[PATCH v3] scsi: core: Fix hang of freezing queue between blocking and running device

[Li Jinlin]

From: Li Jinlin <lijinlin3@xxxxxxxxxx>

We found a hang issue, the test steps are as follows:
  1. blocking device via scsi_device_set_state()
  2. dd if=/dev/sda of=/mnt/t.log bs=1M count=10
  3. echo none > /sys/block/sda/queue/scheduler
  4. echo "running" >/sys/block/sda/device/state

Step 3 and 4 should finish this work after step 4, but they hangs.

  CPU#0               CPU#1                CPU#2
  ---------------     ----------------     ----------------
                                           Step 1: blocking device

                                           Step 2: dd xxxx
                                                  ^^^^^^ get request
                                                         q_usage_counter++

                      Step 3: switching scheculer
                      elv_iosched_store
                        elevator_switch
                          blk_mq_freeze_queue
                            blk_freeze_queue
                              > blk_freeze_queue_start
                                ^^^^^^ mq_freeze_depth++

                              > blk_mq_run_hw_queues
                                ^^^^^^ can't run queue when dev blocked

                              > blk_mq_freeze_queue_wait
                                ^^^^^^ Hang here!!!
                                       wait q_usage_counter==0

  Step 4: running device
  store_state_field
    scsi_rescan_device
      scsi_attach_vpd
        scsi_vpd_inquiry
          __scsi_execute
            blk_get_request
              blk_mq_alloc_request
                blk_queue_enter
                ^^^^^^ Hang here!!!
                       wait mq_freeze_depth==0

    blk_mq_run_hw_queues
    ^^^^^^ dispatch IO, q_usage_counter will reduce to zero

                            blk_mq_unfreeze_queue
                            ^^^^^ mq_freeze_depth--

Step 3 and 4 wait for each other.

To fix this, we need to run queue before rescanning device when the device
state changes to SDEV_RUNNING.

Fixes: f0f82e2476f6 ("scsi: core: Fix capacity set to zero after offlinining device")
Signed-off-by: Li Jinlin <lijinlin3@xxxxxxxxxx>
Signed-off-by: Qiu Laibin <qiulaibin@xxxxxxxxxx>
---
changes since v2 send with Message-ID:
20210809141308.3700854-1-lijinlin3@xxxxxxxxxx

 - Expanded code comment in store_state_field() as suggested by Bart

 drivers/scsi/scsi_sysfs.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index ae9bfc658203..c0d31119d6d7 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -808,12 +808,15 @@ store_state_field(struct device *dev, struct device_attribute *attr,
 	ret = scsi_device_set_state(sdev, state);
 	/*
 	 * If the device state changes to SDEV_RUNNING, we need to
-	 * rescan the device to revalidate it, and run the queue to
-	 * avoid I/O hang.
+	 * run the queue to avoid I/O hang, and rescan the device
+	 * to revalidate it. Running the queue first is necessary
+	 * because another thread may be waiting inside
+	 * blk_mq_freeze_queue_wait() and because that call may be
+	 * waiting for pending I/O to finish.
 	 */
 	if (ret == 0 && state == SDEV_RUNNING) {
-		scsi_rescan_device(dev);
 		blk_mq_run_hw_queues(sdev->request_queue, true);
+		scsi_rescan_device(dev);
 	}
 	mutex_unlock(&sdev->state_mutex);
 
-- 
2.27.0