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 14:31:23 +0100 |
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On 24.01.20 10:12, Greg Kroah-Hartman wrote: > 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.
I think you are talking about doing a "transport_remove_device(dev)" before doing the "device_del(dev)", combined with proper locking.
Will look into that for the memory subsystem ...
Thanks!
-- Thanks,
David / dhildenb
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