On 11/29/17 4:56 PM, Ming Lei wrote:

Now we track legacy requests with .q_usage_counter in commit 055f6e18e08f
("block: Make q_usage_counter also track legacy requests"), but that
commit never runs and drains legacy queue before waiting for this counter
becoming zero, then IO hang is caused in the test of pulling disk during IO.

This patch fixes the issue by draining requests before waiting for
q_usage_counter becoming zero, both Mauricio and chenxiang reported this
issue, and observed that it can be fixed by this patch.

Better late than never, added for 4.15, thanks Ming.

-- 
Jens Axboe