linux-kernel - Re: [PATCH RT 3/4] mips-remove-smp-reserve-lock.patch

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Date:	Thu, 07 Jun 2012 12:47:19 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Ralf Baechle <ralf@...ux-mips.org>
CC:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH RT 3/4] mips-remove-smp-reserve-lock.patch

On 06/07/2012 12:32 PM, Steven Rostedt wrote:
> On Thu, 2012-06-07 at 12:08 -0700, David Daney wrote:
>
>>> Should it go to mainline stable?
>>>
>>
>> I don't think it is necessary.  As far as I know, RT may be the only
>> thing that needs it.
>
> Ah, you're right. As this is just an issue because it is called with
> interrupts disabled (from stop_machine). Although it's interesting that
> the mips code, re-enables interrupts from that function.
>
>  From kernel/cpu.c:
>
> _cpu_down() {
> 	 __stop_machine(take_cpu_down,&tcd_param, cpumask_of(cpu));
>
> take_cpu_down() {
> 	err = __cpu_disable();
>
> kernel/stop_machine.c:
>
> __stop_machine(int (*fn)(void *) ...) {
> 	local_irq_save(flags);
> 	hard_irq_disable();
> 	ret = (*fn)(data);
> 	local_irq_restore(flags);
>
>
> arch/mips/include/asm/smp.h:
>
> static inline int __cpu_disable(void)
> {
>          extern struct plat_smp_ops *mp_ops;     /* private */
>
>          return mp_ops->cpu_disable();
> }
>
> arch/mips/cavium-octeon/smp.c:
>
> octeon_cpu_disable(void) {
>          local_irq_disable();
>          fixup_irqs();
>          local_irq_enable();
>
> struct plat_smp_ops octeon_smp_ops = {
>          .cpu_disable            = octeon_cpu_disable,
>
>
> Is this expected? It causes the cpu notifiers to be called with
> interrupts enabled. Not sure if that's a problem or not.

I am inclined to go with your instinct here.  Probably we shouldn't 
unconditionally local_irq_enable() here.

Perhaps {,raw}_local_irq_save/{,raw}local_irq_restore would be better. 
Or even no local irq enable manipulation...

In any event, I may let Ralf sort it out.

David Daney


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