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>

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 230ef3056b72e72fc1164aa6afdf7a14817e9e7f..eb55492e6875771f4723c92113bed70f7b8aec51 100644 (file)
--- 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();