Recent memory-model work deduces the relationships of RCU read-side
critical sections and grace periods based on the relationships of
accesses within a critical section and accesses preceding and following
the grace period. This commit therefore adds this viewpoint.
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
---
.../RCU/Design/Requirements/Requirements.html | 25 +++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html
b/Documentation/RCU/Design/Requirements/Requirements.html
index a4d3838130e4..81b40cb83435 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -547,7 +547,7 @@ The <tt>rcu_access_pointer()</tt> on line 6 is similar
to
It could reuse a value formerly fetched from this same pointer.
It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
- mash-up of two distince pointer values.
+ mash-up of two distinct pointer values.
It might even use value-speculation optimizations, where it makes
a wrong guess, but by the time it gets around to checking the
value, an update has changed the pointer to match the wrong guess.
@@ -659,6 +659,29 @@ systems with more than one CPU:
In other words, a given instance of <tt>synchronize_rcu()</tt>
can avoid waiting on a given RCU read-side critical section only
if it can prove that <tt>synchronize_rcu()</tt> started first.
+
+ <p>
+ A related question is “When <tt>rcu_read_lock()</tt>
+ doesn't generate any code, why does it matter how it relates
+ to a grace period?”
+ The answer if that it is not the relationship of
+ <tt>rcu_read_lock()</tt> itself that is important, but rather
+ the relationship of the code within the enclosed RCU read-side
+ critical section to the code preceding and following the
+ grace period.
+ If we take this viewpoint, then a given RCU read-side critical
+ section begins before a given grace period when some access
+ preceding the grace period observes the effect of some access
+ within the critical section, in which case none of the accesses
+ within the critical section may observe the effects of any
+ access following the grace period.
+
+ <p>
+ As of late 2016, mathematical models of RCU take this
+ viewpoint, for example, see slides 62 and 63
+ of the
+ <a
href="http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.2016.10.04c.LCE.pdf";>2016
LinuxCon EU</a>
+ presentation.
</font></td></tr>
<tr><td> </td></tr>
</table>
--
2.5.2
