[PATCH v2 3/4] Documentation: LKMM: Add litmus test for RCU GP guarantee where reader stores

[Joel Fernandes (Google)]

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

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 Documentation/litmus-tests/README             |  5 +++
 .../litmus-tests/rcu/RCU+sync+read.litmus     | 37 +++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 Documentation/litmus-tests/rcu/RCU+sync+read.litmus

diff --git a/Documentation/litmus-tests/README b/Documentation/litmus-tests/README
index 84208bc197f2e..79d187f75679d 100644
--- a/Documentation/litmus-tests/README
+++ b/Documentation/litmus-tests/README
@@ -7,3 +7,8 @@ RCU (/rcu directory)
 MP+onceassign+derefonce.litmus
     Demonstrates that rcu_assign_pointer() and rcu_dereference() to
     ensure that an RCU reader will not see pre-initialization garbage.
+
+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
index 0000000000000..f34176720231d
--- /dev/null
+++ b/Documentation/litmus-tests/rcu/RCU+sync+read.litmus
@@ -0,0 +1,37 @@
+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)
-- 
2.25.1.696.g5e7596f4ac-goog