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

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Message-ID: <20090806100443.GE26446@hawkmoon.kerlabs.com>
Date:	Thu, 6 Aug 2009 12:04:43 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Benjamin Blum <bblum@...gle.com>, 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 06/08/09 11:58 +0200, Louis Rilling wrote:
> 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.

Not exactly what I meant, sorry. This would require defining as many sub-classes
as possible (still limited to MAX_LOCK_DEPTH) to support many threads.

> 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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