kcsan: test: Fix flaky test case

If CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n, then we may also see data
races between the writers only. If we get unlucky and never capture a
read-write data race, but only the write-write data races, then the
test_no_value_change* test cases may incorrectly fail.

The second problem is that the initial value needs to be reset, as
otherwise we might actually observe a value change at the start.

Fix it by also looking for the write-write data races, and resetting the
value to what will be written.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/kcsan/kcsan_test.c

@@ -493,17 +493,24 @@ static void test_concurrent_races(struct kunit *test)
 __no_kcsan
 static void test_novalue_change(struct kunit *test)
 {
-	const struct expect_report expect = {
+	const struct expect_report expect_rw = {
 		.access = {
 			{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
 			{ test_kernel_read, &test_var, sizeof(test_var), 0 },
 		},
 	};
+	const struct expect_report expect_ww = {
+		.access = {
+			{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
+			{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
+		},
+	};
 	bool match_expect = false;
 
+	test_kernel_write_nochange(); /* Reset value. */
 	begin_test_checks(test_kernel_write_nochange, test_kernel_read);
 	do {
-		match_expect = report_matches(&expect);
+		match_expect = report_matches(&expect_rw) || report_matches(&expect_ww);
 	} while (!end_test_checks(match_expect));
 	if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY))
 		KUNIT_EXPECT_FALSE(test, match_expect);
@@ -518,17 +525,24 @@ static void test_novalue_change(struct kunit *test)
 __no_kcsan
 static void test_novalue_change_exception(struct kunit *test)
 {
-	const struct expect_report expect = {
+	const struct expect_report expect_rw = {
 		.access = {
 			{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
 			{ test_kernel_read, &test_var, sizeof(test_var), 0 },
 		},
 	};
+	const struct expect_report expect_ww = {
+		.access = {
+			{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
+			{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
+		},
+	};
 	bool match_expect = false;
 
+	test_kernel_write_nochange_rcu(); /* Reset value. */
 	begin_test_checks(test_kernel_write_nochange_rcu, test_kernel_read);
 	do {
-		match_expect = report_matches(&expect);
+		match_expect = report_matches(&expect_rw) || report_matches(&expect_ww);
 	} while (!end_test_checks(match_expect));
 	KUNIT_EXPECT_TRUE(test, match_expect);
 }