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

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Message-ID: <20120111160730.GA24556@redhat.com>
Date:	Wed, 11 Jan 2012 17:07:30 +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 01/06, Mandeep Singh Baines wrote:
>
> Oleg Nesterov (oleg@...hat.com) wrote:
> >
> > > > 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:

(I assume, you mean the lockless case)

> * 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

Well yes, but it is not easy to exactly define what after/before
means in this case.

> 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.

I don't understand how this helps... and how this can work even
ignoring the barriers.

OK, we have the main thream M and the sub-thread T, we are doing

	do {
		do_something(t);
	} while_each_thread(M, t, M);

why we can't miss T if it does exec?

Oleg.

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