Re: [PATCH RFCv2 2/6] mm/memory_hotplug: make add_memory() take the device_hotplug_lock

[Pasha Tatashin <Pavel.Tatashin@microsoft.com>]

On 8/21/18 6:44 AM, David Hildenbrand wrote:
> add_memory() currently does not take the device_hotplug_lock, however
> is aleady called under the lock from
> 	arch/powerpc/platforms/pseries/hotplug-memory.c
> 	drivers/acpi/acpi_memhotplug.c
> to synchronize against CPU hot-remove and similar.
> 
> In general, we should hold the device_hotplug_lock when adding memory
> to synchronize against online/offline request (e.g. from user space) -
> which already resulted in lock inversions due to device_lock() and
> mem_hotplug_lock - see 30467e0b3be ("mm, hotplug: fix concurrent memory
> hot-add deadlock"). add_memory()/add_memory_resource() will create memory
> block devices, so this really feels like the right thing to do.
> 
> Holding the device_hotplug_lock makes sure that a memory block device
> can really only be accessed (e.g. via .online/.state) from user space,
> once the memory has been fully added to the system.
> 
> The lock is not held yet in
> 	drivers/xen/balloon.c
> 	arch/powerpc/platforms/powernv/memtrace.c
> 	drivers/s390/char/sclp_cmd.c
> 	drivers/hv/hv_balloon.c
> So, let's either use the locked variants or take the lock.
> 
> Don't export add_memory_resource(), as it once was exported to be used
> by XEN, which is never built as a module. If somebody requires it, we
> also have to export a locked variant (as device_hotplug_lock is never
> exported).

Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>