How to unblock a thread from a hard isr (IRQF_NO_THREAD)

[Till Straumann <strauman@slac.stanford.edu>]

Hello list.

I have a driver which is designed to do most work in user-space.
The ISR is really simple. It just ACKs/clears the interrupt and
then should unblock a waiting thread.
It seems overkill (and costs me about 5-10us) to use a threaded
interrupt in such a case (hard isr unblocks a kthread whose only
'work' is unblocking another (user) thread).

I have found some references where users try to do a similar
thing via UIO
(e.g.,
http://article.gmane.org/gmane.linux.rt.user/7372/
http://article.gmane.org/gmane.linux.rt.user/7676)

However, the succinct answer was along the line 'that can't work with UIO'.

IMHO it would be very helpful to get a more detailed explanation
as to why it doesn't work (i.e., what things you are and are not
allowed to do from a hard-isr under RT_PREEMPT). I suspect it is
because UIO calls routines such as 'kill_fasync' which use
ordinary (as opposed to raw) spinlocks which means that the
caller could be preempted (under RT_PREEMPT), right?

What *is* the recommended mechanism to wake up a thread from
a hard-isr?

It would IMHO be desirable to modify the UIO driver so that it
is possible to support non-threaded handlers.

Thanks for any enlightenment (and the fantastic rt-preempt patch
in the first place)!

- Till

PS: Please CC me on any replies; I am currently not a subscriber.