linux-kernel - Re: while_each_thread() under rcu_read

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Message-ID: <20100625100809.GB6292@redhat.com>
Date:	Fri, 25 Jun 2010 12:08:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Chris Friesen <chris.friesen@...band.com>,
	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>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: while_each_thread() under rcu_read_lock() is broken?

On 06/24, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> If what we are looking for is a stable list_head that won't disappear
> on us we should be able to put one in sighand_struct or signal_struct
> (I forget which is which at the moment) and have a list_head that
> lives for the life of the longest living thread, and that won't get
> messed up by things like de_thread, and then next_thread could simply
> return NULL when we hit the end of the list.

This was already discussed.

Yes, we can add list_head into signal_struct. But this breaks, say,
thread_group_empty() and the similar code. This is fixeable.

But. We need to convert the code which does while_each_thread(g, t) to
use list_for_each(t, head). And, if g != group_leader this can't work.

Not to mention the increase of sizeof(signal_struct).

Oleg.

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