Re: Reproducible deadlock when usb-storage scsi command timeouts twice

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On Wed, May 03, 2023 at 03:54:45PM +0200, Oliver Neukum wrote:
> On 03.05.23 14:51, Benjamin Block wrote:
> 
> > > usb-storage can do a reset only on the USB device level,
> > > which translates to a bus reset on the SCSI level.
> > > 
> > > And we are supposed to cancel any communication with the device
> > > before that.
> > 
> > Is that a limitation of the devices or drivers? Because then you don't
> > match SCSI semantics for LU reset - which aborts all running commands
> 
> We do not support a LUN reset. That's a limitation of the protocol.
> If something goes wrong you need to reset the whole USB device, which
> corresponds to a host adaptor on the SCSI level.

There is the Bulk-Only Mass Storage Reset request, which is what 
usb-storage issues when asked to perform a device reset.  But few 
devices implement it correctly.

> > on that scope among things. Which might explain the reason/choice behind
> > this unexpected behavior for you.
> 
> For the device a reset presumably does wipe out the command currently
> under execution. The problem is within the driver. It thinks that
> a command is still active. And we are limited to one command at a time
> (on the whole bus - again protocol limitation)
> 
> > On random thought I had: in theory you could implement your own EH
> > strategy handler if the default one doesn't work for you. ATA and SAS do so.
> > [drivers/scsi/scsi_error.c:2285 `shost->transportt->eh_strategy_handler()`]
> > This can re-use parts/all of the existing escalation sequence in
> > `scsi_eh_ready_devs()`.
> > 
> > But that's no short-term fix.
> 
> That looks like using a sledge hammer.

I think the best answer is to accept the patch that started this email 
thread, perhaps with a minor change.  The driver can easily abort the 
currently running command (if any) on its own before starting a reset.

Alan Stern



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