atomic.h: comment on use of atomic_read/set

Add some notes on the use of the relaxed atomic access helpers and their
importance for defined behaviour in C11's multi-threaded memory model.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20160930213106.20186-3-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index c493f89..c4f6950 100644 (file)
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -92,6 +92,12 @@
 /* Weak atomic operations prevent the compiler moving other
  * loads/stores past the atomic operation load/store. However there is
  * no explicit memory barrier for the processor.
+ *
+ * The C11 memory model says that variables that are accessed from
+ * different threads should at least be done with __ATOMIC_RELAXED
+ * primitives or the result is undefined. Generally this has little to
+ * no effect on the generated code but not using the atomic primitives
+ * will get flagged by sanitizers as a violation.
  */
 #define atomic_read(ptr)                              \
     ({                                                \