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>

diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index d93f226327af9bc9b74841b6a1796f3d873b9d90..e282c1166373e325c6ea73372913007a5cf3c390 100644 (file)
--- a/kernel/kcsan/kcsan_test.c
+++ b/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);
 }