> > The formula was more along the line of "do not assume either of these > > cases to hold; use barrier() is you need an unconditional barrier..." > > AFAICT, all current implementations of smp_mb__{before,after}_atomic() > > provides a compiler barrier with either barrier() or "memory" clobber. > > Well, we have two reasonable choices: Say that > smp_mb__{before,after}_atomic will always provide a compiler barrier, > or don't say this. I see no point in saying that the combination of > Before-atomic followed by RMW provides a barrier.
;-/ I'm fine with the first choice. I don't see how the second choice (this proposal/patch) would be consistent with some documentation and with the current implementations; for example, 1) Documentation/atomic_t.txt says: Thus: atomic_fetch_add(); is equivalent to: smp_mb__before_atomic(); atomic_fetch_add_relaxed(); smp_mb__after_atomic(); [...] 2) Some implementations of the _relaxed() variants do not provide any compiler barrier currently. Andrea