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

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Date:	Thu, 31 Jul 2014 10:44:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
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, 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 Thu, Jul 31, 2014 at 07:27:52PM +0200, Oleg Nesterov wrote:
> On 07/31, Paul E. McKenney wrote:
> >
> > On Thu, Jul 31, 2014 at 06:31:38PM +0200, Oleg Nesterov wrote:
> >
> > > But can't we avoid get_task_struct()? This can pin a lot of task_struct's.
> > > Can't we just add list_del_rcu(holdout_list) into __unhash_process() ?
> >
> > If I add the list_del_rcu() there, then I am back to a concurrent list,
> > which I would like to avoid.  Don't get me wrong, it was fun playing with
> > the list-locked stuff, but best to avoid it if we can.
> 
> OK,
> 
> > The nice thing about using get_task_struct to lock
> > them down is that -only- the task_struct itself is locked down -- the
> > task can be reaped and so on.
> 
> I understand. but otoh it would be nice to not pin this memory if the
> task was already (auto)reaped.
> 
> And afaics the number of pinned task_struct's is not bounded. In theory
> it is not even limited by, say, PID_MAX_LIMIT. A thread can exit and reap
> itself right after get_task_struct() but create another running thread
> which can be noticed by rcu_tasks_kthread() too.

Good point!  Maybe this means that I need to have rcu_struct_kthread()
be more energetic if memory runs low, perhaps via an OOM handler.
Would that help?

> > > We only need to ensure that list_add() above can't race with that list_del(),
> > > perhaps we can tolerate lock_task_sighand() ?
> >
> > I am worried about a task that does a voluntary context switch, then exits.
> > This could results in rcu_tasks_kthread() and __unhash_process() both
> > wanting to dequeue at the same time, right?
> 
> Oh yes, I was very wrong. And we do not want to abuse tasklist_lock...
> 
> OK, let me try to read the patch first.

Not a problem, looking forward to your feedback!

							Thanx, Paul

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