1919@mail.hallyn.com>
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Quoting Oleg Nesterov (oleg@redhat.com):
> On 04/16, Oleg Nesterov wrote:
> >
> > I'll investigate and report tomorrow.
> 
> Thanks again Serge, everything is clear.

Phew :)

>  Afaics this needs another
> (almost off-topic) patch.
> 
> Please forget about sys_reboot() for the moment. There is a minor
> and known problem with SIGKILL && SIGNAL_UNKILLABLE. If you send
> SIGKILL from the parent namespace to cinit, this doesn't make it
> SIGNAL_GROUP_EXIT immediately. Only after cinit actually dequeues
> SIGKILL it does do_group_exit() and set this flag. This should be
> fixed anyway, but not now.
> 
> And this explain the problem. Note that your test-case does
> sys_reboot() from the init process, and reboot_pid_ns() does
> 
> 	kill(pid_ns->child_reaper, SIGKILL);
> 
> 	do_exit(0);
> 
> so it sends SIGKILL to itself and exit. This means that cinit
> (current) obviously can't dequeue SIGKILL, and without
> SIGNAL_GROUP_EXIT wait_task_zombie() ignores
> signal->group_exit_code.
> 
> That is why the patch makes a difference.
> 
> But why reboot_pid_ns() does do_exit? Yes, it was copied from
> sys_reboot() and I didn't notice. It should use do_group_exit(),
> and sys_reboot() too. This is the patch I am going to send, but
> I'll try to think a bit more.
> 
> Thanks!
> 
> Oleg.

Great, glad it makes sense!  :)

I'll test when you have a patch ready.  Thanks, Oleg.

-serge