On Sat, Apr 13, 2019 at 11:39:38PM +0200, Andrea Parri wrote: > On Tue, Apr 09, 2019 at 08:01:32AM -0700, Paul E. McKenney wrote: > > On Tue, Apr 09, 2019 at 03:36:18AM +0200, Andrea Parri wrote: > > > > > 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. > > > > But don't all implementations of smp_mb__before_atomic() and > > smp_mb__after_atomic() currently supply a compiler barrier? > > Yes, AFAICS, all implementations of smp_mb__{before,after}_atomic() currently > supply a compiler barrier. > > Nevertheless, there's a difference between: (1) Specify that these barriers > supply a compiler barrier, (2) Specify that (certain) combinations of these > barriers and RMWs supply a compiler barrier, and (3) This patch... ;-) > > FWIW, I'm not aware of current/informal documentation following (the arguably > simpler but slightly stronger) (1). But again (amending my last remark): (1) > and (2) both make sense to me.
Another question is "should the kernel permit smp_mb__{before,after}*() anywhere other than immediately before or after the primitive being strengthened?" Thoughts? Thanx, Paul