[PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Marco Elver <elver@google.com>]

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: data-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: data-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
     ...
    ==================================================================

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

Please let me know if the names make sense, given they do not include a
KCSAN_ prefix.

The names are unique across the kernel. I wouldn't expect another macro
with the same name but different semantics to pop up any time soon. If
there is a dual use to these macros (e.g. another tool that could hook
into it), we could also move it elsewhere (include/linux/compiler.h?).

We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
if it is something that'd be used very widely. It'd be straightforward
to add with the help of these macros, but would need to be added to
include/linux/compiler.h.
---
 include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 21b1d1f214ad5..1a7b51e516335 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -96,4 +96,38 @@ 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 synchronization
+ * logic, 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 synchronization logic, where
+ * violation cannot be detected as a normal data race.
+ *
+ * For example, if a variable is not read nor written by the current thread, nor
+ * should it be touched by any other threads during the current execution phase.
+ *
+ * @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 */
-- 
2.25.0.341.g760bfbb309-goog