kcsan: Add example to data_race() kerneldoc header
Although the data_race() kerneldoc header accurately states what it does,
some of the implications and usage patterns are non-obvious. Therefore,
add a brief locking example and also state how to have KCSAN ignore
accesses while also preventing the compiler from folding, spindling,
or otherwise mutilating the access.
[ paulmck: Apply Bart Van Assche feedback. ]
Reported-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Jens Axboe <axboe@kernel.dk>
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index c00cc6c..9249768 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -194,9 +194,17 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
* This data_race() macro is useful for situations in which data races
* should be forgiven. One example is diagnostic code that accesses
* shared variables but is not a part of the core synchronization design.
+ * For example, if accesses to a given variable are protected by a lock,
+ * except for diagnostic code, then the accesses under the lock should
+ * be plain C-language accesses and those in the diagnostic code should
+ * use data_race(). This way, KCSAN will complain if buggy lockless
+ * accesses to that variable are introduced, even if the buggy accesses
+ * are protected by READ_ONCE() or WRITE_ONCE().
*
- * This macro *does not* affect normal code generation, but is a hint
- * to tooling that data races here are to be ignored.
+ * This macro *does not* affect normal code generation, but is a hint to
+ * tooling that data races here are to be ignored. If code generation must
+ * be protected *and* KCSAN should ignore the access, use both data_race()
+ * and READ_ONCE(), for example, data_race(READ_ONCE(x)).
*/
#define data_race(expr) \
({ \
