linux-kernel - Re: [RFC PATCH] rcu: Change use of __this

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Date:	Fri, 19 Sep 2014 13:57:54 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] rcu: Change use of __this_cpu ops on a bool type

On Fri, Sep 19, 2014 at 01:26:03PM -0400, Pranith Kumar wrote:
> __this_cpu_{read/write}() uses sizeof() to determine the size of the variable.
> Using this operation on a bool type causes sparse to complain.
> 
> I checked the generated code and there is only a slight difference in
> instructions generated.
> 
> I am not sure this warning by sparse is really valid, but let us silence it
> anyways.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

I believe this warning was actually fixed with current versions of
Sparse; there was some discussion and a patch on the linux-sparse
mailing list.  Can you please recheck with the latest Sparse from git,
and if it still breaks, please raise that on linux-sparse?

>  kernel/rcu/tree.c        | 12 ++++++++----
>  kernel/rcu/tree_plugin.h |  6 ++++--
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bc085e5..85f228a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -186,21 +186,25 @@ static int rcu_gp_in_progress(struct rcu_state *rsp)
>   */
>  void rcu_sched_qs(void)
>  {
> -	if (!__this_cpu_read(rcu_sched_data.passed_quiesce)) {
> +	struct rcu_data *rdp = __this_cpu_ptr(&rcu_sched_data);
> +
> +	if (!rdp->passed_quiesce) {
>  		trace_rcu_grace_period(TPS("rcu_sched"),
>  				       __this_cpu_read(rcu_sched_data.gpnum),
>  				       TPS("cpuqs"));
> -		__this_cpu_write(rcu_sched_data.passed_quiesce, 1);
> +		rdp->passed_quiesce = true;
>  	}
>  }
>  
>  void rcu_bh_qs(void)
>  {
> -	if (!__this_cpu_read(rcu_bh_data.passed_quiesce)) {
> +	struct rcu_data *rdp = __this_cpu_ptr(&rcu_bh_data);
> +
> +	if (!rdp->passed_quiesce) {
>  		trace_rcu_grace_period(TPS("rcu_bh"),
>  				       __this_cpu_read(rcu_bh_data.gpnum),
>  				       TPS("cpuqs"));
> -		__this_cpu_write(rcu_bh_data.passed_quiesce, 1);
> +		rdp->passed_quiesce = true;
>  	}
>  }
>  
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 77f9376..98de303 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -143,11 +143,13 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
>   */
>  static void rcu_preempt_qs(void)
>  {
> -	if (!__this_cpu_read(rcu_preempt_data.passed_quiesce)) {
> +	struct rcu_data *rdp = __this_cpu_ptr(&rcu_preempt_data);
> +
> +	if (!rdp->passed_quiesce) {
>  		trace_rcu_grace_period(TPS("rcu_preempt"),
>  				       __this_cpu_read(rcu_preempt_data.gpnum),
>  				       TPS("cpuqs"));
> -		__this_cpu_write(rcu_preempt_data.passed_quiesce, 1);
> +		rdp->passed_quiesce = true;
>  		barrier(); /* Coordinate with rcu_preempt_check_callbacks(). */
>  		current->rcu_read_unlock_special.b.need_qs = false;
>  	}
> -- 
> 2.1.0
> 
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