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

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Message-ID: <26455.1507724399@warthog.procyon.org.uk>
Date:   Wed, 11 Oct 2017 13:19:59 +0100
From:   David Howells <dhowells@...hat.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     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, will.deacon@....com,
        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()

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).

David