Converting dev->mutex into dev->spinlock ?

[Tetsuo Handa]

Hello.

There is a long-standing deadlock problem in driver core code caused by
"struct device"->mutex being marked as "do not apply lockdep checks".

We can make this deadlock visible by applying [1], and we can confirm that
there is a deadlock problem that I think needs to be addressed in core code [2].

Also, since driver developers are taking it for granted that driver callback
functions can behave as if dev->mutex is not held (because possibility of deadlock
was never reported), it would solve many deadlocks in driver code if you can update
driver core code to avoid calling driver callback functions with dev->mutex held
(by e.g. replacing dev->mutex with dev->spinlock and dev->atomic_flags).
But I'm not familiar enough to propose such change...

[1] https://lkml.kernel.org/r/8c3fc3d1-8fed-be22-e0e7-ef1e1ea723ce@xxxxxxxxxxxxxxxxxxx
[2] https://lkml.kernel.org/r/b7bc63c8-bb28-d21d-7c3f-97e4e79a9292@xxxxxxxxxxxxxxxxxxx