linux-kernel - Re: [RFC][PATCH RT 1/6] rt,rcu: Have rcu_read_lock_sched() use locks for PREEMPT

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Date:	Thu, 27 Jun 2013 07:28:09 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Galbraith <bitbucket@...ine.de>
Cc:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clark.williams@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH RT 1/6] rt,rcu: Have rcu_read_lock_sched() use
 locks for PREEMPT_RT

On Thu, 2013-06-27 at 05:52 +0200, Mike Galbraith wrote:
> On Wed, 2013-06-26 at 15:28 -0400, Steven Rostedt wrote:
> 
> > @@ -2491,6 +2491,31 @@ static inline int rcu_blocking_is_gp(voi
> >  	return ret;
> >  }
> >  
> > +#ifdef CONFIG_PREEMPT_RT_FULL
> > +DEFINE_LOCAL_IRQ_LOCK(rcu_sched_lock);
> > +/*
> > + * Real-time allows for synchronize sched to sleep but not migrate.
> > + * This is done via the local locks. When calling synchronize_sched(),
> > + * all the local locks need to be taken and released. This will ensure
> > + * that all users of rcu_read_lock_sched() will be out of their critical
> > + * sections at the completion of this function. synchronize_sched() will
> > + * still perform the normal RCU sched operations to synchronize with
> > + * locations that use disabling preemption or interrupts.
> > + */
> > +static void rcu_synchronize_sched_rt(void)
> > +{
> > +	int cpu;
> > +
> > +	for_each_possible_cpu(cpu) {
> > +		spin_lock(&per_cpu(rcu_sched_lock, cpu).lock);
> > +		spin_unlock(&per_cpu(rcu_sched_lock, cpu).lock);
> > +	}
> > +}
> 
> Does that have to be possible vs online?

No, I was doing the "paranoid" approach, as I didn't know how much
hotplug may be using synchronize_sched() and wanted to make sure I hit
all CPUs. But I think online would be sufficient.

-- Steve


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