Re: [PATCH] ACPI, cpu hotplug: move try_offline_node() after acpi_unmap_lsapic()

[Toshi Kani <toshi.kani@hp.com>]

On Sun, 2013-08-11 at 22:42 +0200, Rafael J. Wysocki wrote:
> On Friday, August 09, 2013 08:11:19 PM Toshi Kani wrote:
> > On Sat, 2013-08-10 at 01:29 +0200, Rafael J. Wysocki wrote:
> > > On Friday, August 09, 2013 04:16:56 PM Toshi Kani wrote:
> > > > On Fri, 2013-08-09 at 15:28 +0800, Tang Chen wrote:
> > > > > On 08/07/2013 12:56 AM, Toshi Kani wrote:
 :
> > > > > > BTW, do you know why try_offline_node() has to use stop_machine()?
> > > > > 
> > > > > try_offline_node() is used to check if the node could be hot-removed
> > > > > after each memory or cpu hot-remove operation.
> > > > > 
> > > > > In memory hot-remove path, we have lock_memory_hotplug() to series all
> > > > > the memory hot-remove options.
> > > > > 
> > > > > But when doing cpu hot-remove,
> > > > > 
> > > > > acpi_processor_remove()
> > > > >   |->try_offline_node()
> > > > > 
> > > > > There is no lock to protect it. I think, when we are going to hot-remove
> > > > > a node, others should not do any memory or cpu hotplug operation. In memory
> > > > > hotplug path, we have lock_memory_hotplug(). But in cpu hotplug path, I
> > > > > didn't find any lock. So we used stop_machine() to call check_cpu_on_node().
> > > > > If we find any cpu still present, we return and do not remove the node.
> > > > 
> > > > CPU/Memory hotplug operations and sysfs eject are serialized with
> > > > acpi_os_hotplug_execute().  CPU online/offline is protected by
> > > > cpu_hotplug_[begin|done]() and [get|put]_online_cpus().  But, yes,
> > > > online/offline and hotplug operations are not serialized.  I tried to
> > > > serialize them before, but that framework was not well received.
> > > 
> > > What about lock_device_hotplug()?  It is taken by both online/offline and
> > > the ACPI hotplug code, isn't it?
> > 
> > Oh, that's right!  I forgot about this one.  Yes, lock_device_hotplug()
> > nicely protects online/offline and hotplug operations. :-)
> > 
> > > > Anyway, it looks to me that cpu_up()->mem_online_node() path can race
> > > > with try_offline_node().
> > > 
> > > It can in principle, but I'm not sure if there's a way to trigger that
> > > race.  Do you have an example?
> > 
> > With lock_device_hotplug(), I agree that we do not have this race
> > condition -- cpu_up() may not run while other hotplug is running.
> > store_online() will be blocked at lock_device_hotplug() in such case.
> > When store_online() acquired the lock, this CPU may have been deleted.
> > So, we still need to make sure that this case is handled properly.
> 
> Yes.
> 
> > I suppose sysfs keeps *dev valid with ref_count (Is that right?).
> 
> Yes, it does.
> 
> > I think cpu_up() needs to check with cpu_present(), not cpu_possible(), at
> > the top.  Otherwise, cpu_to_node(cpu) may return NUMA_NO_NODE (-1), which is
> > probably not a good value for node_online(nid).
> 
> We do cpu_to_node(cpuid) in cpu_subsys_online() before that, so maybe
> it's better to check the result already there and bail out if that's
> negative?
> 
> Something like the patch below.

That looks good to me.

Thanks,
-Toshi