Documentation: kunit: add tips for using current->kunit_test

As of commit 359a3760 ("kunit: support failure from dynamic analysis
tools"), we can use current->kunit_test to find the current kunit test.

Mention this in tips.rst and give an example of how this can be used in
conjunction with `test->priv` to pass around state and specifically
implement something like mocking.
There's a lot more we could go into on that topic, but given that
example is already longer than every other "tip" on this page, we just
point to the API docs and leave filling in the blanks as an exercise to
the reader.

Also give an example of kunit_fail_current_test().
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/tips.rst

@@ -78,8 +78,82 @@ Similarly to the above, it can be useful to add test-specific logic.
 	void test_only_hook(void) { }
 	#endif
 
-TODO(dlatypov@google.com): add an example of using ``current->kunit_test`` in
-such a hook when it's not only updated for ``CONFIG_KASAN=y``.
+This test-only code can be made more useful by accessing the current kunit
+test, see below.
+
+Accessing the current test
+--------------------------
+
+In some cases, you need to call test-only code from outside the test file, e.g.
+like in the example above or if you're providing a fake implementation of an
+ops struct.
+There is a ``kunit_test`` field in ``task_struct``, so you can access it via
+``current->kunit_test``.
+
+Here's a slightly in-depth example of how one could implement "mocking":
+
+.. code-block:: c
+
+	#include <linux/sched.h> /* for current */
+
+	struct test_data {
+		int foo_result;
+		int want_foo_called_with;
+	};
+
+	static int fake_foo(int arg)
+	{
+		struct kunit *test = current->kunit_test;
+		struct test_data *test_data = test->priv;
+
+		KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
+		return test_data->foo_result;
+	}
+
+	static void example_simple_test(struct kunit *test)
+	{
+		/* Assume priv is allocated in the suite's .init */
+		struct test_data *test_data = test->priv;
+
+		test_data->foo_result = 42;
+		test_data->want_foo_called_with = 1;
+
+		/* In a real test, we'd probably pass a pointer to fake_foo somewhere
+		 * like an ops struct, etc. instead of calling it directly. */
+		KUNIT_EXPECT_EQ(test, fake_foo(1), 42);
+	}
+
+
+Note: here we're able to get away with using ``test->priv``, but if you wanted
+something more flexible you could use a named ``kunit_resource``, see :doc:`api/test`.
+
+Failing the current test
+------------------------
+
+But sometimes, you might just want to fail the current test. In that case, we
+have ``kunit_fail_current_test(fmt, args...)`` which is defined in ``<kunit/test-bug.h>`` and
+doesn't require pulling in ``<kunit/test.h>``.
+
+E.g. say we had an option to enable some extra debug checks on some data structure:
+
+.. code-block:: c
+
+	#include <kunit/test-bug.h>
+
+	#ifdef CONFIG_EXTRA_DEBUG_CHECKS
+	static void validate_my_data(struct data *data)
+	{
+		if (is_valid(data))
+			return;
+
+		kunit_fail_current_test("data %p is invalid", data);
+
+		/* Normal, non-KUnit, error reporting code here. */
+	}
+	#else
+	static void my_debug_function(void) { }
+	#endif
+
 
 Customizing error messages
 --------------------------