Date: Tue, 17 Jan 2006 23:37:34 -0800
From: Andrew Morton <akpm@osdl.org>
To: Nathan Lynch <ntl@pobox.com>
Cc: mingo@elte.hu, anton@au1.ibm.com, linux-kernel@vger.kernel.org,
       michael@ellerman.id.au, linuxppc64-dev@ozlabs.org, serue@us.ibm.com,
       paulus@au1.ibm.com
Subject: Re: 2.6.15-mm4 failure on power5
Message-Id: <20060117233734.506c2f2e.akpm@osdl.org>
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Nathan Lynch <ntl@pobox.com> wrote:
>
> Andrew Morton wrote:
> > Ingo Molnar <mingo@elte.hu> wrote:
> > > - so buggy early bootup code which relies on interrupts being 
> > > off might be surprised by it.
> > 
> > I don't think it's necessarily buggy that bootup code needs interrupts
> > disabled.  It _is_ buggy that bootup code which needs interrupts disabled
> > is calling lock_cpu_hotplug().
> 
> I guess I don't understand -- why is it wrong for code that runs only
> in early early bootup, when there is only one process context, to use
> common code to e.g. register a hotplug cpu notifier?

OK, it's not wrong I guess - we're running code which requires
local_irq_disable() and that code is calling functions which do
local_irq_enable() but we know that those functions won't do that because
there cannot be any lock contention.

So it works, and will continue to work, but it's all rather unpleasant, IMO.

>  Should the
> powerpc numa code be made to wait to register its notifier until
> initcall time or something?

I think the powerpc code is busted, really - it shouldn't be keeling over
like that if someone enables local interrupts.  That being said, it's a
good way of detecting accidental interrupt-enablings.

> Yes, which would be why this code never triggered a warning when
> cpucontrol was a semaphore.

Yup.  Perhaps a sane fix which preserves the unpleasant semantics is to do
irqsave in the mutex debug code.
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