linux-kernel - Re: while_each_thread() under rcu_read

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Message-Id: <20100618210801.61B4540162@magilla.sf.frob.com>
Date:	Fri, 18 Jun 2010 14:08:01 -0700 (PDT)
From:	Roland McGrath <roland@...hat.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>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: while_each_thread() under rcu_read_lock() is broken?

I think you're right.  I can't see what would prevent that race.  

So for_each_process and do_each_thread are safe only under
read_lock(&tasklist_lock) and while_each_thread is only safe under
either that or siglock.  (Also a few places using next_thread in
similar loops outside those macros.)

Perhaps we could move those del's from __unhash_process to
__put_task_struct (or just delayed_put_task_struct?) and then
they wouldn't need to be rculist.h calls after all.  But we
would a need careful audit to figure out the assumptions about
being on the list meaning not reaped yet.

I think de_thread() in exec-by-nonleader is the only case where this
can happen, right?  So then perhaps we could make it call release_task
only via call_rcu?

Thanks,
Roland
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