linux-kernel - Re: [PATCH] ib-release-locks-in-the-proper-order

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Date:	Mon, 21 Sep 2009 17:48:36 -0400 (EDT)
From:	John Kacur <jkacur@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	mingo@...e.hu, Roland Dreier <rolandd@...co.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>, tglx@...utronix.de,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ib-release-locks-in-the-proper-order


----- "Steven Rostedt" <rostedt@...dmis.org> wrote:

> On Mon, 2009-09-21 at 21:35 +0200, John Kacur wrote:
> > Please consider the following patch - originally from Steven
> Rostedt. 
> > It solves a problem for rt which is very sensitive to the lock
> ordering. 
> > It should have a no impact on non-rt.
> > 
> > The patch applies to current tip/master - but it is fine with me if
> it 
> > would be more appropriate for one of the infiniband people to take
> it.
> > 
> > Thanks
> > 
> > >From e533f2b9ee9b0bd95aaa4c3369e79b350c9895d3 Mon Sep 17 00:00:00
> 2001
> > From: Steven Rostedt <srostedt@...hat.com>
> > Date: Mon, 21 Sep 2009 21:23:46 +0200
> > Subject: [PATCH] ib: release locks in the proper order
> > 
> > RT is very sensitive to the order locks are taken and released
> > wrt read write locks. We must do
> > 
> >   lock(a);
> >   lock(b);
> >   lock(c);
> > 
> >   [...]
> > 
> >   unlock(c);
> >   unlock(b);
> >   unlock(a);
> > 
> > otherwise bad things can happen.
> > 
> > Signed-off-by: Ken Cox <jkc@...hat.com>
> > Signed-off-by: Clark Williams <williams@...hat.com>
> > Signed-off-by: John Kacur <jkacur@...hat.com>
> 
> The -rt patch doesn't use the multi rwlock code anymore (the reason
> for
> the first patch), and the last revision of that code was able to
> handle
> that too.
> 
> Linus totally ripped into this idea. A lock must be able to handle
> any
> order of unlocking. There should be no technical reason a lock must
> be
> unlocked in reverse order they were locked.
> 
> What exactly is sensitive about this?
> 

Thanks Steve!
I hereby withdraw this patch!!!!
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