Re: [rfc][patch] mm: lockdep page lock

[Jamie Lokier <jamie@shareable.org>]

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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