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

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Message-ID: <15076.1512401936@warthog.procyon.org.uk>
Date:   Mon, 04 Dec 2017 15:38:56 +0000
From:   David Howells <dhowells@...hat.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     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, peterz@...radead.org,
        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()

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

David