Atomics and memory barriers

[Malte Vesper]

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  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L288>  If a caller requires memory barrier semantics around an atomic_t
289  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L289>  operation which does not return a value, a set of interfaces are
290  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L290>  defined which accomplish this:
291  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L291>  
292  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L292>  	void smp_mb__before_atomic(void);
293  <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L293>  	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
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