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

[David Hildenbrand <david@redhat.com>]

On 25.07.19 21:19, Michal Hocko wrote:
> On Thu 25-07-19 16:35:07, David Hildenbrand wrote:
>> On 25.07.19 15:57, Michal Hocko wrote:
>>> On Thu 25-07-19 15:05:02, David Hildenbrand wrote:
>>>> On 25.07.19 14:56, Michal Hocko wrote:
>>>>> On Wed 24-07-19 16:30:17, David Hildenbrand wrote:
>>>>>> We end up calling __add_memory() without the device hotplug lock held.
>>>>>> (I used a local patch to assert in __add_memory() that the
>>>>>>  device_hotplug_lock is held - I might upstream that as well soon)
>>>>>>
>>>>>> [   26.771684]        create_memory_block_devices+0xa4/0x140
>>>>>> [   26.772952]        add_memory_resource+0xde/0x200
>>>>>> [   26.773987]        __add_memory+0x6e/0xa0
>>>>>> [   26.775161]        acpi_memory_device_add+0x149/0x2b0
>>>>>> [   26.776263]        acpi_bus_attach+0xf1/0x1f0
>>>>>> [   26.777247]        acpi_bus_attach+0x66/0x1f0
>>>>>> [   26.778268]        acpi_bus_attach+0x66/0x1f0
>>>>>> [   26.779073]        acpi_bus_attach+0x66/0x1f0
>>>>>> [   26.780143]        acpi_bus_scan+0x3e/0x90
>>>>>> [   26.780844]        acpi_scan_init+0x109/0x257
>>>>>> [   26.781638]        acpi_init+0x2ab/0x30d
>>>>>> [   26.782248]        do_one_initcall+0x58/0x2cf
>>>>>> [   26.783181]        kernel_init_freeable+0x1bd/0x247
>>>>>> [   26.784345]        kernel_init+0x5/0xf1
>>>>>> [   26.785314]        ret_from_fork+0x3a/0x50
>>>>>>
>>>>>> So perform the locking just like in acpi_device_hotplug().
>>>>>
>>>>> While playing with the device_hotplug_lock, can we actually document
>>>>> what it is protecting please? I have a bad feeling that we are adding
>>>>> this lock just because some other code path does rather than with a good
>>>>> idea why it is needed. This patch just confirms that. What exactly does
>>>>> the lock protect from here in an early boot stage.
>>>>
>>>> We have plenty of documentation already
>>>>
>>>> mm/memory_hotplug.c
>>>>
>>>> git grep -C5 device_hotplug mm/memory_hotplug.c
>>>>
>>>> Also see
>>>>
>>>> Documentation/core-api/memory-hotplug.rst
>>>
>>> OK, fair enough. I was more pointing to a documentation right there
>>> where the lock is declared because that is the place where people
>>> usually check for documentation. The core-api documentation looks quite
>>> nice. And based on that doc it seems that this patch is actually not
>>> needed because neither the online/offline or cpu hotplug should be
>>> possible that early unless I am missing something.
>>
>> I really prefer to stick to locking rules as outlined on the
>> interfaces if it doesn't hurt. Why it is not needed is not clear.
>>
>>>
>>>> Regarding the early stage: primarily lockdep as I mentioned.
>>>
>>> Could you add a lockdep splat that would be fixed by this patch to the
>>> changelog for reference?
>>>
>>
>> I have one where I enforce what's documented (but that's of course not
>> upstream and therefore not "real" yet)
> 
> Then I suppose to not add locking for something that is not a problem.
> Really, think about it. People will look at this code and follow the
> lead without really knowing why the locking is needed.
> device_hotplug_lock has its purpose and if the code in question doesn't
> need synchronization for the documented scenarios then the locking
> simply shouldn't be there. Adding the lock just because of a
> non-existing, and IMHO dubious, lockdep splats is just wrong.
> 
> 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.

The only alternative I see to this patch is adding a comment like

/*
 * We end up calling __add_memory() without the device_hotplug_lock
 * held. This is fine as we cannot race with other hotplug activities
 * and userspace trying to online memory blocks.
 */

Personally, I don't think that's any better than just grabbing the lock
as we are told to. (honestly, I don't see how optimizing away the lock
here is of *any* help to optimize our overall memory hotplug locking)

@Rafael, what's your take? lock or comment?

-- 

Thanks,

David / dhildenb