doc: De-emphasize smp_read_barrier_depends

This commit keeps only the historical and low-level discussion of
smp_read_barrier_depends().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Adjusted to allow for David Howells feedback on prior commit. ]

Documentation/RCU/Design/Requirements/Requirements.html

@@ -581,7 +581,8 @@ This guarantee was only partially premeditated.
 DYNIX/ptx used an explicit memory barrier for publication, but had nothing
 resembling <tt>rcu_dereference()</tt> for subscription, nor did it
 have anything resembling the <tt>smp_read_barrier_depends()</tt>
-that was later subsumed into <tt>rcu_dereference()</tt>.
+that was later subsumed into <tt>rcu_dereference()</tt> and later
+still into <tt>READ_ONCE()</tt>.
 The need for these operations made itself known quite suddenly at a
 late-1990s meeting with the DEC Alpha architects, back in the days when
 DEC was still a free-standing company.

Documentation/RCU/rcu_dereference.txt

@@ -122,11 +122,7 @@ o	Be very careful about comparing pointers obtained from
 		Note that if checks for being within an RCU read-side
 		critical section are not required and the pointer is never
 		dereferenced, rcu_access_pointer() should be used in place
-		of rcu_dereference(). The rcu_access_pointer() primitive
-		does not require an enclosing read-side critical section,
-		and also omits the smp_read_barrier_depends() included in
-		rcu_dereference(), which in turn should provide a small
-		performance gain in some CPUs (e.g., the DEC Alpha).
+		of rcu_dereference().
 
 	o	The comparison is against a pointer that references memory
 		that was initialized "a long time ago."  The reason

Documentation/RCU/whatisRCU.txt

@@ -600,8 +600,7 @@ don't forget about them when submitting patches making use of RCU!]
 
 	#define rcu_dereference(p) \
 	({ \
-		typeof(p) _________p1 = p; \
-		smp_read_barrier_depends(); \
+		typeof(p) _________p1 = READ_ONCE(p); \
 		(_________p1); \
 	})