linux-kernel - Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

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Message-ID: <20090805102057.GT29252@hawkmoon.kerlabs.com>
Date:	Wed, 5 Aug 2009 12:20:57 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Paul Menage <menage@...gle.com>
Cc:	Benjamin Blum <bblum@...gle.com>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by
	tgid at once

On 04/08/09 12:28 -0700, Paul Menage wrote:
> On Tue, Aug 4, 2009 at 12:14 PM, Benjamin Blum<bblum@...gle.com> wrote:
> >
> > Hmm, the tasklist_lock section in fork() is entirely inside the
> > read-lock. Presumably then iterating the threadgroup list to take all
> > rwsems is safe from a race in which one thread escapes?
> >
> 
> I think so - the write-side locking would be something like:
> 
> again:
>   read_lock(&tasklist_lock);
>   find first not-yet-locked task t
>   while (t != group_leader) {
>     if (!down_write_trylock(&t->threadgroup_stability_mutex)) {
>       get_task_struct(t);
>       read_unlock(&tasklist_lock);
>       down_write(&t->threadgroup_stability_mutex);
>       put_task_struct(t);
>       goto again;
>     }
>     t = t->next_thread;
>   }
>   read_unlock(&tasklist_lock);

The downside of this is teaching lockdep about this recursive locking. Not that
simple actually... so that such cases are currently handled using a higher-level
lock that prevents races in locking the whole chain (there was one such example
for locking all vmas with KVM). IIUC, the intent here is to avoid such
higher-level lock.

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes

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