linux-kernel - Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}

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Date:   Wed, 24 Apr 2019 10:44:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided
 by smp_mb__{before,after}_atomic()

On Wed, Apr 24, 2019 at 01:29:48AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 23, 2019 at 10:28:31PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 23, 2019 at 01:16:37PM -0700, Paul E. McKenney wrote:
> > 
> > > Agreed, but I thought that one of the ideas going forward was to get
> > > rid of smp_mb__{before,after}_atomic().
> > 
> > It's not one I had considered.. I just wanted to get rid of this
> > 'surprise' behaviour.
> 
> Ah, good point, your patch is in fact a midpoint between those two
> positions.  Just to make sure I understand:
> 
> 1.	Without your patch, smp_mb__{before,after}_atomic() orders
> 	only against the atomic itself.

Right, and that was not intentional.

> 2.	With your patch, smp_mb__{before,after}_atomic() orders against
> 	the atomic itself and the accesses on the other side of that
> 	atomic.  However, it does not order the atomic against the
> 	accesses on the other side of that atomic.

Right. I'll go make a more complete patch, covering all the
architectures.

> 	Putting things between the smp_mb__{before,after}_atomic()
> 	and the atomic is in my opinion a bad idea, but in this case
> 	they are not necessarily ordered.

Agreed, that is an unsupported idiom and it would be good to have
something check for this.

> 3.	Dispensing with smp_mb__{before,after}_atomic() would have
> 	void RMW atomics fully ordered, but I suspect that it results
> 	in ugly performance regressions.
> 
> Or am I still missing something?

I think we're good :-)