linux-kernel - Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan

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Date:   Fri, 26 Jul 2019 09:20:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in
 acpi_scan_init()

On 25.07.19 23:23, Rafael J. Wysocki wrote:
> On Thu, Jul 25, 2019 at 10:49 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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?
> 
> Well, I have ACKed your patch already. :-)

It's never to late to un-ACK if you changed your mind :)

> 
> That said, adding a comment stating that the lock is acquired mostly
> for consistency wouldn't hurt.
> 

I can certainly do that. Thanks!

-- 

Thanks,

David / dhildenb