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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2016-08-10 07:15:15 +0300 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2016-08-22 19:25:33 +0300 |
| commit | ed2bec07fd1aa47f1c06be92c164c13c70fb7a45 (patch) | |
| tree | 42c5a07deb9a7f984b9c6631e86d60dabcdedb68 /Documentation/RCU | |
| parent | e1ef69217f68b8407245e9e353cf88cc2f9ebc18 (diff) | |
| download | linux-ed2bec07fd1aa47f1c06be92c164c13c70fb7a45.tar.xz | |
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')
| -rw-r--r-- | Documentation/RCU/Design/Requirements/Requirements.html | 22 |
1 files changed, 22 insertions, 0 deletions
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 “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> |