linux-kernel - Re: MAX_LOCKDEP_SUBCLASSES too low in 2.6.23-rc7-mm1

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Date:	Mon, 24 Sep 2007 18:37:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...hat.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Dave Hansen <haveblue@...ibm.com>,
	Alexey Dobriyan <adobriyan@...ru>
Subject: Re: MAX_LOCKDEP_SUBCLASSES too low in 2.6.23-rc7-mm1

On Mon, 24 Sep 2007 12:03:58 -0400 Mathieu Desnoyers
<mathieu.desnoyers@...ymtl.ca> wrote:

> Hi Ingo,
> 
> I got the following printk when booting 2.6.23-rc7-mm1 :
> 
> BUG: MAX_LOCKDEP_SUBCLASSES too low!
> turning off the locking correctness validator.
> 
> Is it known/correct situation ?

Known, bug in r/o mounts code:

> static void lock_and_coalesce_cpu_mnt_writer_counts(void)
> {
> 	int cpu;
> 	struct mnt_writer *cpu_writer;
> 
> 	for_each_possible_cpu(cpu) {
> 		cpu_writer = &per_cpu(mnt_writers, cpu);
> 		spin_lock_nested(&cpu_writer->lock, 42);
> 		                                   ^^^^
> 
> What the heck is going on? Given that MAX_LOCKDEP_SUBCLASSES is only 8,
> this reliably turns off lockdep at boot.
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