linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Thu, 7 Aug 2014 08:43:58 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Thu, Aug 07, 2014 at 10:49:21AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 05, 2014 at 02:55:10PM -0700, Paul E. McKenney wrote:
> > +/* Check for nohz_full CPUs executing in userspace. */
> > +static void check_no_hz_full_tasks(void)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	int cpu;
> > +	struct task_struct *t;
> > +
> > +	for_each_online_cpu(cpu) {
> > +		cond_resched();
> > +		rcu_read_lock();
> > +		t = rcu_dynticks_task_cur(cpu);
> > +		if (t == NULL || is_idle_task(t)) {
> > +			rcu_read_unlock();
> > +			continue;
> > +		}
> > +		if (ACCESS_ONCE(t->rcu_tasks_holdout))
> > +			ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> > +		rcu_read_unlock();
> > +	}
> > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > +}
> 
> That's not hotplug safe afaict, and I've no idea if someone pointed that
> out already because people refuse to trim email and I can't be arsed to
> wade through pages and pages of quoting.

Hmmm...  That does look a bit suspicious, now that you mention it.
After all, if a CPU is offline, its idle tasks cannot be on a
trampoline.

							Thanx, Paul

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