Re: [PATCH v3 1/2] scsi: Handle Unit Attention when issuing SCSI command

[James Bottomley <jejb@linux.vnet.ibm.com>]

On Wed, 2016-10-26 at 08:42 -0700, Bart Van Assche wrote:
> On 10/25/2016 04:50 PM, James Bottomley wrote:
> > On Tue, 2016-10-25 at 23:18 +0000, Bart Van Assche wrote:
> > > Anyway, currently the following functions interpret the SCSI 
> > > sense buffer:
> > > * scsi_io_completion() in scsi_lib.c.
> > > * scsi_mode_sense() in scsi_lib.c.
> > > * scsi_test_unit_ready_flags() in scsi_lib.c.
> > > * scsi_probe_lun() in scsi_scan.c.
> > > * scsi_report_lun_scan() in scsi_scan.c.
> > > * ioctl_internal_command() in scsi_ioctl.c.
> > > * sg_rq_end_io() in sg.c.
> > > * scsi_check_sense() in scsi_error.c.
> > > * spi_execute() in scsi_transport_spi.c.
> > > 
> > > Are you sure we should add sense code interpretation code in a 
> > > tenth function in the SCSI core?
> > 
> > In the absence of a better proposal, yes.  I originally looked into
> > better BLOCK_PC error handling in scsi_io_completion, but that has 
> > some knock on problems, so it seems best to leave it alone.
> 
> Can you elaborate on this? Since the sense buffer is available in 
> scsi_io_completion() and since that function already calls 
> scsi_command_normalize_sense() this function seems like a good 
> candidate to me to add retry logic for REQ_TYPE_BLOCK_PC requests.

UAs are used to signal AENs to userspace control processes that use
BLOCK_PC, like CD changers, burners and scanners.  If we eat UAs inside
scsi_io_completion(), we could potentially break any userspace control
system that relies on AENs.

James