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

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Date:   Fri, 26 Jul 2019 10:05:58 +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 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.

Sorry, I can't follow what you are saying here - are you confusing the
function we are talking about with arch_add_memory() ? (where I pulled
out the creation of memory block devices)

-- 

Thanks,

David / dhildenb