sleep/wakeup sequence using tsleep_interlock

[Aggelos Economopoulos <aoiko@xxxxxxxxxxxxxx>]

According to the comment preceding tsleep_interlock() in kern_synch.c:

/*
 * This is a dandy function that allows us to interlock tsleep/wakeup
 * operations with unspecified upper level locks, such as lockmgr locks,
 * simply by holding a critical section.  The sequence is:
 *
 *	(enter critical section)
 *	(acquire upper level lock)
 *	tsleep_interlock(blah)
 *	(release upper level lock)
 *	tsleep(blah, ...)
 *	(exit critical section)

It is not obvious to me why the first two operations can't be reversed (i.e. 
first acquire lock, then enter critical section). Can they?

TIA,
Aggelos