linux-kernel - Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier

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Date:   Wed, 11 Oct 2017 05:54:51 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org,
        mingo@...nel.org, torvalds@...ux-foundation.org,
        mark.rutland@....com, linux-arch@...r.kernel.org,
        peterz@...radead.org, Jonathan Corbet <corbet@....net>,
        Alexander Kuleshov <kuleshovmail@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove
 smp_read_barrier_depends()

On Wed, Oct 11, 2017 at 01:22:17PM +0100, Will Deacon wrote:
> On Wed, Oct 11, 2017 at 01:19:59PM +0100, David Howells wrote:
> > Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > -	node = result.terminal_node.node;
> > > -	smp_read_barrier_depends();
> > > +	node = READ_ONCE(result.terminal_node.node); /* Address dependency. */
> > 
> > The main problem I have with this method of annotation is that it's not
> > obvious there's a barrier there or which side the barrier is.
> > 
> > I think one of the trickiest issues is that a barrier is typically between two
> > things and we're not making it clear what those two things actually are.
> > 
> > Also, I would say that the most natural interpretation of READ_ONCE() is that
> > the implicit barrier comes after the read, e.g.:
> > 
> > 	f = READ_ONCE(stuff->foo);
> > 	/* Implied barrier */
> > 	look_at(f->a);
> > 	look_at(f->b);
> > 
> > I.e. READ_ONCE() prevents stuff->foo from being reread whilst you access f and
> > orders LOAD(stuff->foo) before LOAD(f->a) and LOAD(f->b).
> 
> FWIW, that's exactly what my patches do, this fixup looks a bit weird
> because it removes a prior barrier which suggests that either (a) it's in
> the wrong place to start with, or (b) we're annotating the wrong load.

You lost me on this one.  Here is the side-by-side change, minus the
comment:

node = result.terminal_node.node;		 node = READ_ONCE(result.terminal_node.node);
smp_read_barrier_depends();

The barrier was after the load that got annotated.

Or are you talking about some other fixup?

								Thanx, Paul