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

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Message-ID: <20140802225757.GB8101@linux.vnet.ibm.com>
Date:	Sat, 2 Aug 2014 15:57:57 -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 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.

Good to know.  Any other important tasks I am missing?

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. */
		}
	}

Or is there a better way to handle this?

> But this doesn't really matter, the main problem is that I still think that
> for_each_process_thread() can't really work anyway.

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.  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.

So what else am I missing?

							Thanx, Paul

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