documentation: Subsequent writes ordered by rcu_dereference()

The current memory-barriers.txt does not address the possibility of
a write to a dereferenced pointer.  This should be rare, but when it
happens, we need that write -not- to be clobbered by the initialization.
This commit therefore adds an example showing a data dependency ordering
a later data-dependent write.

Reported-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 6bee0a2c43abe483359553c7fbd6e863f2fee1b9..e9ebeb3b1077eb8a2c1df6cf45374d27037eb70c 100644 (file)
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -555,6 +555,30 @@ 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:
+
+       CPU 1                 CPU 2
+       ===============       ===============
+       { A == 1, B == 2, C = 3, P == &A, Q == &C }
+       B = 4;
+       <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:
+
+       (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.
+
+
 [!] Note that this extremely counterintuitive situation arises most easily on
 machines with split caches, so that, for example, one cache bank processes
 even-numbered cache lines and the other bank processes odd-numbered cache