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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>
This commit is contained in:
Alex Bennée 2016-09-30 22:30:53 +01:00 committed by Paolo Bonzini
parent 23ea7f5794
commit e653bc6b0f
1 changed files with 6 additions and 0 deletions
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6
include/qemu/atomic.h
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@ -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) \
({ \