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

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Date:   Fri, 10 Jan 2020 18:36:24 +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

On 10.01.20 18:33, Dan Williams wrote:
> On Fri, Jan 10, 2020 at 9:29 AM David Hildenbrand <david@...hat.com> wrote:
> [..]
>>> 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.
> 
> That's a large comment block when we could have just taken the lock.
> There's probably many other code paths in the kernel where some locks
> are not necessary before userspace is up, but the code takes the lock
> anyway to minimize the code maintenance burden. Is there really a
> compelling reason to be clever here?

It was a lengthy discussion back then and I was sharing your opinion. I
even had a patch ready to enforce that we are holding the lock (that's
how I identified that specific case in the first place).

-- 
Thanks,

David / dhildenb