Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks()

[Oleg Nesterov]

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.

> > 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.

Oleg.

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