Re: [RFC v2 PATCH 2/4] block: add queue runtime pm callbacks

Alan Stern
2012-05-23T14:58:26Z

On Wed, 23 May 2012, Lin Ming wrote:

> Let's consider below code.
>
> @@ -587,6 +591,11 @@ void __elv_add_request(struct request_queue *q,
> struct request *rq, int where)
> {
> trace_block_rq_insert(q, rq);
>
> + if (!(rq->cmd_flags & REQ_PM))
> + if (q->nr_pending++ == 0 && (q->rpm_status == RPM_SUSPENDED ||
> + q->rpm_status == RPM_SUSPENDING) && q->dev)
> + pm_request_resume(q->dev);
> +
> rq->q = q;
>
> if (rq->cmd_flags & REQ_SOFTBARRIER) {
>
> Block layer reads runtime status and pm core writes this status.
> PM core uses dev->power.lock to protect this status.
>
> I was thinking will it have problem if block layer does not acquire
> dev->power.lock?
> From your explanation below, it seems does not have problem.

I don't think it's a problem, because all you're doing is reading
dev->power.rpm_status -- you're not writing it.

On the other hand, there's nothing really wrong with keeping your own
local copy of rpm_status. You could think of it as being the queue's
status as opposed to the device's status. (Also, some people might
argue that dev->power.rpm_status is supposed to be private to the
runtime PM core and shouldn't be used by other code.)

Alan Stern