Messages in this thread | | | Date | Fri, 24 Jan 2020 10:12:21 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH v1] driver core: check for dead devices before onlining/offlining |
| |
On Fri, Jan 24, 2020 at 10:09:03AM +0100, David Hildenbrand wrote: > On 24.01.20 10:00, Greg Kroah-Hartman wrote: > > On Mon, Jan 20, 2020 at 11:49:09AM +0100, David Hildenbrand wrote: > >> We can have rare cases where the removal of a device races with > >> somebody trying to online it (esp. via sysfs). We can simply check > >> if the device is already removed or getting removed under the dev->lock. > >> > >> E.g., right now, if memory block devices are removed (remove_memory()), > >> we do a: > >> > >> remove_memory() -> lock_device_hotplug() -> mem_hotplug_begin() -> > >> lock_device() -> dev->dead = true > >> > >> Somebody coming via sysfs (/sys/devices/system/memory/memoryX/online) > >> triggers a: > >> > >> lock_device_hotplug_sysfs() -> device_online() -> lock_device() ... > >> > >> So if we made it just before the lock_device_hotplug_sysfs() but get > >> delayed until remove_memory() released all locks, we will continue > >> taking locks and trying to online the device - which is then a zombie > >> device. > >> > >> Note that at least the memory onlining path seems to be protected by > >> checking if all memory sections are still present (something we can then > >> get rid of). We do have other sysfs attributes > >> (e.g., /sys/devices/system/memory/memoryX/valid_zones) that don't do any > >> such locking yet and might race with memory removal in a similar way. For > >> these users, we can then do a > >> > >> device_lock(dev); > >> if (!device_is_dead(dev)) { > >> /* magic /* > >> } > >> device_unlock(dev); > >> > >> Introduce and use device_is_dead() right away. > >> > >> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > >> Cc: "Rafael J. Wysocki" <rafael@kernel.org> > >> Cc: Suzuki K Poulose <suzuki.poulose@arm.com> > >> Cc: Saravana Kannan <saravanak@google.com> > >> Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com> > >> Cc: Dan Williams <dan.j.williams@intel.com> > >> Cc: Michal Hocko <mhocko@kernel.org> > >> Signed-off-by: David Hildenbrand <david@redhat.com> > >> --- > >> > >> Am I missing any obvious mechanism in the device core that handles > >> something like this already? (especially also for other sysfs attributes?) > > > > So is a sysfs attribute causing the device itself to go away? We have > > nope, removal is triggered via the driver, not via a sysfs attribute.
But the idea is the same, it comes from the driver, not the driver core.
> Regarding this patch: Is there anything prohibiting the possible > scenario I document above (IOW, is this patch applicable, or is there > another way to fence it properly (e.g., the "specific call" you mentioned))?
I think it's the same thing, look at how scsi does it.
thanks,
greg k-h
| |