doc: Clarify rcu_assign_pointer() and rcu_dereference() ordering

This commit expands on the ordering properties of rcu_assign_pointer()
and rcu_dereference(), outlining their constraints on CPUs and compilers.

Reported-by: Rao Shoaib <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: paulmckrcu

Documentation/RCU/whatisRCU.rst

@@ -250,21 +250,25 @@ rcu_assign_pointer()
 ^^^^^^^^^^^^^^^^^^^^
 	void rcu_assign_pointer(p, typeof(p) v);
 
-	Yes, rcu_assign_pointer() **is** implemented as a macro, though it
-	would be cool to be able to declare a function in this manner.
-	(Compiler experts will no doubt disagree.)
+	Yes, rcu_assign_pointer() **is** implemented as a macro, though
+	it would be cool to be able to declare a function in this manner.
+	(And there has been some discussion of adding overloaded functions
+	to the C language, so who knows?)
 
 	The updater uses this spatial macro to assign a new value to an
 	RCU-protected pointer, in order to safely communicate the change
 	in value from the updater to the reader.  This is a spatial (as
 	opposed to temporal) macro.  It does not evaluate to an rvalue,
-	but it does execute any memory-barrier instructions required
-	for a given CPU architecture.  Its ordering properties are that
-	of a store-release operation.
-
-	Perhaps just as important, it serves to document (1) which
-	pointers are protected by RCU and (2) the point at which a
-	given structure becomes accessible to other CPUs.  That said,
+	but it does provide any compiler directives and memory-barrier
+	instructions required for a given compile or CPU architecture.
+	Its ordering properties are that of a store-release operation,
+	that is, any prior loads and stores required to initialize the
+	structure are ordered before the store that publishes the pointer
+	to that structure.
+
+	Perhaps just as important, rcu_assign_pointer() serves to document
+	(1) which pointers are protected by RCU and (2) the point at which
+	a given structure becomes accessible to other CPUs.  That said,
 	rcu_assign_pointer() is most frequently used indirectly, via
 	the _rcu list-manipulation primitives such as list_add_rcu().
 
@@ -283,7 +287,11 @@ rcu_dereference()
 	executes any needed memory-barrier instructions for a given
 	CPU architecture.  Currently, only Alpha needs memory barriers
 	within rcu_dereference() -- on other CPUs, it compiles to a
-	volatile load.
+	volatile load.	However, no mainstream C compilers respect
+	address dependencies, so rcu_dereference() uses volatile casts,
+	which, in combination with the coding guidelines listed in
+	rcu_dereference.rst, prevent current compilers from breaking
+	these dependencies.
 
 	Common coding practice uses rcu_dereference() to copy an
 	RCU-protected pointer to a local variable, then dereferences