[PATCH] doc: Update memory-barriers.txt for read-to-write dependencies

[Paul E. McKenney]

The memory-barriers.txt document contains an obsolete passage stating that
smp_read_barrier_depends() is required to force ordering for read-to-write
dependencies.  We now know that this is not required, even for DEC Alpha.
This commit therefore updates this passage to state that read-to-write
dependencies are respected even without smp_read_barrier_depends().

Reported-by: Lance Roy <ldr709@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Jade Alglave <j.alglave@xxxxxxxxx>
Cc: Luc Maranget <luc.maranget@xxxxxxxx>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 9d5e0f853f08..a8a91b9d5a1b 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -594,7 +594,10 @@ between the address load and the data load:
 This enforces the occurrence of one of the two implications, and prevents the
 third possibility from arising.
 
-A data-dependency barrier must also order against dependent writes:
+A data-dependency barrier is not required to order dependent writes
+because the CPUs that the Linux kernel supports don't do writes until
+they are certain (1) that the write will actually happen, (2) of the
+location of the write, and (3) of the value to be written.
 
 	CPU 1		      CPU 2
 	===============	      ===============
@@ -603,19 +606,19 @@ A data-dependency barrier must also order against dependent writes:
 	<write barrier>
 	WRITE_ONCE(P, &B);
 			      Q = READ_ONCE(P);
-			      <data dependency barrier>
 			      *Q = 5;
 
-The data-dependency barrier must order the read into Q with the store
-into *Q.  This prohibits this outcome:
+Therefore, no data-dependency barrier is required to order the read into
+Q with the store into *Q.  In other words, this outcome is prohibited,
+even without a data-dependency barrier:
 
 	(Q == &B) && (B == 4)
 
 Please note that this pattern should be rare.  After all, the whole point
 of dependency ordering is to -prevent- writes to the data structure, along
 with the expensive cache misses associated with those writes.  This pattern
-can be used to record rare error conditions and the like, and the ordering
-prevents such records from being lost.
+can be used to record rare error conditions and the like, and the CPUs'
+naturally occurring ordering prevents such records from being lost.
 
 
 [!] Note that this extremely counterintuitive situation arises most easily on