> Am 11.01.2020 um 14:56 schrieb Qian Cai <cai@xxxxxx>: > > > >> On Jan 11, 2020, at 6:03 AM, David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> So I just remember why I think this (and the previously reported done >> for ACPI DIMMs) are false positives. The actual locking order is >> >> onlining/offlining from user space: >> >> kn->count -> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock >> >> memory removal: >> >> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock -> kn->count >> >> >> This looks like a locking inversion - but it's not. Whenever we come via >> user space we do a mutex_trylock(), which resolves this issue by backing >> up. The device_hotplug_lock will prevent >> >> I have no clue why the device_hotplug_lock does not pop up in the >> lockdep report here. Sounds wrong to me. >> >> I think this is a false positive and not stable material. > > The point is that there are other paths does kn->count —> cpu_hotplug_lock without needing device_hotplug_lock to race with memory removal. > > kmem_cache_shrink_all+0x50/0x100 (cpu_hotplug_lock.rw_sem/mem_hotplug_lock.rw_sem) > shrink_store+0x34/0x60 > slab_attr_store+0x6c/0x170 > sysfs_kf_write+0x70/0xb0 > kernfs_fop_write+0x11c/0x270 ((kn->count) > __vfs_write+0x3c/0x70 > vfs_write+0xcc/0x200 > ksys_write+0x7c/0x140 > system_call+0x5c/0x6 > But not the lock of the memory devices, or am I missing something?
