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

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Date:	Wed, 5 Aug 2009 18:42:18 +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,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by
	tgid at once

On 05/08/09  9:11 -0700, Paul Menage wrote:
> On Wed, Aug 5, 2009 at 3:20 AM, Louis Rilling<Louis.Rilling@...labs.com> wrote:
> >
> > The downside of this is teaching lockdep about this recursive locking. Not that
> > simple actually...
> 
> Don't we just give each thread's lock its own lock class? That's what
> we did for the cgroup hierarchy_mutex.

Given that lock classes must be static and that lockdep only supports a limited
lock depth, this is an issue for processes having many threads.

> 
> > 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.
> 
> cgroup_mutex already fulfills the role of the higher-level lock.

If so (that is, here cgroup_mutex is taken before write-locking all threads'
rw_sem), then enhancing rwsem's interface in a similar way to the
spin_lock_nest_lock() interface could do it. There will still be an issue with
many threads and lockdep limited lock depth though.

Added Peter in CC.

Louis

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