Simplify volatile definition

doc/synchronization-notes.md

@@ -52,3 +52,13 @@ class Node
   # ...
 end
 ```
+## Piggybacking
+
+Any write executed before volatile write based on program-order is visible to
+the volatile read as well, which allows
+[piggybacking](http://stackoverflow.com/questions/8769570/volatile-piggyback-is-this-enough-for-visiblity).
+Because it creates synchronizes-with (JMM term) order between volatile write
+and read, which participates in creating happens-before order.
+
+This trick is used in some of the abstractions, to avoid unnecessary
+synchronization or volatile declarations.

doc/synchronization.md

@@ -34,7 +34,7 @@ time-consuming and error-prone.
 The Ruby memory model is a framework allowing to reason about programs in
 concurrent and parallel environment. It defines what variable writes can be
 observed by a particular variable read, which is essential to be able to
-determine if a program is correct. It is achieved be defining what subset of
+determine if a program is correct. It is achieved by defining what subset of
 all possible program execution orders is allowed.
 
 A memory model sources:
@@ -70,14 +70,9 @@ C++11). This is not a formal model.
 
 Key properties are:
 
--   **volatility (V)** - Any written value is immediately visible to any
-    subsequent volatile read of the same variable. Any write executed before
-    volatile write based on program-order is visible to the read as well, which
-    allows
-    [piggybacking](http://stackoverflow.com/questions/8769570/volatile-piggyback-is-this-enough-for-visiblity).
-    (Same meaning as in Java, it creates synchronizes-with (JMM term) order
-    between write and read, which participates in creating happens-before
-    order.)
+-   **volatility (V)** - A written value is immediately visible to any
+    subsequent volatile read of the same variable on any Thread. (It has same
+    meaning as in Java.)
 -   **atomicity (A)** - Operation is either done or not as a whole.
 -   **serialized (S)** - Operations are serialized in some order (they
     cannot disappear). This is a new property not mentioned in other memory