If a process needs to access some resource such as a buffer cache buffer
that is currently in use, it will issue a sleep() call specifying the
address of the resource it requires.  A swtch() call is made to relinquish
control of the CPU, allowing another process to run.  For example, to wait
on a busy buffer, the following code sequence is made:

    if (bp->b_flags & B_BUSY) {
	  bp->b_flags |= B_WANTED;
	  sleep(bp, PRIBIO);
    }

The address of the structure on which the process is waiting (called the
wait channel) is stored in the p_wchan field of the proc structure.  The
priority argument passed to sleep() will be described in more detail
later in the chapter.  Note for now though that if the priority is greater
than or equal to zero, the process may be awoken from a sleep by a signal.
A value of less than zero prevents this from happening.

When a process is about to relinquish control of a specific resource, it
looks to see if another process is waiting on the resource and issues a
corresponding wakeup() call to signal to the process that the resource is
now available.  In this case, the following code sequence is invoked:

    if (bp->b_flags & B_WANTED)
	  wakeup(bp);

To determine which process is sleeping on the resource, a scan is made
through the proc table issuing a wakeup() call for each process whose
p_wchan field is set to 'bp'.

		-- Steve D. Pate
		   /Unix Filesystems: Evolution, Design, and Implementation/
		   Wiley, 2003