Re: [Patch v4 14/16] x86, irq: Introduce helper to check whether an IOAPIC has been registered

[Thomas Gleixner]

On Thu, 28 Aug 2014, Jiang Liu wrote:

> Introduce acpi_ioapic_registered() to check whether an IOAPIC has already
> been registered, it will be used when enabling IOAPIC hotplug.
> 
> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/io_apic.h |    1 +
>  arch/x86/kernel/acpi/boot.c    |   11 +++++++++++
>  arch/x86/kernel/apic/io_apic.c |   11 +++++++++++
>  include/linux/acpi.h           |    1 +
>  4 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/include/asm/io_apic.h b/arch/x86/include/asm/io_apic.h
> index ce63cf34c93c..0db2b7037e4b 100644
> --- a/arch/x86/include/asm/io_apic.h
> +++ b/arch/x86/include/asm/io_apic.h
> @@ -191,6 +191,7 @@ extern void mp_unmap_irq(int irq);
>  extern int mp_register_ioapic(int id, u32 address, u32 gsi_base,
>  			      struct ioapic_domain_cfg *cfg);
>  extern int mp_unregister_ioapic(u32 gsi_base);
> +extern int mp_ioapic_registered(u32 gsi_base);
>  extern int mp_irqdomain_map(struct irq_domain *domain, unsigned int virq,
>  			    irq_hw_number_t hwirq);
>  extern void mp_irqdomain_unmap(struct irq_domain *domain, unsigned int virq);
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 560a6d1cb0f4..6aa796f77b71 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -810,6 +810,17 @@ int acpi_unregister_ioapic(acpi_handle handle, u32 gsi_base)
>  }
>  EXPORT_SYMBOL(acpi_unregister_ioapic);
>  
> +int acpi_ioapic_registered(acpi_handle handle, u32 gsi_base)
> +{
> +	int ret;
> +
> +	down_write(&acpi_ioapic_rwsem);

Why down_write? You are merily checking whether the thing is
registered already.

> +	ret  = mp_ioapic_registered(gsi_base);
> +	up_write(&acpi_ioapic_rwsem);
> +
> +	return ret;
> +}

So I assume that this is for a particular caller in the apci ioapic
hotplug code and that call site has its own serialization. Otherwise
the return value is not protected at all.

Please add a Docbook comment to that function, and document the
locking rules as thats pretty non obvious.

The such is missing in some other patches as well.

Thanks,

	tglx
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