Re: [PATCH] usb: uas: fix usb subsystem hang after power off hub port

[Bart Van Assche]

On Tue, 2019-04-09 at 10:44 -0400, Alan Stern wrote:
+AD4 On Mon, 8 Apr 2019, Martin K. Petersen wrote:
+AD4 
+AD4 +AD4 
+AD4 +AD4 Alan,
+AD4 +AD4 
+AD4 +AD4 +AD4 So it looks as though the SCSI subsystem doesn't like to have a reset 
+AD4 +AD4 +AD4 handler call scsi+AF8-remove+AF8-host.
+AD4 +AD4 
+AD4 +AD4 Are you talking about a PCI device removal handler or a SCSI error
+AD4 +AD4 handler?
+AD4 
+AD4 The context of this discussion is a USB mass-storage device where the
+AD4 device's port on its upstream hub has been powered off.  The
+AD4 powered-off port causes an executing command to time out.  As a result
+AD4 the SCSI error handler runs and calls the USB reset routine, but the
+AD4 reset fails because the kernel is unable to communicate with the device
+AD4 through the powered-off port.  This causes the USB reset routine to
+AD4 unbind the device from its USB driver, which in turn calls
+AD4 scsi+AF8-remove+AF8-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+AF8-srp.c) follows the
latter approach. See also srp+AF8-queue+AF8-remove+AF8-work().

Bart.