Re: [PATCH v2 03/18] scsi: core: allow libata to complete successful commands via EH

[Niklas Cassel <Niklas.Cassel@wdc.com>]

On Fri, Jan 13, 2023 at 08:57:37AM +0100, Hannes Reinecke wrote:
> On 1/12/23 15:03, Niklas Cassel wrote:
> > In SCSI, we get the sense data as part of the completion, for ATA
> > however, we need to fetch the sense data as an extra step. For an
> > aborted ATA command the sense data is fetched via libata's
> > ->eh_strategy_handler().
> > 
> > For Command Duration Limits policy 0xD:
> > The device shall complete the command without error with the additional
> > sense code set to DATA CURRENTLY UNAVAILABLE.
> > 
> > In order to handle this policy in libata, we intend to send a successful
> > command via SCSI EH, and let libata's ->eh_strategy_handler() fetch the
> > sense data for the good command. This is similar to how we handle an
> > aborted ATA command, just that we need to read the Successful NCQ
> > Commands log instead of the NCQ Command Error log.
> > 
> > When we get a SATA completion with successful commands, ATA_SENSE will
> > be set, indicating that some commands in the completion have sense data.
> > 
> > The sense_valid bitmask in the Sense Data for Successful NCQ Commands
> > log will inform exactly which commands that had sense data, which might
> > be a subset of all the commands that was completed in the same
> > completion. (Yet all will have ATA_SENSE set, since the status is per
> > completion.)
> > 
> > The successful commands that have e.g. a "DATA CURRENTLY UNAVAILABLE"
> > sense data will have a SCSI ML byte set, so scsi_eh_flush_done_q() will
> > not set the scmd->result to DID_TIME_OUT for these commands. However,
> > the successful commands that did not have sense data, must not get their
> > result marked as DID_TIME_OUT by SCSI EH.
> > 
> > Add a new flag SCMD_EH_SUCCESS_CMD, which tells SCSI EH to not mark a
> > command as DID_TIME_OUT, even if it has scmd->result == SAM_STAT_GOOD.
> > 
> > This will be used by libata in a follow-up patch.
> > 
> > Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
> > ---
> >   drivers/scsi/scsi_error.c | 3 ++-
> >   include/scsi/scsi_cmnd.h  | 5 +++++
> >   2 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> > index 2aa2c2aee6e7..51aa5c1e31b5 100644
> > --- a/drivers/scsi/scsi_error.c
> > +++ b/drivers/scsi/scsi_error.c
> > @@ -2165,7 +2165,8 @@ void scsi_eh_flush_done_q(struct list_head *done_q)
> >   			 * scsi_eh_get_sense), scmd->result is already
> >   			 * set, do not set DID_TIME_OUT.
> >   			 */
> > -			if (!scmd->result)
> > +			if (!scmd->result &&
> > +			    !(scmd->flags & SCMD_EH_SUCCESS_CMD))
> >   				scmd->result |= (DID_TIME_OUT << 16);
> >   			SCSI_LOG_ERROR_RECOVERY(3,
> >   				scmd_printk(KERN_INFO, scmd,
> Wouldn't it be better to use '!scmd->result && !scsi_sense_valid(scmd)'
> instead of a new flag?
> After all, if we have a valid sense code we _have_ been able to communicate
> with the device. And as we did so it's questionable whether it should count
> as a command time out ...

Hello Hannes,

Thanks a lot for helping out reviewing this series!

Unfortunately, your suggestion won't work.


Let me explain:

When you get a FIS, the ACT register will have a bit set for each
command that finished, however, all the commands will share a single
STATUS value (since there is just a shared STATUS field in the FIS).

So let's say that tags 0-3 got finished (i.e. bits 0-3 are set in the
ACT field) and the STATUS field has the "Sense Data Available" bit set.

This just tells us that at least one of tags 0-3 has sense data.


In order to know which of these tags that actually has sense data,
we need to read the "Sense Data for Successful NCQ Commands log",
which contains a sense_valid bitmask (which contains one bit for
each of the 32 tags).

So reading the "Sense Data for Successful NCQ Commands log" might
tell us that just tag 0-1 have sense data.

So, libata calls ata_qc_schedule_eh() on tags 0-3, wait until SCSI calls
libata .eh_strategy_handler(). libata .eh_strategy_handler() will read the
"Sense Data for Successful NCQ Commands log", which will see that there is
sense data for tags 0-1, and will add sense data for those commands, and
call scsi_check_sense() for tags 0-1.

ata_eh_finish() will finally be called, to determine what to do with the
commands that belonged to EH.

The code looks like this:
if (qc->flags & ATA_QCFLAG_SENSE_VALID ||
    qc->flags & ATA_QCFLAG_EH_SUCCESS_CMD) {
	ata_eh_qc_complete(qc);
}

So it will call complete for all 4 tags, regardless is they had sense data
or not.


scsi_eh_flush_done_q() will soon be called, and since ata_eh_qc_complete()
sets scmd->retries == scmd->allowed, none of the four commands will be retired.

if (!scmd->result &&
    !(scmd->flags & SCMD_EH_SUCCESS_CMD))
	scmd->result |= (DID_TIME_OUT << 16);

The 2 commands with sense data will not get DID_TIMEOUT,
because scmd->result has the SCSI ML byte set
(which is set by scsi_check_sense()).

The 2 commands without sense data will have scmd->result == 0,
so they will get DID_TIME_OUT set if we don't have the extra
!(scmd->flags & SCMD_EH_SUCCESS_CMD)) condition.


SCSI could add an additional check for:

if (!scmd->result && !scsi_sense_valid(scmd) &&
    !(scmd->flags & SCMD_EH_SUCCESS_CMD))
	scmd->result |= (DID_TIME_OUT << 16);

so that a command with does have sense data, but scsi_check_sense()
did not set any SCSI ML byte, does not get DID_TIME_OUT set.

However, for CDL policy 0xD, which this patch cares about,
we would still need the "&& !(scmd->flags & SCMD_EH_SUCCESS_CMD))",
so at least from a CDL perspective, I don't see how any benefit of
also adding a check for "&& !scsi_sense_valid(scmd)".


Kind regards,
Niklas