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[PULL,11/39] atomic.h: comment on use of atomic_read/set

Message ID 1475859483-32234-12-git-send-email-pbonzini@redhat.com
State New
Headers show

Commit Message

Paolo Bonzini Oct. 7, 2016, 4:57 p.m. UTC 
From: Alex Bennée <alex.bennee@linaro.org>


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>

---
 include/qemu/atomic.h | 6 ++++++
 1 file changed, 6 insertions(+)

-- 
2.7.4
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Patch

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)                              \
     ({                                                \