On Tue, Sep 01, 2015 at 03:12:34PM +0800, Jiang Liu wrote: > Hi Rafael and Tejun, > When running CPU hotplug tests, it triggers an lockdep warning > as follow. The two possible deadlock paths are: > 1) echo x > /sys/devices/system/cpu/cpux/online > ->kernfs_fop_write() > ->kernfs_get_active() > 1.a) ->rwsem_acquire_read(&kn->dep_map, 0, 1, _RET_IP_); > ->cpu_up() > 1.b) ->cpu_hotplug_begin()[lock_map_acquire(&cpu_hotplug.dep_map)] > 2) hardware triggers hotplug evetns > ->acpi_device_hotplug() > ->acpi_processor_remove() > 2.a) ->cpu_hotplug_begin()[lock_map_acquire(&cpu_hotplug.dep_map)] > ->unregister_cpu() > ->device_del() > ->kernfs_remove_by_name_ns() > ->__kernfs_remove() > ->kernfs_drain() > 2.b) ->rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_) > > So there is a possible deadlock scenario among 1.a, 1.b, 2.a and 2.b. > I'm not familiar with kernfs, so could you please help to comment: > 1) whether is a real deadlock issue? Yes, it seems to be. It's highly unlikely but still possible. > 2) any recommended way to get it fixed? This usually happens with "delete" files and it's worked around by performing special self-removal on the file before actually removing the device. I suppose on/offline files would need to turn off active_protection with kernfs_[un]break_active_protection() which should probably grow sysfs and device layer wrappers. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-hotplug" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
