linux-kernel - Re: [rfc][patch] mm: lockdep page lock

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Message-ID: <20100326115457.GB19308@shareable.org>
Date:	Fri, 26 Mar 2010 11:54:57 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nick Piggin <npiggin@...e.de>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [rfc][patch] mm: lockdep page lock

Peter Zijlstra wrote:
> On Fri, 2010-03-26 at 03:18 +0000, Jamie Lokier wrote:
> > Peter Zijlstra wrote:
> > > On Tue, 2010-03-16 at 13:21 +1100, Nick Piggin wrote:
> > > > 
> > > > 
> > > > Agreed (btw. Peter is there any way to turn lock debugging back on?
> > > > it's annoying when cpufreq hotplug code or something early breaks and
> > > > you have to reboot in order to do any testing).
> > > 
> > > Not really, the only way to do that is to get the full system back into
> > > a known (zero) lock state and then fully reset the lockdep state.
> > 
> > How about: Set a variable nr_pending = number of CPUs, run a task on
> > each CPU which disables interrupts, atomically decrements nr_pending
> > and then spins waiting for it to become negative (raw, not counted in
> > lockdep), and whichever one takes it to zero, that task knows there
> > are no locks held, and can reset the lockdep state.  Then sets it to
> > -1 to wake everyone.
> 
> Nope, won't work, you can easily preempt a lock holder.

Doh, yes of course.

I promise to get some sleep before further appearances :-)

-- Jamie
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