"blk-mq: fix tag_get wait task can't be awakened" causes hangs

[Alex Xu (Hello71)]

Hi,

Recently on torvalds master, I/O on USB flash drives started hanging 
here:

task:systemd-udevd   state:D stack:    0 pid:  374 ppid:   347 flags:0x00004000
Call Trace:
 <TASK>
 ? __schedule+0x319/0x4a0
 ? schedule+0x77/0xa0
 ? io_schedule+0x43/0x60
 ? blk_mq_get_tag+0x175/0x2b0
 ? mempool_alloc+0x33/0x170
 ? init_wait_entry+0x30/0x30
 ? __blk_mq_alloc_requests+0x1b4/0x220
 ? blk_mq_submit_bio+0x213/0x490
 ? submit_bio_noacct+0x22c/0x270
 ? xa_load+0x48/0x80
 ? mpage_readahead+0x114/0x130
 ? blkdev_fallocate+0x170/0x170
 ? read_pages+0x48/0x1d0
 ? page_cache_ra_unbounded+0xee/0x1f0
 ? force_page_cache_ra+0x68/0xc0
 ? filemap_read+0x18c/0x9a0
 ? blkdev_read_iter+0x4e/0x120
 ? vfs_read+0x265/0x2d0
 ? ksys_read+0x50/0xa0
 ? do_syscall_64+0x62/0x90
 ? do_user_addr_fault+0x271/0x3c0
 ? asm_exc_page_fault+0x8/0x30
 ? exc_page_fault+0x58/0x80
 ? entry_SYSCALL_64_after_hwframe+0x44/0xae
 </TASK>

mount(8) hangs with a similar backtrace, making the device effectively 
unusable. It does not seem to affect NVMe or SATA attached drives. The 
affected drive does not support UAS. I don't currently have UAS drives 
to test with. The default I/O scheduler is set to noop.

I found that reverting 180dccb0dba4 ("blk-mq: fix tag_get wait 
task can't be awakened") appears to resolve the issue.

Let me know what other information is needed.

Cheers,
Alex.