Re: 2.6.18-rc1-mm1

Andrew Morton
2006-07-09T10:45:31Z

On Sun, 9 Jul 2006 12:26:45 +0200
"Fabio Comolli" <fabio.comolli@gmail.com> wrote:

> =======================================================
> [ INFO: possible circular locking dependency detected ]
> -------------------------------------------------------
> cpuspeed/1520 is trying to acquire lock:
> (&policy->lock){--..}, at: [<c02c130f>] mutex_lock+0x21/0x24
>
> but task is already holding lock:
> (cpucontrol){--..}, at: [<c02c130f>] mutex_lock+0x21/0x24
>
> which lock already depends on the new lock.

Yeah, that's lock_cpu_hotplug(). We've made a complete and utter mess of
that thing.

And I don't know how to fix it, really. Is it a highly-localised innermost
lock? Or a broad-coverage outermost lock? Nobody knows, neither suits.

I'm suspecting is was a bad idea and we should just rip it out altogether.

- If a piece of kernel code is dealing with cpu-local data it needs to be
running atomically, and that'll hold off hot hotplug anyway.

- If a piece of kernel code is dealing with per-cpu data and cannot run
atomically then it should have its own cpu hotplug handlers anyway. It
is up to that code (ie: cpufreq) to provide its own locking against its
own CPU hotplug callback.

Voila, no more lock_cpu_hotplug().