uio regression in 4.18-rc1 due to "uio: Prevent device destruction while fds are open"

[Mike Christie]

Hi Hamish,

I am hitting a regression with your patch:

commit a93e7b331568227500186a465fee3c2cb5dffd1f
Author: Hamish Martin <hamish.martin@xxxxxxxxxxxxxxxxxxx>
Date:   Mon May 14 13:32:23 2018 +1200

    uio: Prevent device destruction while fds are open

The problem is the addition of spin_lock_irqsave in uio_write. This
leads to hitting  uio_write -> copy_from_user -> _copy_from_user ->
might_fault and the logs filling up with sleeping warnings.

I also noticed some uio drivers allocate memory, sleep, grab mutexes
from callouts like open() and release and uio is now doing
spin_lock_irqsave while calling them.

Note target_core_user's irqcontrol looks buggy and was just doing more
work than it should in that callout. I can fix that driver.