Re: [PATCH v3 2/2] ata: libata-scsi: avoid passthrough command starvation

[Niklas Cassel <cassel@kernel.org>]

On Fri, Jan 02, 2026 at 10:10:44AM +0900, Damien Le Moal wrote:
> On 12/31/25 20:39, Niklas Cassel wrote:
> > On Sat, Dec 20, 2025 at 09:21:40AM +0900, Damien Le Moal wrote:
> >> @@ -1702,6 +1779,17 @@ static int ata_scsi_defer(struct ata_port *ap, struct ata_queued_cmd *qc)
> >>  	if (!ret)
> >>  		return 0;
> >>  
> >> +	/*
> >> +	 * We must defer this qc: if this is not an NCQ command, keep this qc
> >> +	 * as a deferred one and wait for all on-going NCQ commands to complete
> >> +	 * before issuing it with the deferred qc work.
> >> +	 */
> >> +	if (!ata_is_ncq(qc->tf.protocol)) {
> >> +		ap->deferred_qc = qc;
> >> +		return SCSI_MLQUEUE_DEVICE_BUSY;
> > 
> > Here you save the qc, and you return SCSI_MLQUEUE_DEVICE_BUSY;
> > SCSI_MLQUEUE_DEVICE_BUSY will cause the block layer to reinsert the request
> > in the queue, so you will get the same request sent to ata_scsi_translate()
> > again, even though you have save it. A little ugly IMO.
> 
> No it will not cause a requeue in this case because this change:
> 
>  	rc = ata_scsi_defer(ap, qc);
> -	if (rc)
> +	if (rc) {
> +		if (qc == ap->deferred_qc)
> +			return 0;
> 
> ensures that we return "0", thus telling the scsi and block layer that the
> command/request was accepted, but we do *not* call ata_qc_issue() for that qc.
> It is deferred and will be issued by the deferred qc work once the queue drains.

I see how I got confused.

In the case where we store the deferred QC, you overload the return value.
But the original return value did not make sense (SCSI_MLQUEUE_DEVICE_BUSY),
as it means ask upper layer to requeue. I suggest that you let ata_scsi_defer()
return some other value in this case.

ATA_DEFER_ACTION_CMD_STORED or something.

Then at the call site:

	rc = ata_scsi_defer(ap, qc);
	if (rc) {
		if (rc == ATA_DEFER_ACTION_CMD_STORED)
			return 0;
		return rc;
	}


Kind regards,
Niklas