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); \
 	})
 

Documentation/circular-buffers.txt

@@ -220,8 +220,7 @@ before it writes the new tail pointer, which will erase the item.
 
 Note the use of READ_ONCE() and smp_load_acquire() to read the
 opposition index.  This prevents the compiler from discarding and
-reloading its cached value - which some compilers will do across
-smp_read_barrier_depends().  This isn't strictly needed if you can
+reloading its cached value.  This isn't strictly needed if you can
 be sure that the opposition index will _only_ be used the once.
 The smp_load_acquire() additionally forces the CPU to order against
 subsequent memory references.  Similarly, smp_store_release() is used

Documentation/memory-barriers.txt

@@ -1818,7 +1818,7 @@ The Linux kernel has eight basic CPU memory barriers:
 	GENERAL		mb()			smp_mb()
 	WRITE		wmb()			smp_wmb()
 	READ		rmb()			smp_rmb()
-	DATA DEPENDENCY	read_barrier_depends()	smp_read_barrier_depends()
+	DATA DEPENDENCY				READ_ONCE()
 
 
 All memory barriers except the data dependency barriers imply a compiler
@@ -2867,7 +2867,10 @@ access depends on a read, not all do, so it may not be relied on.
 
 Other CPUs may also have split caches, but must coordinate between the various
 cachelets for normal memory accesses.  The semantics of the Alpha removes the
-need for coordination in the absence of memory barriers.
+need for hardware coordination in the absence of memory barriers, which
+permitted Alpha to sport higher CPU clock rates back in the day.  However,
+please note that smp_read_barrier_depends() should not be used except in
+Alpha arch-specific code and within the READ_ONCE() macro.
 
 
 CACHE COHERENCY VS DMA