linux-kernel - Re: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb

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Date:	Fri, 18 Jul 2014 23:10:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	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: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of
 rcu_nocb_mask

On Thu, Jul 17, 2014 at 08:11:01PM -0400, Pranith Kumar wrote:
> On 07/17/2014 07:49 PM, Paul E. McKenney wrote:
> > On Thu, Jul 17, 2014 at 05:30:11PM -0400, Pranith Kumar wrote:
> >> Hi Paul,
> >>
> >> This is something similar to what you found yesterday with tick_nohz_full mask.
> >>
> >> --
> >> Pranith
> >>
> >> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
> >> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
> >> cpumask rcu_nocb_mask can be garbage instead of NULL.
> >>
> >> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
> >> have_rcu_nocb_mask.
> >>
> >> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> > 
> > Good catch!  Could you please forward-port this to current rcu/dev?
> > This now includes the fix you mentioned above, which generates conflicts.
> > 
> > Could you please also fix the use in rcu_is_nocb_cpu()?
> > 
> > And please see the comment below.
> > 
> 
> Please find the fixed patch.
> 
> --
> Pranith
> 
> 
> From: Pranith Kumar <bobby.prani@...il.com>
> Date: Thu, 17 Jul 2014 20:02:54 -0400
> Subject: [PATCH 1/1] rcu: Feedback comments
> 
> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
> cpumask rcu_nocb_mask can be garbage instead of NULL.
> 
> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
> have_rcu_nocb_mask.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
>  kernel/rcu/tree_plugin.h | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index ce8c331..8f987c1 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2053,9 +2053,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>  /* Is the specified CPU a no-CBs CPU? */
>  bool rcu_is_nocb_cpu(int cpu)
>  {
> -	if (have_rcu_nocb_mask)
> -		return cpumask_test_cpu(cpu, rcu_nocb_mask);
> -	return false;
> +	if (!have_rcu_nocb_mask)
> +		return false;
> +
> +	return cpumask_test_cpu(cpu, rcu_nocb_mask);

Hmmm...  That was a subtle way to tell me that my feedback was bogus.  ;-)
I guess I need to stop reviewing patches at 2AM local time.

Anyway, you are correct, this was OK to start with, so I am dropping
this hunk.

I queued the other two for 3.18.

							Thanx, Paul

>  }
>  #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
> 
> @@ -2540,7 +2541,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
>  	struct rcu_data *rdp_prev = NULL;
> 
> -	if (rcu_nocb_mask == NULL)
> +	if (!have_rcu_nocb_mask)
>  		return;
>  #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
>  	if (tick_nohz_full_running)
> @@ -2574,9 +2575,9 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
>  static bool init_nocb_callback_list(struct rcu_data *rdp)
>  {
> -	if (rcu_nocb_mask == NULL ||
> -	    !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
> +	if (!rcu_is_nocb_cpu(rdp->cpu))
>  		return false;
> +
>  	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
>  	return true;
>  }
> -- 
> 1.9.1
> 
> 
> --
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