linux-kernel - Re: Q: cgroup: Questions about possible issues in cgroup locking

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Message-ID: <20120106152356.GA23995@redhat.com>
Date:	Fri, 6 Jan 2012 16:23:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Cgroups <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Paul Menage <paul@...lmenage.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking

On 01/04, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@...hat.com) wrote:
> > On 12/21, Frederic Weisbecker wrote:
> > >
> > > On Wed, Dec 21, 2011 at 11:24:13AM -0800, Mandeep Singh Baines wrote:
> > > >
> > > > If you call exec from a thread other than g, g is now unlinked. So
> > > > "t != g" will always be true. If you then pthread_create, you now
> > > > have two threads so "t != __prev" will also always be true. So
> > > > you now have an infinite loop.
> > >
> > > Oh you're right.
> > >
> > > But then we can't use t != t->group_leader because that assumes while_each_thread()
> > > started on the leader.
> >
> > Yes, this can't work.
> >
> > Besides, we need more burriers to rely on the ->group_leader check.
> >
> > See http://marc.info/?t=127688987300002
> >
>
> I went through the thread. Were there any other concerns other than
> requiring that you start with the group_leader and the barrier?
>
> You could modify zap_other_threads to start with the group leader by
> skipping p:
>
> if (p == t)
>    continue;

Yes, we can but there are other while_each_thread(nonleader) users.
Yes we can fix them too but this looks a bit ugly and we need to
change while_each_thread() anyway. And I do not see why this change
will be simpler if we restrict it to group_leader.

And note that zap_other_threads() is fine in any case, it is called
under ->siglock.

> > in particular, http://marc.info/?l=linux-kernel&m=127714242731448
> > I think this should work, but then we should do something with the
> > users like zap_threads().
> >
>
> With that patch, won't you potentially miss the exec thread if an exec
> occurs while you're iterating over the list? Is that OK?

Of course it is not OK ;) Note the "we should do something with" above.

Oleg.

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