linux-kernel - Re: [PATCH RFC tip/core/rcu 0/15] Remove to-be-unneeded smp_read_barrier

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Message-Id: <20171011125636.GX3521@linux.vnet.ibm.com>
Date:   Wed, 11 Oct 2017 05:56:36 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-kernel@...r.kernel.org, mark.rutland@....com,
        linux-arch@...r.kernel.org, viro@...iv.linux.org.uk, tj@...nel.org,
        cl@...ux.com, davem@...emloft.net, peterz@...radead.org,
        mingo@...hat.com, rostedt@...dmis.org, akpm@...ux-foundation.org,
        corbet@....net, james.l.morris@...cle.com,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC tip/core/rcu 0/15] Remove to-be-unneeded
 smp_read_barrier_depends()

On Wed, Oct 11, 2017 at 01:21:03PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > Will Deacon has proposed adding smp_read_barrier_depends() to READ_ONCE(),
> > which would mean that quite a few instances of smp_read_barrier_depends()
> > would become redundant.
> 
> It's not clear from you description where the barrier is added in relation to
> the read: before, after or both?

After, similar to lockless_dereference().  Of course, there is not
really a barrier except for on Alpha.

							Thanx, Paul