I haven't looked very deeply, but...

can code that is running inside splsoftnet() be interrupted by a user
context ?  if so, can the user context then swtich to kernel (via some
system call) and continue along and call splsoftnet() again ?

(and variations on that idea)

I notice in the spl.h there is no "set/test and wait", just a setting
of the flag and was concerned about what this means...does it (with help
from the kernel elsewhere) guarantee a protected region of code ?

Darren