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: > > > > 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@xxxxxxxxxxxxxx> > > > > > > > > The change looks good to me. > > > > > > > > Acked-by: Toshi Kani<toshi.kani@xxxxxx> > > > > > > > > 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. I suppose sysfs keeps *dev valid with ref_count (Is that right?). 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). Thanks, -Toshi -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
