Re: [v2 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM

[David Hildenbrand <david@redhat.com>]

On 25.04.19 14:30, Pavel Tatashin wrote:
>>
>> Yes, also I think you can let go of the device_lock in
>> check_memblocks_offline_cb, lock_device_hotplug() should take care of
>> this (see Documentation/core-api/memory-hotplug.rst - "locking internals")
>>
> Hi David,
> 
> Thank you for your comments. I went through memory-hotplug.rst, and I
> still think that device_lock() is needed here. In this particular case
> it can be replaced with something like READ_ONCE(), but for simplicity
> it is better to have device_lock()/device_unlock() as this is not a
> performance critical code.
> 
> I do not see any lock ordering issues with this code, as we are
> holding lock_device_hotplug() first that prevents userland from
> adding/removing memory during this check.

Yes, lock ordering is not an issue, I rather think that the device
hotplug lock will guard us in all situations. E.g. remove_memory() also
does not use it when checking if all blocks are offline. But you can
leave it in if you think it is needed.

> 
> https://soleen.com/source/xref/linux/arch/powerpc/platforms/powernv/memtrace.c?r=98fa15f3#248
> 
> Here we have a similar code:
> lock_device_hotplug();
>    online_mem_block();
>     device_online()
>      device_lock(dev);
> 
> Pasha
> 

-- 

Thanks,

David / dhildenb