[PATCH tip/core/rcu 2/2] documentation: Record reason for rcu_head two-byte alignment

[Paul E. McKenney]

The __call_rcu() assertion that checks only the bottom bit of the
rcu_head pointer is a bit counter-intuitive in these days of ubiquitous
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@xxxxxxxxxxxxxxxxxx>
---
 .../RCU/Design/Requirements/Requirements.html      | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index ece410f40436..a4d3838130e4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2493,6 +2493,28 @@ or some future &ldquo;lazy&rdquo;
 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>
 
-- 
2.5.2