[PATCH 1/3] LKMM: Add litmus test for RCU GP guarantee where updater frees object

[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>
---
 .../litmus-tests/RCU+sync+free.litmus         | 40 +++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 tools/memory-model/litmus-tests/RCU+sync+free.litmus

diff --git a/tools/memory-model/litmus-tests/RCU+sync+free.litmus b/tools/memory-model/litmus-tests/RCU+sync+free.litmus
new file mode 100644
index 0000000000000..c4682502dd296
--- /dev/null
+++ b/tools/memory-model/litmus-tests/RCU+sync+free.litmus
@@ -0,0 +1,40 @@
+C RCU+sync+free
+
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that an RCU reader can never see a write after
+ * the grace period, if it saw writes that happen before the grace period. This
+ * is a typical pattern of RCU usage, where the write before the grace period
+ * assigns a pointer, and the writes after destroy the object that the pointer
+ * points to.
+ *
+ * This guarantee also implies, an RCU reader can never span a grace period and
+ * is an important RCU grace period memory ordering guarantee.
+ *)
+
+{
+x = 1;
+y = x;
+z = 1;
+}
+
+P0(int *x, int *z, int **y)
+{
+	int r0;
+	int r1;
+
+	rcu_read_lock();
+	r0 = rcu_dereference(*y);
+	r1 = READ_ONCE(*r0);
+	rcu_read_unlock();
+}
+
+P1(int *x, int *z, int **y)
+{
+	rcu_assign_pointer(*y, z);
+	synchronize_rcu();
+	WRITE_ONCE(*x, 0);
+}
+
+exists (0:r0=x /\ 0:r1=0)
-- 
2.25.1.696.g5e7596f4ac-goog