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Subject: Re: [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages
 called w.o. mem_hotplug_lock
To:     Rashmica <rashmica.g@gmail.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-acpi@vger.kernel.org,
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From:   David Hildenbrand <david@redhat.com>
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Date:   Mon, 17 Sep 2018 09:32:27 +0200
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Am 03.09.18 um 02:36 schrieb Rashmica:
> Hi David,
> 
> 
> On 21/08/18 20:44, David Hildenbrand wrote:
> 
>> There seem to be some problems as result of 30467e0b3be ("mm, hotplug:
>> fix concurrent memory hot-add deadlock"), which tried to fix a possible
>> lock inversion reported and discussed in [1] due to the two locks
>> 	a) device_lock()
>> 	b) mem_hotplug_lock
>>
>> While add_memory() first takes b), followed by a) during
>> bus_probe_device(), onlining of memory from user space first took b),
>> followed by a), exposing a possible deadlock.
> 
> Do you mean "onlining of memory from user space first took a),
> followed by b)"? 

Very right, thanks.

> 
>> In [1], and it was decided to not make use of device_hotplug_lock, but
>> rather to enforce a locking order.
>>
>> The problems I spotted related to this:
>>
>> 1. Memory block device attributes: While .state first calls
>>    mem_hotplug_begin() and the calls device_online() - which takes
>>    device_lock() - .online does no longer call mem_hotplug_begin(), so
>>    effectively calls online_pages() without mem_hotplug_lock.
>>
>> 2. device_online() should be called under device_hotplug_lock, however
>>    onlining memory during add_memory() does not take care of that.
>>
>> In addition, I think there is also something wrong about the locking in
>>
>> 3. arch/powerpc/platforms/powernv/memtrace.c calls offline_pages()
>>    without locks. This was introduced after 30467e0b3be. And skimming over
>>    the code, I assume it could need some more care in regards to locking
>>    (e.g. device_online() called without device_hotplug_lock - but I'll
>>    not touch that for now).
> 
> Can you mention that you fixed this in later patches?

Sure!

> 
> 
> The series looks good to me. Feel free to add my reviewed-by:
> 
> Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
> 

Thanks, r-b only for this patch or all of the series?

-- 

Thanks,

David / dhildenb