[RFC PATCH v4 11/12] powerpc: Add a Kconfig and a function to set new soft_enabled mask

[Madhavan Srinivasan]

On Monday 29 August 2016 07:11 AM, Nicholas Piggin wrote:
> On Mon, 29 Aug 2016 00:07:27 +0530
> Madhavan Srinivasan <maddy at linux.vnet.ibm.com> wrote:
>
>> New Kconfig is added "CONFIG_IRQ_DEBUG_SUPPORT" to add a warn_on
>> to alert the usage of soft_irq_set_mask() for disabling lower
>> bitmask interrupts.
>>
>> Have also moved the code under the CONFIG_TRACE_IRQFLAGS in
>> arch_local_irq_restore() to new Kconfig as suggested.
>>
>> Patch also adds a new soft_irq_set_mask() to update paca->soft_enabled.
>>
>> Signed-off-by: Madhavan Srinivasan <maddy at linux.vnet.ibm.com>
>> ---
>>   arch/powerpc/Kconfig              |  4 ++++
>>   arch/powerpc/include/asm/hw_irq.h | 17 +++++++++++++++++
>>   arch/powerpc/kernel/irq.c         |  4 ++--
>>   3 files changed, 23 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 927d2ab2ce08..878f05925340 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -51,6 +51,10 @@ config TRACE_IRQFLAGS_SUPPORT
>>   	bool
>>   	default y
>>   
>> +config IRQ_DEBUG_SUPPORT
>> +	bool
>> +	default n
>> +
>>   config LOCKDEP_SUPPORT
>>   	bool
>>   	default y
>> diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
>> index 415734c07cfa..9f71559ce868 100644
>> --- a/arch/powerpc/include/asm/hw_irq.h
>> +++ b/arch/powerpc/include/asm/hw_irq.h
>> @@ -81,6 +81,23 @@ static inline unsigned long arch_local_irq_disable(void)
>>   	return flags;
>>   }
>>   
>> +static inline unsigned long soft_irq_set_mask(int value)
>> +{
>> +	unsigned long flags, zero;
>> +
>> +#ifdef CONFIG_IRQ_DEBUG_SUPPORT
>> +	WARN_ON(value <= IRQ_DISABLE_MASK_LINUX);
>> +#endif
>> +	asm volatile(
>> +		"li %1,%3; lbz %0,%2(13); stb %1,%2(13)"
>> +		: "=r" (flags), "=&r" (zero)
>> +		: "i" (offsetof(struct paca_struct, soft_enabled)),\
>> +		 "i" (value)
>> +		: "memory");
>> +
>> +	return flags;
>> +}
> One other thing, if we have:
>
> local_irq_save(flags);              // disable LINUX mask -> LINUX
> local_irq_and_pmu_save(flags);      // disable PMU   mask -> LINUX|PMU
> local_irq_and_pmu_restore(flags);   // enable  PMU   mask -> LINUX
> local_irq_restore(flags);           // enable  LINUX mask -> NONE
>
> Then the nested code that re-enables PMUs will not replay PMU interrupt
> until the outer irq is re-enabled. I don't *think* this is a problem,
> but it probably should be commented.

Yes. That is right. Will comment.

>
> Which brings us to arch_local_irq_restore() (from another patch):
>
>
> @@ -208,7 +209,7 @@ notrace void arch_local_irq_restore(unsigned long en)
>
>   	/* Write the new soft-enabled value */
>   	set_soft_enabled(en);
> -	if (!en)
> +	if (en == IRQ_DISABLE_MASK_LINUX)
>   		return;
>   	/*
>   	 * From this point onward, we can take interrupts, preempt,
>
> I think this is a bit buggy for some cases of nested disables. For the
> above it is okay, but if we have:
>
> local_irq_and_pmu_save(flags);
> local_irq_and_pmu_save(flags);
> local_irq_and_pmu_restore(flags);
> local_irq_and_pmu_restore(flags);
>
>
> The first restore will restore LINUX|PMU mask, which will not be caught
> by this check.

Nice catch :). Yes, this should be non-zero check.

Also looking more closing, another possible issue is with this seq,

local_irq_and_pmu_save(flags);
local_irq_save(flags);

Currently we do just store in the set_soft_enabled(), which will
reset the PMI bit in the above seq. So need to rework on the 
set_soft_enabled().

Maddy
> Testing instead for non-zero (any IRQ bits masked) should work. We should
> probably also add an IRQ_DEBUG_SUPPORT check to ensure LINUX bit is always
> one of the outer-most bits to be cleared (theoretically we could support
> masking other bits without masking LINUX, but let's disallow that until a
> compelling use case shows up).
>
> Thanks,
> Nick
>