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

[Greg Kroah-Hartman]

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@xxxxxxxxxxxxxxxxxxx>
> >> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> >> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> >> Cc: Saravana Kannan <saravanak@xxxxxxxxxx>
> >> Cc: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> ---
> >>
> >> 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