documentation: Record reason for rcu_head two-byte alignment

There is an assertion in __call_rcu() that checks only the bottom
bit of the rcu_head pointer, rather than the bottom two (as might be
expected for 32-bit systems) or the bottom three (as might be expected
for 64-bit systems).  This choice might be a bit surprising in these days
of ubiquitous 32-bit and 64-bit systems.  This commit therefore records
the reason for this odd alignment check, namely that m68k guarantees
only two-byte alignment despite being a 32-bit architectures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/Design/Requirements/Requirements.html

@@ -2493,6 +2493,28 @@ or some future “lazy”
 variant of <tt>call_rcu()</tt> that might one day be created for
 energy-efficiency purposes.
 
+<p>
+That said, there are limits.
+RCU requires that the <tt>rcu_head</tt> structure be aligned to a
+two-byte boundary, and passing a misaligned <tt>rcu_head</tt>
+structure to one of the <tt>call_rcu()</tt> family of functions
+will result in a splat.
+It is therefore necessary to exercise caution when packing
+structures containing fields of type <tt>rcu_head</tt>.
+Why not a four-byte or even eight-byte alignment requirement?
+Because the m68k architecture provides only two-byte alignment,
+and thus acts as alignment's least common denominator.
+
+<p>
+The reason for reserving the bottom bit of pointers to
+<tt>rcu_head</tt> structures is to leave the door open to
+&ldquo;lazy&rdquo; callbacks whose invocations can safely be deferred.
+Deferring invocation could potentially have energy-efficiency
+benefits, but only if the rate of non-lazy callbacks decreases
+significantly for some important workload.
+In the meantime, reserving the bottom bit keeps this option open
+in case it one day becomes useful.
+
 <h3><a name="Performance, Scalability, Response Time, and Reliability">
 Performance, Scalability, Response Time, and Reliability</a></h3>