Messages in this thread | | | Subject | Re: [PATCH v1] driver core: check for dead devices before onlining/offlining | From | David Hildenbrand <> | Date | Fri, 24 Jan 2020 10:09:03 +0100 |
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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.
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))?
-- Thanks,
David / dhildenb
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