+ NB. there's a bug in here. If a thread is inside an
+ unsafePerformIO, and inside blockAsyncExceptions# (there is an
+ unblockAsyncExceptions_ret on the stack), and it is blocked in an
+ interruptible operation, and it receives an exception, then the
+ unsafePerformIO thunk will be updated with a stack object
+ containing the unblockAsyncExceptions_ret frame. Later, when
+ someone else evaluates this thunk, the blocked exception state is
+ not restored, and the result is that unblockAsyncExceptions_ret
+ will attempt to unblock exceptions in the current thread, but it'll
+ find that the CurrentTSO->blocked_exceptions is NULL. Hence, we
+ work around this by checking for NULL in awakenBlockedQueue().
+