> > Though this does raise the issue that it probably makes sense to
> > have the "hardware" layer suspended while the upper layers'
> > "virtual devices" don't know about it. That's not the assumption
> > I think we've been using so far...
>
> No, and I won't be surprised if it comes back later to bite us!
Storage is special because SCSI hides open/close.
[..]
> You're correct that resume will fail-fast. But down(&udev->serialize)
> won't! That's where the blockage is: inside locktree -- the task is
> waiting to lock the device. Once the device has been locked and the
> resume code gets a chance to execute it will indeed fail quickly.
>
> Diagrammatically:
>
> usb-storage control thread khubd
> -------------------------- -----
> dequeue next SCSI command
> start to perform the command,
> but the device has been
> suspended
> lock the parent hub
> get port connect change event
> call usb_disconnect
> lock the device
> call usb_resume_device
> call locktree: Blocks trying
> to lock the parent hub
> call usb_storage:disconnect()
> wait for control thread to exit:
> Deadlock!
>
> Testing for udev->state != USB_STATE_NOTATTACHED prior to doing the down()
> will help but won't completely eliminate the window. I'm beginning to
> think that locktree() needs to fail without blocking indefinitely if the
> device is marked NOTATTACHED.
That would result in a monstrously complex thing with timeouts.
I'd suggest to reduce complexity and move to a global rwsem for now.
If there's contention we can care about it later.
Independent from that, you might use a workqueue triggered by
queuecommand() itself.
> The other possibility is for the usb_storage disconnect routine to exit
> _without_ waiting for the control thread to finish. Until module unload
> support is improved, that will almost certainly lead to oopses.
Eek. Don't do that.
Regards
Oliver
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