FAILED: Patch "ata: libata-scsi: avoid Non-NCQ command starvation" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 0ea84089dbf62a92dc7889c79e6b18fc89260808 Mon Sep 17 00:00:00 2001
From: Damien Le Moal <[email protected]>
Date: Wed, 17 Dec 2025 16:40:48 +0900
Subject: [PATCH] ata: libata-scsi: avoid Non-NCQ command starvation

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.

---
 drivers/ata/libata-core.c |  5 +++
 drivers/ata/libata-eh.c   |  6 +++
 drivers/ata/libata-scsi.c | 93 +++++++++++++++++++++++++++++++++++++++
 drivers/ata/libata.h      |  2 +
 include/linux/libata.h    |  3 ++
 5 files changed, 109 insertions(+)

index b961054817845..e888f24456929 100644
@@ -5644,6 +5644,7 @@ 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);

@@ -6256,6 +6257,10 @@ 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);

index f4c9541d1910e..72a22b6c96821 100644
@@ -918,6 +918,12 @@ 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;
 

index 89dd0ae2b9918..9b4e578ad07ec 100644
@@ -166,6 +166,8 @@ 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);

index aa88244f3d838..d612d4c1660a7 100644
@@ -899,6 +899,9 @@ 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() */
 

2.51.0