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SubjectRe: [PATCH] usb: uas: fix usb subsystem hang after power off hub port
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().


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