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

Alan Stern
2012-06-05T17:27:45Z

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: TEXT/PLAIN; charset=UTF-8, Size: 1745 bytes --]

On Tue, 22 May 2012, Lin Ming wrote:

> > Next, you have to change the parts of the block layer responsible for
> > taking a request from the queue and handing it to the lower-level
> > driver (both peek and get).  If q->rpm_status is RPM_SUSPENDED, they
> > shouldn't do anything -- act as though the queue is empty.  If
> > q->rpm_status is RPM_SUSPENDING or RPM_RESUMING, they should hand over
> > the request only if it has the REQ_PM flag set.

I just realized this isn't quite right. In fact, when the status is
RPM_SUSPENDING or RPM_RESUMING you need to search through the queue for
the first request with REQ_PM set.

Otherwise you can end up in a deadlock. For example, suppose the
device is suspended and a normal I/O request arrives. It gets put on
the queue and a runtime resume is issued. The sd resume method then
submits a "spin-up drive" command to the queue with REQ_PM set, and it
doesn't return until this command is finished. But the command won't
even start, because it isn't at the head of the queue -- the I/O
request is, and it prevents the "spin-up" command from running.

There are two other issues you also need to address. The first is
simple: Make sure your changes don't cause any harm if CONFIG_PM or
CONFIG_PM_RUNTIME is set to N.

Second, near the end of scsi_pm.c:scsi_bus_resume_common(), you can see
that during resume from a system sleep, the device automatically gets
set back to RPM_ACTIVE. You want the queue's runtime status to remain
in sync with the device's status; therefore you need to call
blk_pre_runtime_resume near the start of this function and
blk_post_runtime_resume near the end. (This is the drawback of having
separate rpm_status fields for the queue and the device.)

Alan Stern