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SubjectRe: Lockdep problem
Hi,

2008/3/8, Peter Zijlstra <peterz@infradead.org>:
>
> On Sat, 2008-03-08 at 20:10 +0300, Dmitry wrote:
> > Hi,
> >
> > I've ran into the lockdep problem: I'm hitting the condition at the
> > lockdep.c:2437:
> > if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
> > return 0;
> >
> > What does it mean and how to overcome it? Dumbly increasing constants
> > at kernel/lockdep_internals.h didn't help.
>
>
> It means you have more lockdep classes (lock instantiation sites,
> explicit class keys etc..) that we have static storage reserved for.
>
> One way to quickly achieve this is load/unload modules. Zapping the
> classes on module unload does not make the space available again.
>
> Sadly most setups these days try to load every module under the sun,
> wasting valuable boot time, and causing this head-ache.

Unfortunately this happens before modules loading --- it's kernel itself.

>
> Someone reported to me increasing MAX_LOCKDEP_KEYS_BITS to 15 (IIRC)
> made his distro build work again. Of course, cycle reloading a module
> will eventually run you out of space anyway.
>
>
> > The warning is pretty fatal for me because it happens when I'm locking
> > clocks lock, so when serial console output calls
> > clk_enable/clk_disable the kernel deadlocks.
>
>
> Not sure I fully understand what you mean here, but I've never seen this
> result in anything but a warning, the kernel will just continue..
>
> Or are you trying to debug a deadlock and can't because lockdep runs out
> of classes before it gets to the offending code?

Actually no and yes. IIUC, the call chain looks like this (it's with
modified clk's functions, so
it may be a bit hard to understand. If you wish, search for "clocklib"
patches posted to LKML
nearly a month ago).
kernel -> clk_enable() takes global clock spinlock -> my function
which calls another spin_lock which causes lockdep warning.
If I disable global clocks spinlock locking, I get the lockdep
warning. Otherwise, I see no output. This seems to be related to the
fact that I have pxa serial console enabled which in turn also uses
clk_enable/clk_disable to manage its serial port.


>
> Something you can do is build a kernel without (or very few modules) - a
> good idea anyway as turning off all that unused stuff will significantly
> improve build times too :-)

I have a plenty of modules configured to build, but as I hope this
doesn't count, since they aren't loaded

--
With best wishes
Dmitry


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