Hello,
I have been reading up on atomics and struggle to grasp when exactly I need explicit memory barriers.

While the documentation (Documentat/atomic_ops.txt), talks about operations needing explicit barriers both sides, I assume this is only half true.
If I for instance only want to use an atomic as a flag to show that some operation is complete I reckon that a barrier before is sufficient.

Furthermore I found the following lines:

288 If a caller requires memory barrier semantics around an atomic_t
289 operation which does not return a value, a set of interfaces are
290 defined which accomplish this:
291 
292 	void smp_mb__before_atomic(void);
293 	void smp_mb__after_atomic(void);

note how it refers to "operation which does not return a value", why can't I use these for atomic operations that do return a value?
What should I use instead, normal barriers like mb?

Please enlighten me
Malte