linux-kernel - Re: [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies smp_barrier

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Message-ID: <20171204215448.GY3326@worktop>
Date:   Mon, 4 Dec 2017 22:54:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies
 smp_barrier_depends()

On Mon, Dec 04, 2017 at 10:52:15AM -0800, Paul E. McKenney wrote:
> On Mon, Dec 04, 2017 at 03:38:56PM +0000, David Howells wrote:
> > Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > -	Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q);
> > > +	Q = READ_ONCE(P); D = READ_ONCE(*Q);
> > >  
> > >       the CPU will issue the following memory operations:
> > >  
> > >  	Q = LOAD P, D = LOAD *Q
> > 
> > The CPU may now issue two barriers in addition to the loads, so should we show
> > this?  E.g.:
> > 
> > 	Q = LOAD P, BARRIER, D = LOAD *Q, BARRIER
> 
> Good point!  How about as shown in the updated patch below?

Humm, I thought the idea was to completely remove read_barrier_depends
from the lkmm and memory-barriers.txt, making it an Alpha implementation
detail.