Re: [PATCH v5 2/2] ata: libata-scsi: avoid Non-NCQ command starvation

[Igor Pylypiv <ipylypiv@google.com>]

On Sun, Jan 04, 2026 at 05:22:03PM +0900, Damien Le Moal wrote:
> When a non-NCQ command is issued while NCQ commands are being executed,
> ata_scsi_qc_issue() indicates to the SCSI layer that the command issuing
> should be deferred by returning SCSI_MLQUEUE_XXX_BUSY.  This command
> deferring is correct and as mandated by the ACS specifications since
> NCQ and non-NCQ commands cannot be mixed.
> 
> However, in the case of a host adapter using multiple submission queues,
> when the target device is under a constant load of NCQ commands, there
> are no guarantees that requeueing the non-NCQ command will be executed
> later and it may be deferred again repeatedly as other submission queues
> can constantly issue NCQ commands from different CPUs ahead of the
> non-NCQ command. This can lead to very long delays for the execution of
> non-NCQ commands, and even complete starvation for these commands in the
> worst case scenario.
> 
> Since the block layer and the SCSI layer do not distinguish between
> queueable (NCQ) and non queueable (non-NCQ) commands, libata-scsi SAT
> implementation must ensure forward progress for non-NCQ commands in the
> presence of NCQ command traffic. This is similar to what SAS HBAs with a
> hardware/firmware based SAT implementation do.
> 
> Implement such forward progress guarantee by limiting requeueing of
> non-NCQ commands from ata_scsi_qc_issue(): when a non-NCQ command is
> received and NCQ commands are in-flight, do not force a requeue of the
> non-NCQ command by returning SCSI_MLQUEUE_XXX_BUSY and instead return 0
> to indicate that the command was accepted but hold on to the qc using
> the new deferred_qc field of struct ata_port.
> 
> This deferred qc will be issued using the work item deferred_qc_work
> running the function ata_scsi_deferred_qc_work() once all in-flight
> commands complete, which is checked with the port qc_defer() callback
> return value indicating that no further delay is necessary. This check
> is done using the helper function ata_scsi_schedule_deferred_qc() which
> is called from ata_scsi_qc_complete(). This thus excludes this mechanism
> from all internal non-NCQ commands issued by ATA EH.
> 
> When a port deferred_qc is non NULL, that is, the port has a command
> waiting for the device queue to drain, the issuing of all incoming
> commands (both NCQ and non-NCQ) is deferred using the regular busy
> mechanism. This simplifies the code and also avoids potential denial of
> service problems if a user issues too many non-NCQ commands.
> 
> Finally, whenever ata EH is scheduled, regardless of the reason, a
> deferred qc is always requeued so that it can be retried once EH
> completes. This is done by calling the function
> ata_scsi_requeue_deferred_qc() from ata_eh_set_pending(). This avoids
> the need for any special processing for the deferred qc in case of NCQ
> error, link or device reset, or device timeout.
> 
> Reported-by: Xingui Yang <yangxingui@huawei.com>
> Reported-by: Igor Pylypiv <ipylypiv@google.com>
> Fixes: 42f22fe36d51 ("scsi: pm8001: Expose hardware queues for pm80xx")
> Cc: stable@vger.kernel.org
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Tested-by: Igor Pylypiv <ipylypiv@google.com>

Thanks again for fixing this issue, Damien!

Best,
Igor

> ---
>  drivers/ata/libata-core.c |  5 +++
>  drivers/ata/libata-eh.c   |  6 +++
>  drivers/ata/libata-scsi.c | 89 +++++++++++++++++++++++++++++++++++++++
>  drivers/ata/libata.h      |  2 +
>  include/linux/libata.h    |  3 ++
>  5 files changed, 105 insertions(+)
> 
> diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
> index 09d8c035fcdf..447d8dc666a4 100644
> --- a/drivers/ata/libata-core.c
> +++ b/drivers/ata/libata-core.c
> @@ -5561,6 +5561,7 @@ struct ata_port *ata_port_alloc(struct ata_host *host)
>  	mutex_init(&ap->scsi_scan_mutex);
>  	INIT_DELAYED_WORK(&ap->hotplug_task, ata_scsi_hotplug);
>  	INIT_DELAYED_WORK(&ap->scsi_rescan_task, ata_scsi_dev_rescan);
> +	INIT_WORK(&ap->deferred_qc_work, ata_scsi_deferred_qc_work);
>  	INIT_LIST_HEAD(&ap->eh_done_q);
>  	init_waitqueue_head(&ap->eh_wait_q);
>  	init_completion(&ap->park_req_pending);
> @@ -6173,6 +6174,10 @@ static void ata_port_detach(struct ata_port *ap)
>  		}
>  	}
>  
> +	/* Make sure the deferred qc work finished. */
> +	cancel_work_sync(&ap->deferred_qc_work);
> +	WARN_ON(ap->deferred_qc);
> +
>  	/* Tell EH to disable all devices */
>  	ap->pflags |= ATA_PFLAG_UNLOADING;
>  	ata_port_schedule_eh(ap);
> diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
> index 2586e77ebf45..b90b17f680f8 100644
> --- a/drivers/ata/libata-eh.c
> +++ b/drivers/ata/libata-eh.c
> @@ -917,6 +917,12 @@ static void ata_eh_set_pending(struct ata_port *ap, bool fastdrain)
>  
>  	ap->pflags |= ATA_PFLAG_EH_PENDING;
>  
> +	/*
> +	 * If we have a deferred qc, requeue it so that it is retried once EH
> +	 * completes.
> +	 */
> +	ata_scsi_requeue_deferred_qc(ap);
> +
>  	if (!fastdrain)
>  		return;
>  
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index 5b6b5f1ff3c7..4aeffd6a5640 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -1658,8 +1658,73 @@ static void ata_qc_done(struct ata_queued_cmd *qc)
>  	done(cmd);
>  }
>  
> +void ata_scsi_deferred_qc_work(struct work_struct *work)
> +{
> +	struct ata_port *ap =
> +		container_of(work, struct ata_port, deferred_qc_work);
> +	struct ata_queued_cmd *qc;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(ap->lock, flags);
> +
> +	/*
> +	 * If we still have a deferred qc and we are not in EH, issue it. In
> +	 * such case, we should not need any more deferring the qc, so warn if
> +	 * qc_defer() says otherwise.
> +	 */
> +	qc = ap->deferred_qc;
> +	if (qc && !ata_port_eh_scheduled(ap)) {
> +		WARN_ON_ONCE(ap->ops->qc_defer(qc));
> +		ap->deferred_qc = NULL;
> +		ata_qc_issue(qc);
> +	}
> +
> +	spin_unlock_irqrestore(ap->lock, flags);
> +}
> +
> +void ata_scsi_requeue_deferred_qc(struct ata_port *ap)
> +{
> +	struct ata_queued_cmd *qc = ap->deferred_qc;
> +	struct scsi_cmnd *scmd;
> +
> +	/*
> +	 * If we have a deferred qc when a reset occurs or NCQ commands fail,
> +	 * do not try to be smart about what to do with this deferred command
> +	 * and simply retry it by completing it with DID_SOFT_ERROR.
> +	 */
> +	if (!qc)
> +		return;
> +
> +	scmd = qc->scsicmd;
> +	ap->deferred_qc = NULL;
> +	ata_qc_free(qc);
> +	scmd->result = (DID_SOFT_ERROR << 16);
> +	scsi_done(scmd);
> +}
> +
> +static void ata_scsi_schedule_deferred_qc(struct ata_port *ap)
> +{
> +	struct ata_queued_cmd *qc = ap->deferred_qc;
> +
> +	/*
> +	 * If we have a deferred qc, then qc_defer() is defined and we can use
> +	 * this callback to determine if this qc is good to go, unless EH has
> +	 * been scheduled.
> +	 */
> +	if (!qc)
> +		return;
> +
> +	if (ata_port_eh_scheduled(ap)) {
> +		ata_scsi_requeue_deferred_qc(ap);
> +		return;
> +	}
> +	if (!ap->ops->qc_defer(qc))
> +		queue_work(system_highpri_wq, &ap->deferred_qc_work);
> +}
> +
>  static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
>  {
> +	struct ata_port *ap = qc->ap;
>  	struct scsi_cmnd *cmd = qc->scsicmd;
>  	u8 *cdb = cmd->cmnd;
>  	bool have_sense = qc->flags & ATA_QCFLAG_SENSE_VALID;
> @@ -1689,6 +1754,8 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
>  	}
>  
>  	ata_qc_done(qc);
> +
> +	ata_scsi_schedule_deferred_qc(ap);
>  }
>  
>  static int ata_scsi_qc_issue(struct ata_port *ap, struct ata_queued_cmd *qc)
> @@ -1698,6 +1765,16 @@ static int ata_scsi_qc_issue(struct ata_port *ap, struct ata_queued_cmd *qc)
>  	if (!ap->ops->qc_defer)
>  		goto issue;
>  
> +	/*
> +	 * If we already have a deferred qc, then rely on the SCSI layer to
> +	 * requeue and defer all incoming commands until the deferred qc is
> +	 * processed, once all on-going commands complete.
> +	 */
> +	if (ap->deferred_qc) {
> +		ata_qc_free(qc);
> +		return SCSI_MLQUEUE_DEVICE_BUSY;
> +	}
> +
>  	/* Check if the command needs to be deferred. */
>  	ret = ap->ops->qc_defer(qc);
>  	switch (ret) {
> @@ -1716,6 +1793,18 @@ static int ata_scsi_qc_issue(struct ata_port *ap, struct ata_queued_cmd *qc)
>  	}
>  
>  	if (ret) {
> +		/*
> +		 * We must defer this qc: if this is not an NCQ command, keep
> +		 * this qc as a deferred one and report to the SCSI layer that
> +		 * we issued it so that it is not requeued. The deferred qc will
> +		 * be issued with the port deferred_qc_work once all on-going
> +		 * commands complete.
> +		 */
> +		if (!ata_is_ncq(qc->tf.protocol)) {
> +			ap->deferred_qc = qc;
> +			return 0;
> +		}
> +
>  		/* Force a requeue of the command to defer its execution. */
>  		ata_qc_free(qc);
>  		return ret;
> diff --git a/drivers/ata/libata.h b/drivers/ata/libata.h
> index 0e7ecac73680..60a675df61dc 100644
> --- a/drivers/ata/libata.h
> +++ b/drivers/ata/libata.h
> @@ -165,6 +165,8 @@ void ata_scsi_sdev_config(struct scsi_device *sdev);
>  int ata_scsi_dev_config(struct scsi_device *sdev, struct queue_limits *lim,
>  		struct ata_device *dev);
>  int __ata_scsi_queuecmd(struct scsi_cmnd *scmd, struct ata_device *dev);
> +void ata_scsi_deferred_qc_work(struct work_struct *work);
> +void ata_scsi_requeue_deferred_qc(struct ata_port *ap);
>  
>  /* libata-eh.c */
>  extern unsigned int ata_internal_cmd_timeout(struct ata_device *dev, u8 cmd);
> diff --git a/include/linux/libata.h b/include/linux/libata.h
> index 39534fafa36a..c5b27d97dfaf 100644
> --- a/include/linux/libata.h
> +++ b/include/linux/libata.h
> @@ -903,6 +903,9 @@ struct ata_port {
>  	u64			qc_active;
>  	int			nr_active_links; /* #links with active qcs */
>  
> +	struct work_struct	deferred_qc_work;
> +	struct ata_queued_cmd	*deferred_qc;
> +
>  	struct ata_link		link;		/* host default link */
>  	struct ata_link		*slave_link;	/* see ata_slave_link_init() */
>  
> -- 
> 2.52.0
>