linux-kernel - Re: [PATCH v2 2/2] rcu: Check the return value of rcu_nocb

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Date:	Fri, 25 Jul 2014 11:06:32 -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 v2 2/2] rcu: Check the return value of rcu_nocb_mask
 cpumask allocation

On Fri, Jul 25, 2014 at 08:25:51AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 24, 2014 at 08:37:32PM -0400, Pranith Kumar wrote:
> > This commit checks the return value of the zalloc_cpumask_var() used for
> > allocating cpumask for rcu_nocb_mask.
> > 
> > Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> 
> Hmmm...  I saw the check in the previous patch, but didn't see removal
> of the later have_rcu_nocb_mask check.  Please see below.

And events overtook this one.  Turns out that commit b58cc46c5f6b
(rcu: Don't offload callbacks unless specifically requested) was not
one of my best efforts.

I will adapt your patch 2/2 in this series and apply it with your
Signed-off-by.  Patch 1/2 is obsoleted by the current fix for
commit b58cc46c5f6b.

							Thanx, Paul

> > ---
> > v2: no change from v1
> > 
> >  kernel/rcu/tree_plugin.h | 23 ++++++++++++-----------
> >  1 file changed, 12 insertions(+), 11 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 520538a..9c9a01c 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -54,7 +54,10 @@ static void __init rcu_bootup_announce_oddness_nocb(void)
> >  {
> >  #ifndef CONFIG_RCU_NOCB_CPU_NONE
> >  	if (!have_rcu_nocb_mask) {
> > -		zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
> > +		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> > +			pr_info("rcu_nocb_mask allocation failed\n");
> > +			return;
> > +		}
> >  		have_rcu_nocb_mask = true;
> >  	}
> >  #ifdef CONFIG_RCU_NOCB_CPU_ZERO
> > @@ -66,17 +69,15 @@ static void __init rcu_bootup_announce_oddness_nocb(void)
> >  	cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> >  #endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> >  #endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
> > -	if (have_rcu_nocb_mask) {
> > -		if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> > -			pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
> > -			cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > -				    rcu_nocb_mask);
> > -		}
> > -		cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
> > -		pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
> > -		if (rcu_nocb_poll)
> > -			pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
> > +	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> 
> What happens if CONFIG_RCU_NOCB_CPU_NONE=y and the rcu_nocbs= boot
> parameter is not specified and we get here?
> 
> In order to get visible failures when testing, build with
> CONFIG_CPUMASK_OFFSTACK=y.
> 
> > +		pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
> > +		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > +				rcu_nocb_mask);
> >  	}
> > +	cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
> > +	pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
> > +	if (rcu_nocb_poll)
> > +		pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
> >  }
> > 
> >  /*
> > -- 
> > 2.0.1
> > 

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