Re: [PATCH v12 07/13] task_isolation: add debug boot flag

[Chris Metcalf]

On 5/19/2016 1:54 PM, Peter Zijlstra wrote:
> So the 'simple' thing is:
>
> 	struct rq *rq = cpu_rq(cpu);
> 	struct task_struct *task;
>
> 	raw_spin_lock_irq(&rq->lock);
> 	task = rq->curr;
> 	get_task_struct(task);
> 	raw_spin_unlock_irq(&rq->lock);
>
> Because by holding rq->lock, the remote CPU cannot schedule and the
> current task_must_  still be valid.

I will plan to use that idiom in the next patch series.  Thanks!

> And note; the above can result in a task which already has PF_EXITING
> set.

I think that should be benign though; we may generate an unnecessary
warning, but somebody was doing something that could have resulted in
interrupting an isolated task anyway, so warning about it is reasonable.  And
presumably PF_EXITING just means we don't wake any threads and leave
the signal queued, but that gets flushed when the task finally exits.

> The complex thing is described in the linked thread and will likely make
> your head hurt.

I read the linked thread and was entertained. :-)  I suspect locking the
runqueue may be the more robust solution anyway, and since this is
presumably not a hot path, it seems easier to reason about this way.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com