mmc: block: bonnie++ runs with errors on arc/hsdk board

[Evgeniy Didin]

Hello Adrian,

I have discovered, that beggining with 4.16-rc1 bonnie++ benchmark
runs with errors on arc/hsdk board. After bisecting between 4.15 and 4.16-rc1,
I have found that errors started after 
commit 81196976ed94 (mmc: block: Add blk-mq support).

Error message is like:

| # bonnie++ -u root -r 256 -s 512 -x 1 -d  /mnt 
| Using uid:0, gid:0.
| Writing with putc()...random: crng init done
| done
| Writing intelligently...INFO: task kworker/u8:0:5 blocked for more than 10 seconds.
|      Not tainted 4.15.0-rc3-00012-g81196976ed94-dirty #1
| "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
| kworker/u8:0    D    0     5      2 0x00000000
| Workqueue: writeback wb_workfn (flush-179:0)
|
| Stack Trace:
|  __switch_to+0x0/0xac
|  __schedule+0x1b8/0x738
|  io_schedule+0x5c/0xc0
|  bit_wait_io+0xc/0x48
|  out_of_line_wait_on_bit+0x78/0xc0
|  do_get_write_access+0x1aa/0x4cc
|  jbd2_journal_get_write_access+0x32/0x74
|  __ext4_journal_get_write_access+0x3a/0xac
|  ext4_mb_mark_diskspace_used+0x66/0x5b0
|  ext4_mb_new_blocks+0x1ee/0x830
|  ext4_ext_map_blocks+0x504/0xcac
|  ext4_map_blocks+0x262/0x5e8
|  mpage_map_and_submit_extent+0xb8/0x648
|  ext4_writepages+0x5ce/0x6b4
|  do_writepages+0x20/0x84
|  __writeback_single_inode+0x2a/0x154
|  wb_writeback+0x538/0xae0
|  wb_workfn+0x17c/0x334
|  process_one_work+0x1a6/0x350
|  worker_thread+0xf2/0x478
|  kthread+0x120/0x13c
|  ret_from_fork+0x18/0x1c

There are some details of hsdk_defconfig presented bellow:

SOFTLOCKUP_DETECTOR=y
DETECT_HUNG_TASK=y
DEFAULT_HUNG_TASK_TIMEOUT=10
MMC_DW=y
MMC_DW_PLTFM=y

I have also tested bonnie++ on Wandboard on v4.16-rc5 with the same
DEFAULT_HUNG_TASK_TIMEOUT=10, but there were no errors.

Even though it is not critical error and we see bonnie++ test proceeds to the end,
still it is strange, that some process is running in kernel space for a long time (at least 10sec).
What is strange, before metioned commit I can't reproduce this behaviour.

I am wondering is this expected behaviour? 

Best regards,
Evgeniy Didin