I have several possible entry points into a code path in a driver which can
decide whether to ltsleep for an event or poll. If I know I've entered via an
interrupt (e.g., the device interrupt service routine)- I can mark state so
that I don't attempt to ltsleep.

But it turns out that there is a normal callback via hardclock (in scsipi)
that I have no means of distinguishing from any other call. How can I tell if
I'm on the interrupt stack and it's not safe to call ltsleep?

-matt