Messages in this thread | | | Date | Fri, 24 Jan 2020 10:00:52 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH v1] driver core: check for dead devices before onlining/offlining |
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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 problems with that in the past, look at how the scsi layer handled it, I think there's a specific call you should be making instead of trying to rely on this "dead" flag.
thanks,
greg k-h
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