linux-kernel - Re: [PATCH 1/1] rcu: remove remaining read-modify-write ACCESS

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Date:	Tue, 8 Jul 2014 15:09:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] rcu: remove remaining read-modify-write
 ACCESS_ONCE() calls

On Tue, Jul 08, 2014 at 05:46:50PM -0400, Pranith Kumar wrote:
> Change the remaining uses of ACCESS_ONCE() so that each ACCESS_ONCE() either does a load or a store, but not both.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

Queued for 3.18, thank you Pranith!

								Thanx, Paul

> ---
>  kernel/rcu/tree.c        | 6 ++++--
>  kernel/rcu/tree_plugin.h | 8 +++++---
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index dac6d20..c356bf6 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1700,7 +1700,8 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
>  	if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
>  		raw_spin_lock_irq(&rnp->lock);
>  		smp_mb__after_unlock_lock();
> -		ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
> +		ACCESS_ONCE(rsp->gp_flags) =
> +			ACCESS_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS;
>  		raw_spin_unlock_irq(&rnp->lock);
>  	}
>  	return fqs_state;
> @@ -2514,7 +2515,8 @@ static void force_quiescent_state(struct rcu_state *rsp)
>  		raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
>  		return;  /* Someone beat us to it. */
>  	}
> -	ACCESS_ONCE(rsp->gp_flags) |= RCU_GP_FLAG_FQS;
> +	ACCESS_ONCE(rsp->gp_flags) =
> +		ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS;
>  	raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
>  	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
>  }
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 637a8a9..f87b88c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -897,7 +897,8 @@ void synchronize_rcu_expedited(void)
> 
>  	/* Clean up and exit. */
>  	smp_mb(); /* ensure expedited GP seen before counter increment. */
> -	ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
> +	ACCESS_ONCE(sync_rcu_preempt_exp_count) =
> +					sync_rcu_preempt_exp_count + 1;
>  unlock_mb_ret:
>  	mutex_unlock(&sync_rcu_preempt_exp_mutex);
>  mb_ret:
> @@ -2307,8 +2308,9 @@ static int rcu_nocb_kthread(void *arg)
>  			list = next;
>  		}
>  		trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
> -		ACCESS_ONCE(rdp->nocb_p_count) -= c;
> -		ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
> +		ACCESS_ONCE(rdp->nocb_p_count) = rdp->nocb_p_count - c;
> +		ACCESS_ONCE(rdp->nocb_p_count_lazy) =
> +						rdp->nocb_p_count_lazy - cl;
>  		rdp->n_nocbs_invoked += c;
>  	}
>  	return 0;
> -- 
> 1.9.1
> 

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