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

[Jiang Liu <jiang.liu@linux.intel.com>]

On 2014/9/9 20:37, Thomas Gleixner wrote:
> 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@linux.intel.com>
>> ---
>>  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.
Yeah, a down_read() should be enough:)

>> +	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.
How about this comments about locking rules?
/*
 * Locks related to IOAPIC hotplug
 * Hotplug side:
 *      ->lock_device_hotplug() //device_hotplug_lock
 *              ->acpi_ioapic_rwsem
 *                      ->ioapic_lock
 * Interrupt mapping side:
 *      ->acpi_ioapic_rwsem
 *              ->ioapic_mutex
 *                      ->ioapic_lock
 */
Regards!
Gerry
> 
> The such is missing in some other patches as well.
> 
> Thanks,
> 
> 	tglx
>