linux-kernel - Re: [PATCH] procfs: Do not release pid_ns->proc

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Message-ID: <20100618162734.GB7404@redhat.com>
Date:	Fri, 18 Jun 2010 18:27:34 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org,
	Louis Rilling <Louis.Rilling@...labs.com>
Subject: Re: [PATCH] procfs: Do not release pid_ns->proc_mnt too early

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.

Oleg.

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