blk-mq: fix hang in bt_get()

Avoid that if there are fewer hardware queues than CPU threads that
bt_get() can hang. The symptoms of the hang were as follows:

* All tags allocated for a particular hardware queue.
* (nr_tags) pending commands for that hardware queue.
* No pending commands for the software queues associated with that
  hardware queue.

Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 230ef3056b72..eb55492e6875 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -254,6 +254,13 @@ static int bt_get(struct blk_mq_alloc_data *data,
                 if (tag != -1)
                         break;
 
+                /*
+                 * We're out of tags on this hardware queue, kick any
+                 * pending IO submits before going to sleep waiting for
+                 * some to complete.
+                 */
+                blk_mq_run_hw_queue(hctx, false);
+
                 blk_mq_put_ctx(data->ctx);
 
                 io_schedule();