Difference between atomic operations and memory barriers

[Leonidas .]

Trying to understand difference between atomic ops and memory barriers.

I was thinking all atomic operations must be using barrier internally,
but I read
somewhere that only some of them use barriers. Sorry for being vague here.

Operations before call to smp_mb() will not be re-ordered and all cpus will see
consistent value after the variable is updated.

E.g.
void * ptr = (void *) str;
smb_mb();

Will this not atomically update ptr? Ptr will be seen by all cpu's in
same state
after it has been assigned str, right?

-Leo.
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