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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2016-08-09 21:15:15 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2016-08-22 09:25:33 -0700
commited2bec07fd1aa47f1c06be92c164c13c70fb7a45 (patch)
tree42c5a07deb9a7f984b9c6631e86d60dabcdedb68 /Documentation/RCU/Design
parente1ef69217f68b8407245e9e353cf88cc2f9ebc18 (diff)
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>
Diffstat (limited to 'Documentation/RCU/Design')
-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.html22
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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>