Documentation: LKMM: Add litmus test for RCU GP guarantee where updater frees object

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/rcu/RCU+sync+free.litmus b/Documentation/litmus-tests/rcu/RCU+sync+free.litmus
new file mode 100644 (file)
index 0000000..4ee67e1
--- /dev/null
+++ b/Documentation/litmus-tests/rcu/RCU+sync+free.litmus
@@ -0,0 +1,42 @@
+C RCU+sync+free
+
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that an RCU reader can never see a write that
+ * follows a grace period, if it did not see writes that precede that grace
+ * period.
+ *
+ * This is a typical pattern of RCU usage, where the write before the grace
+ * period assigns a pointer, and the writes following the grace period destroy
+ * the object that the pointer used to point to.
+ *
+ * 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 = 1;
+int *y = &x;
+int 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)