Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Lai Jiangshan
2014-08-04T00:36:15Z

On 08/04/2014 06:05 AM, Paul E. McKenney wrote:
> 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?

An alternative solution:
srcu_read_lock() before exit_notify(), srcu_read_unlock() after the last preempt_disable()
in the do_exit, and synchronize_srcu() in rcu_tasks_kthread().