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

[Rashmica Gupta <rashmica.g@xxxxxxxxx>]

On Mon, 2018-09-17 at 09:32 +0200, David Hildenbrand wrote:
> 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@xxxxxxxxx>
> > 
> 
> Thanks, r-b only for this patch or all of the series?

Sorry, I somehow missed this. To all of the series.
> 

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