On Thu, Apr 13, 2017 at 06:24:09PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 09:10:42AM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 13, 2017 at 11:18:32AM +0200, Peter Zijlstra wrote:
> > > On Wed, Apr 12, 2017 at 09:55:43AM -0700, Paul E. McKenney wrote:
> > > > However, a little future-proofing is a good thing,
> > > > especially given that smp_mb__before_atomic() is only required to
> > > > provide acquire semantics rather than full ordering. This commit
> > > > therefore adds smp_mb__after_atomic() after the atomic_long_inc()
> > > > in sync_exp_work_done().
> > >
> > > Oh!? As far as I'm away the smp_mb__{before,after}_atomic() really must
> > > provide full MB, no confusion about that.
> > >
> > > We have other primitives for acquire/release.
> >
> > Hmmm... Rechecking atomic_ops.txt, it does appear that you are quite
> > correct. Adding Will and Dmitry on CC, but dropping this patch for now.
>
> I'm afraid that document is woefully out dated. I'm surprised it says
> anything on the subject.
And there is some difference of opinion. Some believe that the
smp_mb__before_atomic() only guarantees acquire and smp_mb__after_atomic()
only guarantees release, but all current architectures provide full
ordering, as you noted and as stated in atomic_ops.txt.
How do we decide?
Once we do decide, atomic_ops.txt of course needs to be updated accordingly.
Thanx, Paul
