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Subject: Re: [rfc][patch] mm: lockdep page lock
From: Peter Zijlstra <peterz@infradead.org>
To: Jamie Lokier <jamie@shareable.org>
Cc: Nick Piggin <npiggin@suse.de>, Jan Kara <jack@suse.cz>,
       Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
       linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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Date: Fri, 26 Mar 2010 07:54:07 +0100
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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.