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From: ebiederm@xmission.com (Eric W. Biederman)
To: will_schmidt@vnet.ibm.com
Subject: Re: [PATCH 1/3] [PATCH i386] during VM oom condition,
	kill all threads in process group
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Date: Fri, 08 Jun 2007 16:48:41 -0600
In-Reply-To: <1181337147.21409.42.camel@farscape.rchland.ibm.com> (Will
	Schmidt's message of "Fri, 08 Jun 2007 16:12:26 -0500")
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Cc: linuxppc-dev@ozlabs.org, Andrew Morton <akpm@linux-foundation.org>,
	Oleg Nesterov <oleg@tv-sign.ru>, linux-kernel@vger.kernel.org,
	Anton Blanchard <anton@samba.org>
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Will Schmidt <will_schmidt@vnet.ibm.com> writes:

> On Fri, 2007-06-08 at 12:32 -0700, Andrew Morton wrote:
>> On Fri, 08 Jun 2007 14:19:18 -0500
>> Will Schmidt <will_schmidt@vnet.ibm.com> wrote:
>> 
>> > > > > zap_other_threads() requires tasklist_lock.
>> > 
>
>> In fact, it's probably the case that rcu_read_lock() is now sufficient
>> locking coverage for zap_other_threads() (cc's people).
>> 
>> It had better be, because do_group_exit() forgot to take tasklist_lock.  It
>> is perhaps relying upon spin_lock()'s hidden rcu_read_lock() properties
>> without so much as a code comment, which would be somewhat nasty of it.
>
>> You could perhaps just call do_group_exit() from within the fault
>> handler,
>> btw.
>
> Yup, so looks like I can actually replace the existing do_exit() call
> with do_group_exit().   I'll sit on this for a bit to give other folks a
> chance to comment on which lock call is sufficient, read_lock() or
> rcu_read_lock(), etc;  and do_group_exit()'s issue with taking
> tasklist_lock. 

No.  The rcu_read_lock is not sufficient.
Yes.  sighand->siglock is enough, and we explicitly take it in
do_group_exit before calling zap_other_threads.

Unless I have completely miss-understood this thread.

Eric