linux-kernel - Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu

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Message-ID: <20140801181905.GH4784@linux.vnet.ibm.com>
Date:	Fri, 1 Aug 2014 11:19:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, bobby.prani@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks()

On Fri, Aug 01, 2014 at 05:53:10PM +0200, Oleg Nesterov wrote:
> On 07/30, Paul E. McKenney wrote:
> >
> > +		rcu_read_lock();
> > +		for_each_process_thread(g, t) {
> > +			if (t != current && ACCESS_ONCE(t->on_rq) &&
> > +			    !is_idle_task(t)) {
> > +				t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> > +				t->rcu_tasks_holdout = 1;
> > +				list_add(&t->rcu_tasks_holdout_list,
> > +					 &rcu_tasks_holdouts);
> > +			}
> > +		}
> > +		rcu_read_unlock();
> 
> Wait, unless I missed something this can't work...
> 
> The problem is, once the exiting task passes exit_notify() it can
> be removed from rcu lists.
> 
> Now suppose that (say) proc_exit_connector() has a probe, and this
> task has jumped into trampoline and it was preempted there.
> 
> No?

OK, this sounds to me like another vote for get_task_struct() as used
in the v3 series.  Or am I missing something?

							Thanx, Paul

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