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

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Date:	Thu, 12 Jan 2012 18:07:28 +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

Hi Mandeep,

On 01/11, Mandeep Singh Baines wrote:
> > >
> > > #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?
> >
>
> So for:
>
> struct task *M; /* assuming this is passed in to us */
> struct task *L = M->group_leader;

L == M

> do {
> 	do_something(T);
> } while_each_thread(M, T, L);
>
> Here is my thinking.
>
> If some thread K does exec, you won't miss it because:
>
> 1) Ignoring the group_leader check, you'll visit K just by following
>    next_thread(). That's the case today and is what you except
>    when iterating over an rcu_list.
> 2) (t->group_leader == o) will fail iff t is the exec thread.
>    Since we test t->group_leader before re-assigning it (t=next_thread()),
>    the test will fail only after visiting the exec thread. So you'll
>    visit the exec thread and then terminate the loop.

Still can't understand... Lets look at this trivial example again.

We start from the main thread M, it is ->group_leader. There is
another thread T in this thread group. We are doing

	OLD = M;

	t = M;
	do {
		do_smth(t);
	}
	while (t->group_leader == OLD && ((t = next_thread(t)) != M);

The first iteration does do_smth(M).

T calls de_thread() and, in particular, it does M->group_leader = T
(see "leader->group_leader = tsk" in de_thread).

after that t->group_leader == OLD fails. t == M, its group_leader == T.
do_smth(T) won't be called.

No?

Oleg.

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