Re: [PATCH] slub: Fix sysfs circular locking dependency

[KAMEZAWA Hiroyuki]

On Thu, 6 Jan 2011 12:10:59 -0600 (CST)
Christoph Lameter <cl@xxxxxxxxx> wrote:

> On Thu, 6 Jan 2011, David Rientjes wrote:
> 
> > > Thanks for testing. David, does Christoph's patch look OK to you?
> > >
> >
> > I think it certainly fixes the problem at hand, but I think we also need
> > to do lock_memory_hotplug() for memory hotplug in
> > slab_mem_going_online_callback() to make show_slab_objects() consistent
> > when being printed during concurrent node hot-add since it sets bits in
> > N_NORMAL_MEMORY.  The MEM_OFFLINE callback is already handled at a higher
> > level by taking the lock in the hotplug layer, but we need to protect the
> > MEM_GOING_ONLINE and MEM_CANCEL_ONLINE callbacks if slub_lock is no longer
> > used to protect node arrays (which was admittedly always convenient since
> > it's typically associated with an iteration through slab_caches).
> 
> Hmm, I would have expected the callbacks all to be done under hotplug
> locking.
> 
> The MEM_GOING_ONLINE callback is not that critical since a node that is
> coming online presumably has only a minimal set of objects necessary for
> potential future allocations.
> 
> slab_mem_going_online_callback() etc already take the slub_lock since they
> have to iterate over the list of slab caches in existence. We could take
> the hotplug lock there as well.
> 
> Kame-san: Can you enlighten us on hotplug locking? And also check this
> patch?
> 

IIRC, lock_memory_hotplug() is a new lock in 2.6.37 added by Kosaki

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=20d6c96b5f1cad5c5da4641945ec17a1d9a1afc8

as bugfix.

This lock is for avoiding race with hwpoison and memory hotplug and original
lock (before replacement) was for avoiding race with hibernation. online_pages()
was out of lock because it just makes PG_reserved page to be free page, not racy
with hibernation.

But, nice catch. I think MEM_GOING_ONLINE, MEM_ONLINE should be done under
lock_memory_hotplug. So, could you update your patch and modify online_pages() ?

IIUC, online_pages() is an user interface function and there will be no downside
to insert lock there. online_pages() should be serialized.

Thanks,
-Kame





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