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SubjectRe: [PATCH] USB: serial: fix sysfs-attribute removal deadlock
On Fri, 25 Apr 2014, Li Zhong wrote:

> > No, this isn't self removal. The driver-attribute (not device-attribute)
> > store operation simply grabs a lock that is also held while the driver
> > is being deregistered at module unload. Taking a reference to the module
> > in this case will prevent deregistration while store is running.
> >
> > But it seems like this can be solved for usb-serial by simply not
> > holding the lock while deregistering.
>
> I didn't look carefully about this lock.
>
> But I'm not sure whether there are such requirements for driver
> attributes:
>
> some lock needs be grabbed in the driver attributes store callbacks, and
> the same lock also needs to be grabbed during driver unregister.

In this case, the lock does _not_ need to be grabbed during driver
unregister. The driver grabs the lock, but it doesn't need to.

> If we have such requirements currently or in the future, I think they
> could all be solved by breaking active protection after get the module
> reference.

No! That would be very bad.

Unloading modules is quite different from unbinding drivers. After the
driver is unbound, its attribute callback routines can continue to run.
But after a driver module has been unloaded, its attribute callback
routines _cannot_ run because they aren't present in memory any more.

If we allowed a module to be unloaded while one of its callbacks was
running (because active protection was broken), imagine what would
happen...

Alan Stern



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