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Message-ID: <20150108094102.GD29390@twins.programming.kicks-ass.net>
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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