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>
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/* Weak atomic operations prevent the compiler moving other
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* loads/stores past the atomic operation load/store. However there is
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* no explicit memory barrier for the processor.
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*
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* The C11 memory model says that variables that are accessed from
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* different threads should at least be done with __ATOMIC_RELAXED
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* primitives or the result is undefined. Generally this has little to
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* no effect on the generated code but not using the atomic primitives
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* will get flagged by sanitizers as a violation.
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*/
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#define atomic_read(ptr) \
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({ \
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