usb: uas: fix usb subsystem hang after power off hub port

[Bart Van Assche <bvanassche@acm.org>]

On Tue, 2019-04-09 at 10:44 -0400, Alan Stern wrote:
> On Mon, 8 Apr 2019, Martin K. Petersen wrote:
> 
> > 
> > Alan,
> > 
> > > So it looks as though the SCSI subsystem doesn't like to have a reset 
> > > handler call scsi_remove_host.
> > 
> > Are you talking about a PCI device removal handler or a SCSI error
> > handler?
> 
> The context of this discussion is a USB mass-storage device where the
> device's port on its upstream hub has been powered off.  The
> powered-off port causes an executing command to time out.  As a result
> the SCSI error handler runs and calls the USB reset routine, but the
> reset fails because the kernel is unable to communicate with the device
> through the powered-off port.  This causes the USB reset routine to
> unbind the device from its USB driver, which in turn calls
> scsi_remove_host -- while the error handler is still running.

From which context does that unbind happen? From inside a SCSI EH callback
or from the context of a workqueue? I think the former is not allowed but
that the latter is allowed. The SRP initiator driver (ib_srp.c) follows the
latter approach. See also srp_queue_remove_work().

Bart.