Documentation: LKMM: Add litmus test for RCU GP guarantee where reader stores

This adds an example for the important RCU grace period guarantee, which
shows an RCU reader can never span a grace period.

Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/litmus-tests/README b/Documentation/litmus-tests/README
new file mode 100644 (file)
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--- /dev/null
+++ b/Documentation/litmus-tests/README
@@ -0,0 +1,11 @@
+============
+LITMUS TESTS
+============
+
+RCU (/rcu directory)
+--------------------
+
+RCU+sync+read.litmus
+RCU+sync+free.litmus
+    Both the above litmus tests demonstrate the RCU grace period guarantee
+    that an RCU read-side critical section can never span a grace period.

diff --git a/Documentation/litmus-tests/rcu/RCU+sync+read.litmus b/Documentation/litmus-tests/rcu/RCU+sync+read.litmus
new file mode 100644 (file)
index 0000000..f341767
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+++ b/Documentation/litmus-tests/rcu/RCU+sync+read.litmus
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+C RCU+sync+read
+
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that after a grace period, an RCU updater always
+ * sees all stores done in prior RCU read-side critical sections. Such
+ * read-side critical sections would have ended before the grace period ended.
+ *
+ * This is one implication of the RCU grace-period guarantee, which says (among
+ * other things) that an RCU read-side critical section cannot span a grace period.
+ *)
+
+{
+int x = 0;
+int y = 0;
+}
+
+P0(int *x, int *y)
+{
+       rcu_read_lock();
+       WRITE_ONCE(*x, 1);
+       WRITE_ONCE(*y, 1);
+       rcu_read_unlock();
+}
+
+P1(int *x, int *y)
+{
+       int r0;
+       int r1;
+
+       r0 = READ_ONCE(*x);
+       synchronize_rcu();
+       r1 = READ_ONCE(*y);
+}
+
+exists (1:r0=1 /\ 1:r1=0)