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

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Message-ID: <20120104193614.GF9511@google.com>
Date:	Wed, 4 Jan 2012 11:36:14 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Mandeep Singh Baines <msb@...omium.org>,
	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

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;

> 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?

Regards,
Mandeep

> Oleg.
> 
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