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

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Message-ID: <CAF1ivSbSv+pWFxgf8LrHdamxSQZuJh5S5_0L82DWfy34Si7iLg@mail.gmail.com>
Date:	Wed, 23 May 2012 23:59:07 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Jens Axboe <axboe@...nel.dk>, Jeff Moyer <jmoyer@...hat.com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [RFC v2 PATCH 2/4] block: add queue runtime pm callbacks

On Wed, May 23, 2012 at 10:58 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> 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.)

Agree.
So I'd like to keep local copy of rpm_status.
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