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



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