documentation: Clarify control-dependency pairing

This commit explicitly states that control dependencies pair normally
with other barriers, and gives an example of such pairing.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

1 files changed, 29 insertions, 13 deletions

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index ca2387ef27ab..6974f1c2b4e1 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -592,9 +592,9 @@ See also the subsection on "Cache Coherency" for a more thorough example.
 CONTROL DEPENDENCIES
 --------------------
 
-A control dependency requires a full read memory barrier, not simply a data
-dependency barrier to make it work correctly.  Consider the following bit of
-code:
+A load-load control dependency requires a full read memory barrier, not
+simply a data dependency barrier to make it work correctly.  Consider the
+following bit of code:
 
         q = ACCESS_ONCE(a);
         if (q) {
@@ -615,14 +615,15 @@ case what's actually required is:
         }
 
 However, stores are not speculated.  This means that ordering -is- provided
-in the following example:
+for load-store control dependencies, as in the following example:
 
         q = ACCESS_ONCE(a);
         if (q) {
                 ACCESS_ONCE(b) = p;
         }
 
-Please note that ACCESS_ONCE() is not optional!  Without the
+Control dependencies pair normally with other types of barriers.
+That said, please note that ACCESS_ONCE() is not optional!  Without the
 ACCESS_ONCE(), might combine the load from 'a' with other loads from
 'a', and the store to 'b' with other stores to 'b', with possible highly
 counterintuitive effects on ordering.
@@ -813,6 +814,8 @@ In summary:
       barrier() can help to preserve your control dependency.  Please
       see the Compiler Barrier section for more information.
 
+  (*) Control dependencies pair normally with other types of barriers.
+
   (*) Control dependencies do -not- provide transitivity.  If you
       need transitivity, use smp_mb().
 
@@ -823,14 +826,14 @@ SMP BARRIER PAIRING
 When dealing with CPU-CPU interactions, certain types of memory barrier should
 always be paired.  A lack of appropriate pairing is almost certainly an error.
 
-General barriers pair with each other, though they also pair with
-most other types of barriers, albeit without transitivity.  An acquire
-barrier pairs with a release barrier, but both may also pair with other
-barriers, including of course general barriers.  A write barrier pairs
-with a data dependency barrier, an acquire barrier, a release barrier,
-a read barrier, or a general barrier.  Similarly a read barrier or a
-data dependency barrier pairs with a write barrier, an acquire barrier,
-a release barrier, or a general barrier:
+General barriers pair with each other, though they also pair with most
+other types of barriers, albeit without transitivity.  An acquire barrier
+pairs with a release barrier, but both may also pair with other barriers,
+including of course general barriers.  A write barrier pairs with a data
+dependency barrier, a control dependency, an acquire barrier, a release
+barrier, a read barrier, or a general barrier.  Similarly a read barrier,
+control dependency, or a data dependency barrier pairs with a write
+barrier, an acquire barrier, a release barrier, or a general barrier:
 
         CPU 1                 CPU 2
         ===============       ===============
@@ -850,6 +853,19 @@ Or:
                               <data dependency barrier>
                               y = *x;
 
+Or even:
+
+        CPU 1                 CPU 2
+        ===============       ===============================
+        r1 = ACCESS_ONCE(y);
+        <general barrier>
+        ACCESS_ONCE(y) = 1;   if (r2 = ACCESS_ONCE(x)) {
+                                 <implicit control dependency>
+                                 ACCESS_ONCE(y) = 1;
+                              }
+
+        assert(r1 == 0 || r2 == 0);
+
 Basically, the read barrier always has to be there, even though it can be of
 the "weaker" type.