From e653bc6b0ff645c25b8a2eb607c18a5c98b59db6 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Alex=20Benn=C3=A9e?= Date: Fri, 30 Sep 2016 22:30:53 +0100 Subject: atomic.h: comment on use of atomic_read/set MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Message-Id: <20160930213106.20186-3-alex.bennee@linaro.org> Signed-off-by: Paolo Bonzini --- include/qemu/atomic.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index c493f89..c4f6950 100644 --- 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) \ ({ \ -- cgit v1.1