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. Anyway, it looks to me that cpu_up()->mem_online_node() path can race with try_offline_node(). And I do not think stop_machine() protects this case, either. try_offline_node() stops the execution of num_online_node() during stop_machine(), but then lets it continues to run after that. I think they need to be protected by a lock. I will look further to see if I can come up with some solution. 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
