linux-kernel - Re: [PATCH 1/4] rcu: Don't check irq nesting from rcu idle entry/exit

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Date:	Tue, 29 Nov 2011 01:26:01 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 1/4] rcu: Don't check irq nesting from rcu idle entry/exit

On Mon, Nov 28, 2011 at 04:22:44PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 28, 2011 at 10:24:44PM +0100, Frederic Weisbecker wrote:
> > rcu_idle_enter() and rcu_idle_exit() don't need to check
> > the irq nesting as they are not called from irq.
> > 
> > Only check dyntick_nesting from rcu_irq_enter() and
> > rcu_irq_exit().
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Josh Triplett <josh@...htriplett.org>
> > ---
> >  kernel/rcutree.c |   18 ++++++++----------
> >  1 files changed, 8 insertions(+), 10 deletions(-)
> 
> I have queued this.  I had to update it to deal with a recent conflicting
> update, please see below for the new version.
> 
> 							Thanx, Paul

Hmm, seems there is a problem. Look below:

> 
> ------------------------------------------------------------------------
> 
> rcu: Don't check irq nesting from rcu idle entry/exit
> 
> Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
> not need to check for nesting.  This commit therefore moves nesting
> checks from rcu_idle_enter_common() to rcu_irq_exit() and from
> rcu_idle_exit_common() to rcu_irq_enter().
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index bf085d7..54d9b4f 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -350,10 +350,6 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
>   */
>  static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
>  {
> -	if (rdtp->dynticks_nesting) {
> -		trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
> -		return;
> -	}
>  	trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
>  	if (!is_idle_task(current)) {
>  		struct task_struct *idle = idle_task(smp_processor_id());
> @@ -426,7 +422,10 @@ void rcu_irq_exit(void)
>  	oldval = rdtp->dynticks_nesting;
>  	rdtp->dynticks_nesting--;
>  	WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
> -	rcu_idle_enter_common(rdtp, oldval);
> +	if (rdtp->dynticks_nesting)
> +		trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
> +	else
> +		rcu_idle_enter_common(rdtp, oldval);
>  	local_irq_restore(flags);
>  }
>  
> @@ -439,10 +438,6 @@ void rcu_irq_exit(void)
>   */
>  static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
>  {
> -	if (oldval) {
> -		trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
> -		return;
> -	}
>  	smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
>  	atomic_inc(&rdtp->dynticks);
>  	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
> @@ -519,6 +514,10 @@ void rcu_irq_enter(void)
>  	rdtp->dynticks_nesting++;
>  	WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
>  	rcu_idle_exit_common(rdtp, oldval);

It's now calling rcu_idle_exit_common twice?

> +	if (oldval)
> +		trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
> +	else
> +		rcu_idle_exit_common(rdtp, oldval);
>  	local_irq_restore(flags);
>  }
>  
> 
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