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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>
This commit is contained in:
Paul E. McKenney 2016-08-09 21:15:15 -07:00
parent e1ef69217f
commit ed2bec07fd
1 changed files with 22 additions and 0 deletions
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22
Documentation/RCU/Design/Requirements/Requirements.html
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@ -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>