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 07:22:30 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Thu, Jan 08, 2015 at 10:41:02AM +0100, Peter Zijlstra wrote:
> 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?

Indeed we did!  But that was after I did this commit back on October 29th.

I am planning a bulk change to READ_ONCE() and ASSIGN_ONCE() either as
the last patch for 3.20 or as the first one for 3.21.  Probably as the
first for 3.21 to minimize rebasing hassles with any needed 3.20 fixes.

							Thanx, Paul

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