linux-kernel - Re: while_each_thread() under rcu_read

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Message-ID: <20100628234358.GJ2357@linux.vnet.ibm.com>
Date:	Mon, 28 Jun 2010 16:43:58 -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 Fri, Jun 25, 2010 at 11:55:48AM +0200, Oleg Nesterov wrote:
> On 06/24, Paul E. McKenney wrote:
> >
> > On Thu, Jun 24, 2010 at 11:57:02PM +0200, Oleg Nesterov wrote:
> > > On 06/24, Paul E. McKenney wrote:
> > > >
> > > > 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?
> 
> I think, yes. We can miss them in any case, they can go away before
> while_each_thread(g, t) starts the scan.
> 
> If g == group_leader (old or new), then we should notice this thread
> at least.
> 
> Otherwise we can miss them all, with or without next_thread_careful().

Just to be sure that we are actually talking about the same scenario...

Suppose that a task group is lead by 2908 and has member 2909, 2910,
2911, and 2912.  Suppose that 2910 does pthread_exit() just as some
other task is "ls"ing the relevant /proc entry.  Is it really OK for
"ls" to show 2909 but not 2911 and 2912, even though 2911 and 2912
were alive and kicking the entire time?

							Thanx, Paul
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