Re: [PATCH] procfs: Do not release pid_ns->proc_mnt too early

From: Louis Rilling <Louis.Rilling@kerlabs.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Pavel Emelyanov <xemul@parallels.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Pavel Emelyanov <xemul@openvz.org>,
	Linux Containers <containers@lists.osdl.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] procfs: Do not release pid_ns->proc_mnt too early
Date: Mon, 21 Jun 2010 13:11:27 +0200	[thread overview]
Message-ID: <20100621111127.GI16877@hawkmoon.kerlabs.com> (raw)
In-Reply-To: <20100618162734.GB7404@redhat.com>

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On 18/06/10 18:27 +0200, Oleg Nesterov wrote:
> On 06/18, Louis Rilling wrote:
> >
> > On 17/06/10 23:36 +0200, Oleg Nesterov wrote:
> > > On 06/17, Eric W. Biederman wrote:
> > > >
> > > > The task->children isn't changed until __unhash_process() which runs
> > > > after flush_proc_task().
> > >
> > > Yes. But this is only the current implementation detail.
> > > It would be nice to cleanup the code so that EXIT_DEAD tasks are
> > > never sit in ->children list.
> > >
> > > > So we should be able to come up with
> > > > a variant of do_wait() that zap_pid_ns_processes can use that does
> > > > what we need.
> > >
> > > See above...
> > >
> > > Even if we modify do_wait() or add the new variant, how the caller
> > > can wait for EXIT_DEAD tasks? I don't think we want to modify
> > > release_task() to do __wake_up_parent() or something similar.
> >
> > Indeed, I was thinking about calling __wake_up_parent() from release_task()
> > once parent->children becomes empty.
> >
> > Not sure about the performance impact though. Maybe some WAIT_NO_CHILDREN flag
> > in parent->signal could limit it. But if EXIT_DEAD children are removed from
> > ->children before release_task(), I'm afraid that this becomes impossible.
> 
> Thinking more, even the current do_wait() from zap_pid_ns_processes()
> is not really good. Suppose that some none-init thread is ptraced, then
> zap_pid_ns_processes() will hange until the tracer does do_wait() or
> exits.

Is this really a bad thing? If somebody ptraces a task in a pid namespace, that
sounds reasonable to have this namespace (and it's init task) pinned.

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes

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