Re: [PATCH v3 7/9] scsi_transport_spi: Freeze request queues instead of quiescing

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On 11/22/20 11:02 PM, Hannes Reinecke wrote:
> On 11/23/20 4:17 AM, Bart Van Assche wrote:
>> Instead of quiescing the request queues involved in domain validation,
>> freeze these. As a result, the struct request_queue pm_only member is no
>> longer set during domain validation. That will allow to modify
>> scsi_execute() such that it stops setting the BLK_MQ_REQ_PREEMPT flag.
>> Three additional changes in this patch are that scsi_mq_alloc_queue() is
>> exported, that scsi_device_quiesce() is no longer exported and that
>> scsi_target_{quiesce,resume}() have been changed into
>> scsi_target_{freeze,unfreeze}().
>>
> The description is now partially wrong, as scsi_mq_alloc_queue() is
> certainly not exported (why would it?).
> 
> And it also glosses over the main idea of this patch, namely that not
> only sdev->request_queue is frozen, but also a completely _new_
> request queue is allocated to send SPI DV requests over.
> 
> Please modify the description.

Hi Hannes,

Thanks for the feedback. Please take a look at the attached patch.

>> +    q2 = scsi_mq_alloc_queue(sdev);
>> +
>> +    if (q2) {
>> +        /*
>> +         * Freeze the target such that no other subsystem can submit
>> +         * SCSI commands to 'sdev'. Submitting SCSI commands through
>> +         * q2 may trigger the SCSI error handler. The SCSI error
>> +         * handler must be able to handle a frozen sdev->request_queue
>> +         * and must also use blk_mq_rq_from_pdu(q2)->q instead of
>> +         * sdev->request_queue if it would be necessary to access q2
>> +         * directly.
>> +         */
> 
> ... 'it would be necessary' indicates that it doesn't do so, currently.
> And the SPI DV code most certainly _is_ submitting SCSI commands.
> So doesn't the above imply that SCSI EH will fail to work correctly?
> And if so, why isn't it fixed with some later patch in this series?
> Or how do you plan to address it?
> Hmm?

Before SPI domain validation starts, sdev->request_queue is frozen which
means that DV will only start after all pending SCSI commands have
finished, including potential error handling for these commands.

As far as I know there is only one sdev->request_queue access in the
SCSI error handler, namely in scsi_eh_lock_door(). Patch 5/9 from this
series makes the blk_get_request() call in that function use
BLK_MQ_REQ_NOWAIT so the SCSI error handler should still work fine if a
DV command fails. Does this answer your question?

Thanks,

Bart.
>From 66156c468f418da09512ef321913627ddf0da20d Mon Sep 17 00:00:00 2001
From: Bart Van Assche <bvanassche@xxxxxxx>
Date: Sun, 30 Aug 2020 09:35:23 -0700
Subject: [PATCH] scsi_transport_spi: Freeze request queues instead of
 quiescing

Instead of quiescing the request queues involved in domain validation,
freeze these. As a result, the struct request_queue pm_only member is no
longer set during domain validation. That will allow to modify
scsi_execute() such that it stops setting the BLK_MQ_REQ_PREEMPT flag.

Attach a secondary request queue for submitting DV requests while the
primary request queue is frozen.

Change scsi_target_{quiesce,resume}() into scsi_target_{freeze,unfreeze}().

Unexport scsi_device_quiesce().

Reviewed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Woody Suwalski <terraluna977@xxxxxxxxx>
Cc: Can Guo <cang@xxxxxxxxxxxxxx>
Cc: Stanley Chu <stanley.chu@xxxxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Tested-by: Stan Johnson <userm57@xxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 drivers/scsi/scsi_lib.c           | 21 ++++----
 drivers/scsi/scsi_priv.h          |  2 +
 drivers/scsi/scsi_transport_spi.c | 88 ++++++++++++++++++++++---------
 include/scsi/scsi_device.h        |  6 +--
 4 files changed, 76 insertions(+), 41 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index b5449efc7283..fef4708f3778 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2555,7 +2555,6 @@ scsi_device_quiesce(struct scsi_device *sdev)
 
 	return err;
 }
-EXPORT_SYMBOL(scsi_device_quiesce);
 
 /**
  *	scsi_device_resume - Restart user issued commands to a quiesced device.
@@ -2584,30 +2583,30 @@ void scsi_device_resume(struct scsi_device *sdev)
 EXPORT_SYMBOL(scsi_device_resume);
 
 static void
-device_quiesce_fn(struct scsi_device *sdev, void *data)
+device_freeze_fn(struct scsi_device *sdev, void *data)
 {
-	scsi_device_quiesce(sdev);
+	blk_mq_freeze_queue(sdev->request_queue);
 }
 
 void
-scsi_target_quiesce(struct scsi_target *starget)
+scsi_target_freeze(struct scsi_target *starget)
 {
-	starget_for_each_device(starget, NULL, device_quiesce_fn);
+	starget_for_each_device(starget, NULL, device_freeze_fn);
 }
-EXPORT_SYMBOL(scsi_target_quiesce);
+EXPORT_SYMBOL(scsi_target_freeze);
 
 static void
-device_resume_fn(struct scsi_device *sdev, void *data)
+device_unfreeze_fn(struct scsi_device *sdev, void *data)
 {
-	scsi_device_resume(sdev);
+	blk_mq_unfreeze_queue(sdev->request_queue);
 }
 
 void
-scsi_target_resume(struct scsi_target *starget)
+scsi_target_unfreeze(struct scsi_target *starget)
 {
-	starget_for_each_device(starget, NULL, device_resume_fn);
+	starget_for_each_device(starget, NULL, device_unfreeze_fn);
 }
-EXPORT_SYMBOL(scsi_target_resume);
+EXPORT_SYMBOL(scsi_target_unfreeze);
 
 /**
  * scsi_internal_device_block_nowait - try to transition to the SDEV_BLOCK state
diff --git a/drivers/scsi/scsi_priv.h b/drivers/scsi/scsi_priv.h
index e34755986b47..18485595762a 100644
--- a/drivers/scsi/scsi_priv.h
+++ b/drivers/scsi/scsi_priv.h
@@ -95,6 +95,8 @@ extern int scsi_mq_setup_tags(struct Scsi_Host *shost);
 extern void scsi_mq_destroy_tags(struct Scsi_Host *shost);
 extern void scsi_exit_queue(void);
 extern void scsi_evt_thread(struct work_struct *work);
+extern int scsi_device_quiesce(struct scsi_device *sdev);
+extern void scsi_device_resume(struct scsi_device *sdev);
 struct request_queue;
 struct request;
 
diff --git a/drivers/scsi/scsi_transport_spi.c b/drivers/scsi/scsi_transport_spi.c
index 959990f66865..bc02c865e0cc 100644
--- a/drivers/scsi/scsi_transport_spi.c
+++ b/drivers/scsi/scsi_transport_spi.c
@@ -983,6 +983,22 @@ spi_dv_device_internal(struct scsi_device *sdev, struct request_queue *q,
 	}
 }
 
+/*
+ * Allocate a request queue without setting sdev->request_queue such that it
+ * can be used as a secondary SCSI device queue.
+ */
+static struct request_queue *scsi_mq_alloc_queue(struct scsi_device *sdev)
+{
+	struct request_queue *q = blk_mq_init_queue(&sdev->host->tag_set);
+
+	if (IS_ERR(q))
+		return NULL;
+
+	q->queuedata = sdev;
+	__scsi_init_queue(sdev->host, q);
+	blk_queue_flag_set(QUEUE_FLAG_SCSI_PASSTHROUGH, q);
+	return q;
+}
 
 /**	spi_dv_device - Do Domain Validation on the device
  *	@sdev:		scsi device to validate
@@ -997,59 +1013,79 @@ void
 spi_dv_device(struct scsi_device *sdev)
 {
 	struct scsi_target *starget = sdev->sdev_target;
+	struct request_queue *q2;
 	u8 *buffer;
 	const int len = SPI_MAX_ECHO_BUFFER_SIZE*2;
 
 	/*
-	 * Because this function and the power management code both call
-	 * scsi_device_quiesce(), it is not safe to perform domain validation
-	 * while suspend or resume is in progress. Hence the
-	 * lock/unlock_system_sleep() calls.
+	 * Because this function creates a new request queue that is not
+	 * visible to the rest of the system, this function must be serialized
+	 * against suspend, resume and runtime power management. Hence the
+	 * lock/unlock_system_sleep() and scsi_autopm_{get,put}_device()
+	 * calls.
 	 */
 	lock_system_sleep();
 
+	if (scsi_autopm_get_device(sdev))
+		goto unlock_system_sleep;
+
 	if (unlikely(spi_dv_in_progress(starget)))
-		goto unlock;
+		goto put_autopm;
 
 	if (unlikely(scsi_device_get(sdev)))
-		goto unlock;
-
-	spi_dv_in_progress(starget) = 1;
+		goto put_autopm;
 
 	buffer = kzalloc(len, GFP_KERNEL);
 
 	if (unlikely(!buffer))
-		goto out_put;
-
-	/* We need to verify that the actual device will quiesce; the
-	 * later target quiesce is just a nice to have */
-	if (unlikely(scsi_device_quiesce(sdev)))
-		goto out_free;
-
-	scsi_target_quiesce(starget);
+		goto put_sdev;
 
 	spi_dv_pending(starget) = 1;
+
 	mutex_lock(&spi_dv_mutex(starget));
+	if (unlikely(spi_dv_in_progress(starget)))
+		goto clear_pending;
+
+	spi_dv_in_progress(starget) = 1;
 
 	starget_printk(KERN_INFO, starget, "Beginning Domain Validation\n");
 
-	spi_dv_device_internal(sdev, sdev->request_queue, buffer);
+	q2 = scsi_mq_alloc_queue(sdev);
+
+	if (q2) {
+		/*
+		 * Freeze the target such that no other subsystem can submit
+		 * SCSI commands to 'sdev'. Submitting SCSI commands through
+		 * q2 may trigger the SCSI error handler. The SCSI error
+		 * handler must be able to handle a frozen sdev->request_queue
+		 * and must also use blk_mq_rq_from_pdu(q2)->q instead of
+		 * sdev->request_queue if it would be necessary to access q2
+		 * directly.
+		 */
+		scsi_target_freeze(starget);
+		spi_dv_device_internal(sdev, q2, buffer);
+		blk_cleanup_queue(q2);
+		scsi_target_unfreeze(starget);
+	}
 
 	starget_printk(KERN_INFO, starget, "Ending Domain Validation\n");
 
-	mutex_unlock(&spi_dv_mutex(starget));
-	spi_dv_pending(starget) = 0;
-
-	scsi_target_resume(starget);
-
 	spi_initial_dv(starget) = 1;
+	spi_dv_in_progress(starget) = 0;
+
+clear_pending:
+	spi_dv_pending(starget) = 0;
+	mutex_unlock(&spi_dv_mutex(starget));
 
- out_free:
 	kfree(buffer);
- out_put:
-	spi_dv_in_progress(starget) = 0;
+
+put_sdev:
 	scsi_device_put(sdev);
-unlock:
+
+put_autopm:
+	scsi_autopm_put_device(sdev);
+
+unlock_system_sleep:
 	unlock_system_sleep();
 }
 EXPORT_SYMBOL(spi_dv_device);
diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index f47fdf9cf788..dc193d7f479a 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -423,10 +423,8 @@ extern struct scsi_event *sdev_evt_alloc(enum scsi_device_event evt_type,
 extern void sdev_evt_send(struct scsi_device *sdev, struct scsi_event *evt);
 extern void sdev_evt_send_simple(struct scsi_device *sdev,
 			  enum scsi_device_event evt_type, gfp_t gfpflags);
-extern int scsi_device_quiesce(struct scsi_device *sdev);
-extern void scsi_device_resume(struct scsi_device *sdev);
-extern void scsi_target_quiesce(struct scsi_target *);
-extern void scsi_target_resume(struct scsi_target *);
+extern void scsi_target_freeze(struct scsi_target *);
+extern void scsi_target_unfreeze(struct scsi_target *);
 extern void scsi_scan_target(struct device *parent, unsigned int channel,
 			     unsigned int id, u64 lun,
 			     enum scsi_scan_mode rescan);

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