On Wed, 2013-08-14 at 21:21 -0400, KOSAKI Motohiro wrote: > (8/12/13 3:34 PM), Toshi Kani wrote: > > lock_device_hotplug() serializes hotplug & online/offline operations. > > The lock is held in common sysfs online/offline interfaces and ACPI > > hotplug code paths. > > > > try_offline_node() off-lines a node if all memory sections and cpus > > are removed on the node. It is called from acpi_processor_remove() > > and acpi_memory_remove_memory()->remove_memory() paths, both of which > > are in the ACPI hotplug code. > > > > try_offline_node() calls stop_machine() to stop all cpus while checking > > all cpu status with the assumption that the caller is not protected from > > CPU hotplug or CPU online/offline operations. However, the caller is > > always serialized with lock_device_hotplug(). Also, the code needs to > > be properly serialized with a lock, not by stopping all cpus at a random > > place with stop_machine(). > > > > This patch removes the use of stop_machine() in try_offline_node() and > > adds comments to try_offline_node() and remove_memory() that > > lock_device_hotplug() is required. > > This patch need more verbose explanation. check_cpu_on_node() traverse cpus > and cpu hotplug seems to use cpu_hotplug_driver_lock() instead of lock_device_hotplug(). As described: | lock_device_hotplug() serializes hotplug & online/offline operations. | The lock is held in common sysfs online/offline interfaces and ACPI | hotplug code paths. And here are their code paths. - CPU & Mem online/offline via sysfs online store_online()->lock_device_hotplug() - Mem online via sysfs state: store_mem_state()->lock_device_hotplug() - ACPI CPU & Mem hot-add: acpi_scan_bus_device_check()->lock_device_hotplug() - ACPI CPU & Mem hot-delete: acpi_scan_hot_remove()->lock_device_hotplug() > That said, the race is not happen against another memeory happen. It's likely happen > another cpu hotplug. So commenting remove_memory() doesn't make much sense. This lock scheme protects from both CPU and memory hotplug, and requires the caller to call lock_device_hotplug(). Thanks, -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>