panic: Assertion in_epoch(net_epoch_preempt) failed at ... src/sys/net/if.c:3694

[Ryan Stone]

I haven't found any good references on the subject, but here's my understanding:

- epoch_enter() and epoch_exit() are very inexpensive operations
(cheaper than mtx, rw_lock or rm_lock operations) that are use to mark
read-only critical sections
- epoch_wait() guarantees that no threads that were in the critical
section when it was first called are still in the critical section
when it completes

With this guarantee, you can safely destroy an object with the
following procedure:

1. Atomically remove all global pointers to the object (e.g. remove it
from any lists that the critical sections might look it up in).  This
must be done atomically because read-only threads can be concurrently
running in the critical section.  This guarantees that no more threads
can get a pointer to it.
2. Call epoch_wait() to drain all threads that already held pointers
to it before step 1.
3. You now hold the only pointer to the object, so you are free to
destroy it as you please.