Prevent non-NCQ command starvation

Factor out of ata_scsi_translate() the code handling queued command deferral using the port qc_defer callback and inssuing the queued command with ata_qc_issue() into the new function ata_scsi_qc_issue(), and simplify the goto used in ata_scsi_translate().
While at it, also add a lockdep annotation to check that the port lock is held when ata_scsi_translate() is called.

No functional changes.

Cc [email protected]

Signed-off-by Damien Le Moal <[email protected]>

Factor out of ata_scsi_translate() the code handling queued command deferral using the port qc_defer callback and inssuing the queued command with ata_qc_issue() into the new function ata_scsi_qc_issue(), and simplify the goto used in ata_scsi_translate().
While at it, also add a lockdep annotation to check that the port lock is held when ata_scsi_translate() is called.

No functional changes.

Cc [email protected]

Signed-off-by Damien Le Moal<[email protected]>

---
  drivers/ata/libata-scsi.c | 81 ++++++++++++++++++++++++---------------
  1 file changed, 50 insertions(+), 31 deletions(-)

drivers/ata/libata-scsi.c +0 -0

index 721d3f270c8e..5b6b5f1ff3c7 100644
@@ -1691,6 +1691,42 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
  	ata_qc_done(qc);

drivers/ata/libata-scsi.c +4 -0

+	ret = ap->ops->qc_defer(qc);
+	switch (ret) {
+	case 0:
+		break;

Factor out of ata_scsi_translate() the code handling queued command deferral using the port qc_defer callback and inssuing the queued

command with ata_qc_issue() into the new function ata_scsi_qc_issue(), and simplify the goto used in ata_scsi_translate().
While at it, also add a lockdep annotation to check that the port lock is held when ata_scsi_translate() is called.

No functional changes.

Cc [email protected]

Signed-off-by Damien Le Moal <[email protected]>

drivers/ata/libata-scsi.c +18 -0

@@ -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;

drivers/ata/libata-scsi.c +15 -0

+
+	/*
+	 * 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);
+}

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 <[email protected]>

Reported-by Igor Pylypiv <[email protected]>

Fixes 42f22fe36d51 ("scsi: pm8001: Expose hardware queues for pm80xx")

Cc [email protected]

Signed-off-by Damien Le Moal <[email protected]>

---

  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(+)

drivers/ata/libata-core.c +5 -0

index 09d8c035fcdf..447d8dc666a4 100644
@@ -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);

drivers/ata/libata-eh.c +6 -0

index 2586e77ebf45..b90b17f680f8 100644
@@ -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;
 

drivers/ata/libata-scsi.c +37 -0

index 5b6b5f1ff3c7..4aeffd6a5640 100644
@@ -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;
+

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 <[email protected]>

Reported-by Igor Pylypiv <[email protected]>

Fixes 42f22fe36d51 ("scsi: pm8001: Expose hardware queues for pm80xx")

Cc [email protected]

Signed-off-by Damien Le Moal <[email protected]>

---
 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(+)

drivers/ata/libata-core.c +5 -0

index 09d8c035fcdf..447d8dc666a4 100644
@@ -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);

drivers/ata/libata-eh.c +6 -0

index 2586e77ebf45..b90b17f680f8 100644
@@ -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;
 

drivers/ata/libata-scsi.c +89 -0

index 5b6b5f1ff3c7..4aeffd6a5640 100644
@@ -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;

drivers/ata/libata.h +2 -0

index 0e7ecac73680..60a675df61dc 100644
@@ -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);

include/linux/libata.h +3 -0

index 39534fafa36a..c5b27d97dfaf 100644
@@ -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() */
 

Signature · 1 line

2.52.0

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 <[email protected]>

Reported-by Igor Pylypiv <[email protected]>

Fixes 42f22fe36d51 ("scsi: pm8001: Expose hardware queues for pm80xx")

Cc [email protected]

Signed-off-by Damien Le Moal <[email protected]>

---
  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(+)

drivers/ata/libata-core.c +5 -0

index 09d8c035fcdf..447d8dc666a4 100644
@@ -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);

drivers/ata/libata-eh.c +6 -0

index 2586e77ebf45..b90b17f680f8 100644
@@ -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;
 

drivers/ata/libata-scsi.c +89 -0

index 5b6b5f1ff3c7..4aeffd6a5640 100644
@@ -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;

drivers/ata/libata.h +2 -0

index 0e7ecac73680..60a675df61dc 100644
@@ -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);

include/linux/libata.h +3 -0

index 39534fafa36a..c5b27d97dfaf 100644
@@ -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() */
 

This small patch series addresses potential command starvation issues with non-NCQ commands issued to a device accessed through a multi-queue libsas HBA. In such setup, a non-NCQ command issued to a device may suffer long delays and even complete starvation if the device is under a constant workload of NCQ commands issued from multiple queues.