Re: [Xen-devel] [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock

[Pasha Tatashin <Pavel.Tatashin@xxxxxxxxxxxxx>]

On 8/21/18 6:44 AM, 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.
> 
> 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).
> 
> Now that we hold the device_hotplug_lock when
> - adding memory (e.g. via add_memory()/add_memory_resource())
> - removing memory (e.g. via remove_memory())
> - device_online()/device_offline()
> 
> We can move mem_hotplug_lock usage back into
> online_pages()/offline_pages().
> 
> Why is mem_hotplug_lock still needed? Essentially to make
> get_online_mems()/put_online_mems() be very fast (relying on
> device_hotplug_lock would be very slow), and to serialize against
> addition of memory that does not create memory block devices (hmm).
> 
> [1] http://driverdev.linuxdriverproject.org/pipermail/ driverdev-devel/
>     2015-February/065324.html
> 
> This patch is partly based on a patch by Vitaly Kuznetsov.

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