linux-kernel - Re: [PATCH UPDATED 03/10] threadgroup: extend threadgroup

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Message-ID: <20111205184315.GJ627@google.com>
Date:	Mon, 5 Dec 2011 10:43:15 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	paul@...lmenage.org, rjw@...k.pl, lizf@...fujitsu.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, matthltc@...ibm.com,
	akpm@...ux-foundation.org, oleg@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH UPDATED 03/10] threadgroup: extend threadgroup_lock()
 to cover exit and exec

Hello, Frederic.

On Fri, Dec 02, 2011 at 05:28:03PM +0100, Frederic Weisbecker wrote:
> But I don't think it's very useful to protect against irq_exit_thread(),
> what happens there is purely of internal irq interest.
> 
> Then right after, PF_EXITING is set before any interesting change.
> Isn't it possible to simply lock this flag setting? IIRC, as soon
> as the PF_EXITING flag is set, you ignore the task for attachment.

I think that's technically possible but it does introduce another
class of tasks - the dying ones.  e.g. If a task has PF_EXITING set
and the containing process is migrating, we'll have to migrate all
tasks but the dying one and cgroup ->exit callbacks can be called on
the lonely task after the migration is complete.  It's kinda messy and
if someone makes a wrong assumption there, the bug is gonna be even
more difficult to reproduce / track down than now.  Yes, smaller scope
locking is nicer but I would like to avoid api weirdities like that.

Thanks.

-- 
tejun
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