From 4046235e92daa21b2f2aeedce63b0bf7b8dcf1ff Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Alex=20Benn=C3=A9e?= Date: Mon, 26 Feb 2018 05:03:53 -0500 Subject: [PATCH] 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. Backports commit e653bc6b0ff645c25b8a2eb607c18a5c98b59db6 from qemu --- qemu/include/qemu/atomic.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/qemu/include/qemu/atomic.h b/qemu/include/qemu/atomic.h index f0fdef11..6fe5a06b 100644 --- a/qemu/include/qemu/atomic.h +++ b/qemu/include/qemu/atomic.h @@ -57,6 +57,12 @@ void _ReadWriteBarrier(void); /* 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) \ ({ \