Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work_done()

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 09:57:55AM -0700, Paul E. McKenney wrote:
> 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.

Which 'some' think it only provides acquire/release ?

I made very sure -- when I renamed/audited/wrote all this -- that they
indeed do a full memory barrier.

> How do we decide?

I say its a full mb, always was.

People used it to create acquire/release _like_ constructs, because we
simply didn't have anything else.

Also, I think Linus once opined that acquire/release is part of a
store/load (hence smp_store_release/smp_load_acquire) and not a barrier.

> Once we do decide, atomic_ops.txt of course needs to be updated accordingly.

There was so much missing there that I didn't quite know where to start.