kcsan, compiler_types: Introduce __data_racy type qualifier

Based on the discussion at [1], it would be helpful to mark certain
variables as explicitly "data racy", which would result in KCSAN not
reporting data races involving any accesses on such variables. To do
that, introduce the __data_racy type qualifier:

	struct foo {
		...
		int __data_racy bar;
		...
	};

In KCSAN-kernels, __data_racy turns into volatile, which KCSAN already
treats specially by considering them "marked". In non-KCSAN kernels the
type qualifier turns into no-op.

The generated code between KCSAN-instrumented kernels and non-KCSAN
kernels is already huge (inserted calls into runtime for every memory
access), so the extra generated code (if any) due to volatile for few
such __data_racy variables are unlikely to have measurable impact on
performance.

Link: https://lore.kernel.org/all/CAHk-=wi3iondeh_9V2g3Qz5oHTRjLsOpoy83hb58MVh=nRZe0A@mail.gmail.com/ [1]
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Marco Elver <elver@google.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/dev-tools/kcsan.rst

@@ -91,6 +91,16 @@ the below options are available:
   behaviour when encountering a data race is deemed safe.  Please see
   `"Marking Shared-Memory Accesses" in the LKMM`_ for more information.
 
+* Similar to ``data_race(...)``, the type qualifier ``__data_racy`` can be used
+  to document that all data races due to accesses to a variable are intended
+  and should be ignored by KCSAN::
+
+    struct foo {
+        ...
+        int __data_racy stats_counter;
+        ...
+    };
+
 * Disabling data race detection for entire functions can be accomplished by
   using the function attribute ``__no_kcsan``::
 

include/linux/compiler_types.h

@@ -273,9 +273,16 @@ struct ftrace_likely_data {
  * disable all instrumentation. See Kconfig.kcsan where this is mandatory.
  */
 # define __no_kcsan __no_sanitize_thread __disable_sanitizer_instrumentation
+/*
+ * Type qualifier to mark variables where all data-racy accesses should be
+ * ignored by KCSAN. Note, the implementation simply marks these variables as
+ * volatile, since KCSAN will treat such accesses as "marked".
+ */
+# define __data_racy volatile
 # define __no_sanitize_or_inline __no_kcsan notrace __maybe_unused
 #else
 # define __no_kcsan
+# define __data_racy
 #endif
 
 #ifndef __no_sanitize_or_inline

kernel/kcsan/kcsan_test.c

@@ -304,6 +304,7 @@ static long test_array[3 * PAGE_SIZE / sizeof(long)];
 static struct {
 	long val[8];
 } test_struct;
+static long __data_racy test_data_racy;
 static DEFINE_SEQLOCK(test_seqlock);
 static DEFINE_SPINLOCK(test_spinlock);
 static DEFINE_MUTEX(test_mutex);
@@ -358,6 +359,8 @@ static noinline void test_kernel_write_uninstrumented(void) { test_var++; }
 
 static noinline void test_kernel_data_race(void) { data_race(test_var++); }
 
+static noinline void test_kernel_data_racy_qualifier(void) { test_data_racy++; }
+
 static noinline void test_kernel_assert_writer(void)
 {
 	ASSERT_EXCLUSIVE_WRITER(test_var);
@@ -1009,6 +1012,19 @@ static void test_data_race(struct kunit *test)
 	KUNIT_EXPECT_FALSE(test, match_never);
 }
 
+/* Test the __data_racy type qualifier. */
+__no_kcsan
+static void test_data_racy_qualifier(struct kunit *test)
+{
+	bool match_never = false;
+
+	begin_test_checks(test_kernel_data_racy_qualifier, test_kernel_data_racy_qualifier);
+	do {
+		match_never = report_available();
+	} while (!end_test_checks(match_never));
+	KUNIT_EXPECT_FALSE(test, match_never);
+}
+
 __no_kcsan
 static void test_assert_exclusive_writer(struct kunit *test)
 {
@@ -1424,6 +1440,7 @@ static struct kunit_case kcsan_test_cases[] = {
 	KCSAN_KUNIT_CASE(test_read_plain_atomic_rmw),
 	KCSAN_KUNIT_CASE(test_zero_size_access),
 	KCSAN_KUNIT_CASE(test_data_race),
+	KCSAN_KUNIT_CASE(test_data_racy_qualifier),
 	KCSAN_KUNIT_CASE(test_assert_exclusive_writer),
 	KCSAN_KUNIT_CASE(test_assert_exclusive_access),
 	KCSAN_KUNIT_CASE(test_assert_exclusive_access_writer),