We found a hang, the steps to reproduce  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 complete after step 4, but they hang.

  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--

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

Link: https://lore.kernel.org/r/20210824025921.3277629-1-lijinlin3@huawei.com
Fixes: f0f82e24

 ("scsi: core: Fix capacity set to zero after offlinining device")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Li Jinlin <lijinlin3@huawei.com>
Signed-off-by: Qiu Laibin <qiulaibin@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>