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

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Date:	Thu, 6 Aug 2009 11:58:54 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Benjamin Blum <bblum@...gle.com>
Cc:	Paul Menage <menage@...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 17:01 -0700, Benjamin Blum wrote:
> On Wed, Aug 5, 2009 at 9:42 AM, Louis Rilling<Louis.Rilling@...labs.com> wrote:
> > 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.
> 
> If we make the locks per-thread, then we can use plain mutexes instead
> of rwsems since the only reader will ever be the owning thread itself,
> and we can use mutex_lock_nested.

mutex_lock_nested is not enough, since this would require putting each thread's
mutex in a different class. Again, something like mutex_lock_nest_lock() is
the solution, especially since Peter's recent improvement.

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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