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

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Message-ID: <20120106182535.GJ9511@google.com>
Date:	Fri, 6 Jan 2012 10:25:35 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	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

Oleg Nesterov (oleg@...hat.com) wrote:
> 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.
> 

So requirements should be something like this:

* Any task alive for the duration of the iteration MUST be visited
* No task should be visited more than once
* Any task born or exiting after starting the iteration MAY be skipped
* You can start at any task in the thread group

Would something like this work:

#define while_each_thread(g, t, o) \
	while (t->group_leader == o && (t = next_thread(t)) != g)

Where o should have the value of g->group_leader.

Regards,
Mandeep

> Oleg.
> 
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