linux-kernel - Re: [PATCH] mm/memory_hotplug: Fix remove

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Date:   Fri, 10 Jan 2020 18:29:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/memory_hotplug: Fix remove_memory() lockdep splat

>>> Why make someone dig for the reasons this lock is sufficient?
>>
>> I think 5 LOC of comment are too much for something that is documented
>> e.g., in Documentation/core-api/memory-hotplug.rst ("Locking
>> Internals"). But whatever you prefer.
> 
> Sure, lets beef up that doc to clarify this case and refer to it.

Referring is a good idea. We should change the "is advised" for the device_online()
to a "is required" or similar. Back then I wasn't sure how it all worked in
detail...

>>>> I properly documented the semantics of
>>>> add_memory_block_devices()/remove_memory_block_devices() already (that
>>>> they need the device hotplug lock).
>>>
>>> I see that, but I prefer lockdep_assert_held() in the code rather than
>>> comments. I'll send a patch to fix that up.
>>
>> That won't work as early boot code from ACPI won't hold it while it adds
>> memory. And we decided (especially Michal :) ) to keep it like that.
> 
> So then the comment is actively misleading for that case. I would
> expect an explicit _unlocked path for that case with a comment about
> why it's special. Is there already a comment to that effect somewhere?
> 

__add_memory() - the locked variant - is called from the same ACPI location
either locked or unlocked. I added a comment back then after a longe
discussion with Michal:

drivers/acpi/scan.c:
	/*
	 * Although we call __add_memory() that is documented to require the
	 * device_hotplug_lock, it is not necessary here because this is an
	 * early code when userspace or any other code path cannot trigger
	 * hotplug/hotunplug operations.
	 */


It really is a special case, though.

-- 
Thanks,

David / dhildenb