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

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Date:	Sun, 3 Aug 2014 15:05:35 -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 v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Sun, Aug 03, 2014 at 03:33:18PM +0200, Oleg Nesterov wrote:
> On 08/02, Paul E. McKenney wrote:
> >
> > On Sat, Aug 02, 2014 at 04:56:16PM +0200, Oleg Nesterov wrote:
> > > On 07/31, 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)) {
> > >
> > > I didn't notice this check before, but it is not needed. for_each_process()
> > > can see the idle threads, there are not on process/thread lists.
>     ^^^^^^^
> 
> argh, I mean't "can't see" of course...
> 
> > Good to know.  Any other important tasks I am missing?
> 
> Nothing else.

Whew!  ;-)

> > I am guessing that I need to do something like this:
> >
> > 	for_each_process(g) {
> > 		/* Do build step. */
> > 		for_each_thread(g, t) {
> > 			if (g == t)
> > 				continue;
> > 			/* Do build step. */
> > 		}
> > 	}
> 
> Sorry, I don't understand... This is equivalent to
> 
> 	for_each_process_thread(g, t) {
> 		/* Do build step. */
> 	}

OK, got it.

> > Your point about exiting tasks I get, and I believe I can solve it.
> > I am hoping that the above sort of construction takes care of the
> > idle threads.
> 
> It is simple to find the idle threads, just
> 
> 	for_each_cpu(cpu) {
> 		do_something(cpu_rq(cpu)->idle);
> 	}
> 
> but it is not clear to me what else you need to do with the idle threads.
> Probably not too much, they mostly run under preempt_disable().

OK, looks easy enough.  And yes, one good question is what, if anything,
we need to do with the idle threads.

> > I might also need to do something to handle changes in
> > process/thread hierarchy -- but hopefully without having to read-acquire
> > the task-list lock.
> 
> It seems that you need another global list, a task should be visible on that
> list until exit_rcu().

As in create another global list that all tasks are added to when created
and then remved from when they exit?

							Thanx, Paul

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