On Wed, Jun 6, 2012 at 2:27 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > 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. At least for SCSI sd driver, the PM request is always queued at the head. scsi_execute() blk_execute_rq(req->q, NULL, req, 1); "1" means the request will be inserted at the head. blk_execute_rq_nowait() spin_lock_irq(q->queue_lock); __elv_add_request(q, rq, where); __blk_run_queue(q) spin_unlock_irq(q->queue_lock); So the request is inserted at the head and executed immediatly. > > 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 I am travelling through June 10. I'll investigate these when I'm back. Thanks, Lin Ming -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
