path: root/include
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authorAlex Bennée <alex.bennee@linaro.org>2016-09-30 22:30:53 +0100
committerPaolo Bonzini <pbonzini@redhat.com>2016-10-04 10:00:25 +0200
commite653bc6b0ff645c25b8a2eb607c18a5c98b59db6 (patch)
tree2a558ef7e18bf2a423d2a52ebc7bfb9135e95f01 /include
parent23ea7f57949f2f5934f4d5bbc29fe321b3a7067b (diff)
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>
Diffstat (limited to 'include')
1 files changed, 6 insertions, 0 deletions
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index c493f89059..c4f6950fcb 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) \
({ \