Thanks to Dru <andru@treshna.com>, who provided an easy way to reproduce
the problem.

What we have in lib/rwsem.c:__rwsem_do_wake():
	int woken, loop;
	^^^
and several lines below:
	loop = woken;
	woken *= RWSEM_ACTIVE_BIAS-RWSEM_WAITING_BIAS;
	woken -= RWSEM_ACTIVE_BIAS;

However, rw_semaphore->count is 64-bit on Alpha, so
RWSEM_WAITING_BIAS has been defined as -0x0000000100000000L.
Obviously, this blows up in the write contention case.