Re: [PATCH v1] driver core: check for dead devices before onlining/offlining

[David Hildenbrand <david@redhat.com>]

On 24.01.20 14:31, David Hildenbrand wrote:
> 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 ...

... looking into transports, it most probably does not apply here, hmm ...

-- 
Thanks,

David / dhildenb