Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init()

[David Hildenbrand <david@redhat.com>]

On 26.07.19 10:31, Michal Hocko wrote:
> On Fri 26-07-19 10:05:58, David Hildenbrand wrote:
>> On 26.07.19 09:57, Michal Hocko wrote:
>>> On Thu 25-07-19 22:49:36, David Hildenbrand wrote:
>>>> On 25.07.19 21:19, Michal Hocko wrote:
>>> [...]
>>>>> We need to rationalize the locking here, not to add more hacks.
>>>>
>>>> No, sorry. The real hack is calling a function that is *documented* to
>>>> be called under lock without it. That is an optimization for a special
>>>> case. That is the black magic in the code.
>>>
>>> OK, let me ask differently. What does the device_hotplug_lock actually
>>> protects from in the add_memory path? (Which data structures)
>>>
>>> This function is meant to be used when struct pages and node/zone data
>>> structures should be updated. Why should we even care about some device
>>> concept here? This should all be handled a layer up. Not all memory will
>>> have user space API to control online/offline state.
>>
>> Via add_memory()/__add_memory() we create memory block devices for all
>> memory. So all memory we create via this function (IOW, hotplug) will
>> have user space APIs.
> 
> Ups, I have mixed add_memory with add_pages which I've had in mind while
> writing that. Sorry about the confusion.

No worries :)

> 
> Anyway, my dislike of the device_hotplug_lock persists. I would really
> love to see it go rather than grow even more to the hotplug code. We
> should be really striving for mem hotplug internal and ideally range
> defined locking longterm. 

Yes, and that is a different story, because it will require major
changes to all add_memory() users. (esp, due to the documented race
conditions). Having that said, memory hotplug locking is not ideal yet.

-- 

Thanks,

David / dhildenb