[PATCH] block: fix potential IO hang when turning off io_poll

QUEUE_FLAG_POLL flag will be cleared when turning off 'io_poll', while
at that moment there may be IOs stuck in hw queue uncompleted. The
following polling routine won't help reap these IOs, since blk_poll()
will return immediately because of cleared QUEUE_FLAG_POLL flag. Thus
these IOs will hang until they finnaly time out. The hang out can be
observed by 'fio --engine=io_uring iodepth=1', while turning off
'io_poll' at the same time.

To fix this, freeze and flush the request queue first when turning off
'io_poll'.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
 block/blk-sysfs.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index b513f1683af0..10d74741c002 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c

@@ -430,8 +430,11 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
 
 	if (poll_on)
 		blk_queue_flag_set(QUEUE_FLAG_POLL, q);
-	else
+	else {
+		blk_mq_freeze_queue(q);
 		blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
+		blk_mq_unfreeze_queue(q);
+	}
 
 	return ret;
 }

-- 
2.27.0