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

Jiang Liu
2014-09-10T02:46:43Z

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
>