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 13:22:17 +0100
From:   Will Deacon <will.deacon@....com>
To:     David Howells <dhowells@...hat.com>
Cc:     paulmck@...ux.vnet.ibm.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 01:19:59PM +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.

Will