System hang when changing scheduler for null_blk device

Issue

• System hang when changing scheduler for null_blk device

[  370.007085] INFO: task kworker/u8:2:409 blocked for more than 120 seconds.
[  370.008506]       Not tainted 4.18.0-48.el8.x86_64 #1
[  370.009852] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  370.011268] kworker/u8:2    D    0   409      2 0x80000000
[  370.011283] Workqueue: writeback wb_workfn (flush-251:0)
[  370.011287] Call Trace:
[  370.011299]  ? __schedule+0x254/0x840
[  370.011306]  schedule+0x28/0x70
[  370.011313]  io_schedule+0x12/0x40
[  370.011320]  wbt_wait+0x202/0x300
[  370.011325]  ? wb_timer_fn+0x3b0/0x3b0
[  370.011332]  rq_qos_throttle+0x31/0x40
[  370.011342]  blk_mq_make_request+0x12b/0x560
[  370.011351]  generic_make_request+0x1a1/0x400
[  370.011358]  submit_bio+0x45/0x140
[  370.011364]  ? guard_bio_eod+0x32/0x100
[  370.011369]  submit_bh_wbc+0x163/0x190
[  370.011376]  __block_write_full_page+0x22a/0x440
[  370.011381]  ? end_buffer_async_read+0x120/0x120
[  370.011386]  ? check_disk_change+0x60/0x60
[  370.011393]  __writepage+0x19/0x50
[  370.011398]  write_cache_pages+0x1dc/0x450
[  370.011403]  ? __wb_calc_thresh+0x130/0x130
[  370.011414]  generic_writepages+0x56/0x90
[  370.011421]  do_writepages+0x41/0xd0
[  370.011428]  ? blk_mq_sched_dispatch_requests+0x11e/0x170
[  370.011434]  __writeback_single_inode+0x3d/0x360
[  370.011441]  writeback_sb_inodes+0x1e3/0x450
[  370.011451]  __writeback_inodes_wb+0x5d/0xb0
[  370.011458]  wb_writeback+0x25f/0x2f0
[  370.011466]  wb_workfn+0x32b/0x4a0
[  370.011475]  ? __switch_to_asm+0x34/0x70
[  370.011480]  ? __switch_to_asm+0x40/0x70
[  370.011485]  ? __switch_to_asm+0x34/0x70
[  370.011490]  ? __switch_to_asm+0x40/0x70
[  370.011494]  ? __switch_to_asm+0x34/0x70
[  370.011498]  ? __switch_to+0xa3/0x490
[  370.011503]  ? __switch_to_asm+0x34/0x70
[  370.011511]  process_one_work+0x1a7/0x360
[  370.011517]  worker_thread+0x30/0x390
[  370.011523]  ? pwq_unbound_release_workfn+0xd0/0xd0
[  370.011527]  kthread+0x112/0x130
[  370.011533]  ? kthread_bind+0x30/0x30
[  370.011539]  ret_from_fork+0x1f/0x40