allow selwakeup to be called from interrupt filter context.

[adrian]

Summary

The Linux DRM 4.7 work done by Kip minimises changes to the linux code
and one of those changes is that linux interrupts are interrupts, not
deferred.

So driver code that wants to do things like call selwakeup for waking
up sockets (ie, what DRM is doing) is doing it from the interrupt
context, and not deferred into an ithread or tasklet.

This change allows drivers to do it from interrupt filter context.

[jhb]

Running arbitrary code from filters is going to break much more than just selwakeup(). FreeBSD's kernel isn't really designed for that model.

Also, have you run this with WITNESS_SKIPSPIN disabled? The lock ordering of spin locks is rather finicky, especially with regards to _sleep() and cv_wait(). msleep_spin() goes to special hoops separate from _sleep() to handle spin locks. cv_wait* do not and assume a non-spin lock. If you are ktracing a thread that sleeps on st_wait this probably panics in a kernel with INVARIANTS and/or WITNESS.

[imp]

Looks like jhb's reaciton is controller, but tagging for srcmgr to review.