linux-kernel - Re: while_each_thread() under rcu_read

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Message-Id: <20100708235925.A71DA4A967@magilla.sf.frob.com>
Date:	Thu,  8 Jul 2010 16:59:25 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>
Subject: Re: while_each_thread() under rcu_read_lock() is broken?

> We can't do call_rcu(release_task),
> we can't take tasklist for writing in the softirq context. But even
> if we could, this can't help in fact or I missed something.

Ah, I had missed that constraint of call_rcu.  (It's not mentioned in the
kerneldoc comment directly, and is only buried in a couple of brief
mentions in Documentation/RCU/.)  I was thinking that would suffice because
it does it between rcu_read_lock critical sections, but I guess it actually
only does it after the last one and doesn't prevent a new one from having
started on another CPU.  (And we can't use it anyway.)


Thanks,
Roland
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