kcsan: Introduce ASSERT_EXCLUSIVE_*() macros

Introduces ASSERT_EXCLUSIVE_WRITER() and ASSERT_EXCLUSIVE_ACCESS(), which
may be used to assert properties of synchronization logic, where
violation cannot be detected as a normal data race.

Examples of the reports that may be generated:

    ==================================================================
    BUG: KCSAN: assert: race in test_thread / test_thread

    write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
     test_thread+0x8d/0x111
     debugfs_write.cold+0x32/0x44
     ...

    assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
     test_thread+0xa3/0x111
     debugfs_write.cold+0x32/0x44
     ...
    ==================================================================

    ==================================================================
    BUG: KCSAN: assert: race in test_thread / test_thread

    assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
     test_thread+0xb9/0x111
     debugfs_write.cold+0x32/0x44
     ...

    read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
     test_thread+0x77/0x111
     debugfs_write.cold+0x32/0x44
     ...
    ==================================================================
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/kcsan-checks.h

@@ -96,4 +96,44 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
 	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
 #endif
 
+/**
+ * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
+ *
+ * Assert that there are no other threads writing @var; other readers are
+ * allowed. This assertion can be used to specify properties of concurrent code,
+ * where violation cannot be detected as a normal data race.
+ *
+ * For example, if a per-CPU variable is only meant to be written by a single
+ * CPU, but may be read from other CPUs; in this case, reads and writes must be
+ * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
+ * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
+ * race condition. Using this macro allows specifying this property in the code
+ * and catch such bugs.
+ *
+ * @var variable to assert on
+ */
+#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
+	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
+
+/**
+ * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
+ *
+ * Assert that no other thread is accessing @var (no readers nor writers). This
+ * assertion can be used to specify properties of concurrent code, where
+ * violation cannot be detected as a normal data race.
+ *
+ * For example, in a reference-counting algorithm where exclusive access is
+ * expected after the refcount reaches 0. We can check that this property
+ * actually holds as follows:
+ *
+ *	if (refcount_dec_and_test(&obj->refcnt)) {
+ *		ASSERT_EXCLUSIVE_ACCESS(*obj);
+ *		safely_dispose_of(obj);
+ *	}
+ *
+ * @var variable to assert on
+ */
+#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
+	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
+
 #endif /* _LINUX_KCSAN_CHECKS_H */