linux-kernel - Re: while_each_thread() under rcu_read

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Date:	Thu, 24 Jun 2010 20:41:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Jerome Marchand <jmarchan@...hat.com>,
	Mandeep Singh Baines <msb@...gle.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: while_each_thread() under rcu_read_lock() is broken?

On Thu, Jun 24, 2010 at 11:57:02PM +0200, Oleg Nesterov wrote:
> On 06/24, Paul E. McKenney wrote:
> >
> > On Wed, Jun 23, 2010 at 05:24:21PM +0200, Oleg Nesterov wrote:
> > > It is very possible that I missed something here, my only point is
> > > that I think it would be safer to assume nothing about the leaderness.
> >
> > It is past time that I list out my assumptions more carefully.  ;-)
> >
> > First, what "bad things" can happen to a reader scanning a thread
> > group?
> 
> (I assume you mean the lockless case)

You are quite right -- I should have stated that explicitly.

> Currently, the only bad thing is that while_each_thread(g) can loop
> forever if we race with exec(), or exit() if g is not leader.
> 
> And, to simplify, let's consider the same example again
> 
> 	t = g;
> 	do {
> 		printk("pid %d\n", t->pid);
> 	} while_each_thread(g, t);
> 
> 
> > 1.	The thread-group leader might do exec(), destroying the old
> > 	list and forming a new one.  In this case, we want any readers
> > 	to stop scanning.
> 
> I'd say, it is not that we want to stop scanning, it is OK to stop
> scanning after we printed g->pid

Fair enough.

> > 2.	Some other thread might do exec(), destroying the old list and
> > 	forming a new one.  In this case, we also want any readers to
> > 	stop scanning.
> 
> The same.
> 
> If the code above runs under for_each_process(g) or it did
> "g = find_task_by_pid(tgid)", we will see either new or old leader
> and print its pid at least.

OK.

> > 3.	The thread-group leader might do pthread_exit(), removing itself
> > 	from the thread group
> 
> No. It can exit, but it won't be removed from thread group. It will
> be zombie untill all sub-threads disappear.

This does make things easier!  Whew!!!  ;-)

> > 4.	Some other thread might do pthread_exit(), removing itself
> > 	from the thread group, and again might do so while the hapless
> > 	reader is referencing that thread.  In this case, we want
> > 	the hapless reader to continue scanning the remainder of the
> > 	thread group.
> 
> Yes.
> 
> But, if that thread was used as a starting point g, then
> 
> 	before the patch:	loop forever
> 	after the patch:	break

So it is OK to skip some of the other threads in this case, even
though they were present throughout the whole procedure?

> > 5.	The thread-group leader might do exit(), destroying the old
> > 	list without forming a new one.  In this case, we want any
> > 	readers to stop scanning.
> >
> > 6.	Some other thread might do exit(), destroying the old list
> > 	without forming a new one.  In this case, we also want any
> > 	readers to stop scanning.
> 
> Yes. But again, it is fine to print more pids as far as we know it
> is safe to iterate over the exiting thread group. However,
> next_thread_careful() can stop earlier compared to next_thread().
> Either way, we can miss none/some/most/all threads if we race with
> exit_group().

Yes, if there is an exit(), it makes sense that you might not see all
of the threads -- they could reasonably have disappeared before you
got done listing them.

> > Anything else I might be missing?
> 
> I think this is all.

OK, thank you (and Roland) for the tutorial!

							Thanx, Paul
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