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

[Nicholas Piggin]

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.

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.

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