Commit-ID: ed2bec07fd1aa47f1c06be92c164c13c70fb7a45 Gitweb: http://git.kernel.org/tip/ed2bec07fd1aa47f1c06be92c164c13c70fb7a45 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Tue, 9 Aug 2016 21:15:15 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Mon, 22 Aug 2016 09:25:33 -0700 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@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 ece410f..a4d3838 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/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 +“lazy” 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> -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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