linux-kernel - Re: [PATCH] ACPI, cpu hotplug: move try_offline_node() after acpi_unmap

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Date:	Sat, 10 Aug 2013 01:29:08 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Toshi Kani <toshi.kani@...com>
Cc:	Tang Chen <tangchen@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	rafael.j.wysocki@...el.com, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wen Congyang <wency@...fujitsu.com>
Subject: Re: [PATCH] ACPI, cpu hotplug: move try_offline_node() after acpi_unmap_lsapic()

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:
> > > On Tue, 2013-08-06 at 19:11 +0900, Yasuaki Ishimatsu wrote:
> > >> try_offline_node() checks that all cpus related with removed node have been
> > >> removed by using cpu_present_bits. If all cpus related with removed node have
> > >> been removed, try_offline_node() clears the node information.
> > >>
> > >> But try_offline_node() called from acpi_processor_remove() never clears
> > >> the node information. For disabling cpu_present_bits, acpi_unmap_lsapic()
> > >> need be called. But acpi_unmap_lsapic() is called after try_offline_node()
> > >> runs. So when try_offline_node() runs, the cpu's cpu_present_bits is always
> > >> set.
> > >>
> > >> This patch moves try_offline_node() after acpi_unmap_lsapic().
> > >>
> > >> Signed-off-by: Yasuaki Ishimatsu<isimatu.yasuaki@...fujitsu.com>
> > >
> > > The change looks good to me.
> > >
> > > Acked-by: Toshi Kani<toshi.kani@...com>
> > >
> > > 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?

> 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?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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