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From: Arnd Bergmann <arnd@arndb.de>
To: linuxppc-dev@ozlabs.org
Subject: Re: [RFC] [patch 4/6 -rt] powerpc 2.6.20-rt8: fix a runtime warnings for xmon
Date: Wed, 7 Mar 2007 12:06:51 +0100
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Cc: Ingo Molnar <mingo@elte.hu>, Tsutomu OWA <tsutomu.owa@toshiba.co.jp>,
       linux-kernel@vger.kernel.org
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On Wednesday 07 March 2007, Ingo Molnar wrote:
> i'm not an xmon expert, but maybe it might make more sense to first 
> disable preemption, then interrupts - otherwise you could be preempted 
> right after having disabled these interrupts (and be scheduled to 
> another CPU, etc.). What is the difference between local_irq_save() and 
> the above 'disable interrupts' sequence? If it's not the same and 
> xmon_core() relied on having hardirqs disabled then it might make sense 
> to do a local_irq_save() there, instead of a preempt_disable().

Since relatively recently, powerpc does no longer actually disable
the hardware interrupts with local_irq_disable(), but rather sets
a per-cpu flag that will be checked if an actual interrupt comes
in as part of the critical section.

The mtmsr() sequence in xmon corresponds to hard_irq_disable()
and should probably changed to that, but then you still need
the extra preempt_disable() / preempt_enable().

I think you're right about the sequence having to be
1. preempt_disable()
2. hard_irq_disable()
3.
4. hard_irq_enable()
5. preempt_enable()

	Arnd <><