linux-kernel - Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless accesses with ACCESS

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Date:	Thu, 8 Jan 2015 10:41:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless
 accesses with ACCESS_ONCE()

On Wed, Jan 07, 2015 at 09:32:20AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> This commit prevents random compiler optimizations by applying
> ACCESS_ONCE() to lockless accesses.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcu/tree_plugin.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3ec85cb5d544..d59913ef8360 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1127,7 +1127,8 @@ static int rcu_boost(struct rcu_node *rnp)
>  	struct task_struct *t;
>  	struct list_head *tb;
>  
> -	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
> +	if (ACCESS_ONCE(rnp->exp_tasks) == NULL &&
> +	    ACCESS_ONCE(rnp->boost_tasks) == NULL)
>  		return 0;  /* Nothing left to boost. */

Didn't we just obsolete ACCESS_ONCE with that {READ,WRITE}_ONCE stuff?
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