linux-kernel - Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work

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Date:   Thu, 13 Apr 2017 09:57:55 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com, dvyukov@...gle.com,
        will.deacon@....com
Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to
 sync_exp_work_done()

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