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

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Date:   Thu, 25 Jul 2019 15:05:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        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 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

Regarding the early stage: primarily lockdep as I mentioned.

-- 

Thanks,

David / dhildenb