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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index ae39c7f3d83dc02e437c7590caf2e948d7f599a0..0f4f0c8a7825637a93ee43e74b68ae0b3dad6f9f 100644 (file)
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -434,10 +434,13 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
        if (ret < 0)
                return ret;
 
-       if (poll_on)
+       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;
 }