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

[Lai Jiangshan]

On 08/04/2014 10:56 PM, Peter Zijlstra wrote:
> On Mon, Aug 04, 2014 at 02:25:15PM +0200, Peter Zijlstra wrote:
>> On Mon, Aug 04, 2014 at 04:50:44AM -0700, Paul E. McKenney wrote:
>>> OK, I will bite...
>>>
>>> What kinds of tasks are on a runqueue, but neither ->on_cpu nor
>>> PREEMPT_ACTIVE?
>>
>> Userspace tasks, they don't necessarily get PREEMPT_ACTIVE when
>> preempted. Now obviously you're not _that_ interested in userspace tasks
>> for this, so that might be ok.
>>
>> But the main point was, you cannot use ->on_cpu or PREEMPT_ACTIVE
>> without holding rq->lock.
> 
> Hmm, maybe you can, we have the context switch in between setting
> ->on_cpu and clearing PREEMPT_ACTIVE and vice-versa.
> 
> The context switch (obviously) provides a full barrier, so we might be
> able to -- with careful consideration -- read these two separate values
> and construct something usable from them.
> 
> Something like:
> 
> 	task_preempt_count(tsk) & PREEMPT_ACTIVE
	the @tsk is running on_cpu, the above result is false.
> 	smp_rmb();
> 	tsk->on_cpu
	now the @tsk is preempted, the above result also is false.

	so it is useless if we fetch the preempt_count and on_cpu in two separated
instructions.  Maybe it would work if we also take tsk->nivcsw in consideration.
(I just noticed that tsk->n[i]vcsw are the version numbers for the tsk->on_cpu)

bool task_on_cpu_or_preempted(tsk)
{
	unsigned long saved_nivcsw;

	saved_nivcsw = task->nivcsw;
	if (tsk->on_cpu)
		return true;

	smp_rmb();

	if (task_preempt_count(tsk) & PREEMPT_ACTIVE)
		return true;

	smp_rmb();

	if (tsk->on_cpu || task->nivcsw != saved_nivcsw)
		return true;

	return false;
}

> 
> And because we set PREEMPT_ACTIVE before clearing on_cpu, this should
> race the right way (err towards the inclusive side).
> 
> Obviously that wants a big fat comment...

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