blk-mq: don't lose requests if a stopped queue restarts
Normally if driver is busy to dispatch a request the logic is like below:
block layer: driver:
__blk_mq_run_hw_queue
a. blk_mq_stop_hw_queue
b. rq add to ctx->dispatch
later:
1. blk_mq_start_hw_queue
2. __blk_mq_run_hw_queue
But it's possible step 1-2 runs between a and b. And since rq isn't in
ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
there are no subsequent requests kick in.
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 76f460e..e68b71b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -858,6 +858,16 @@
spin_lock(&hctx->lock);
list_splice(&rq_list, &hctx->dispatch);
spin_unlock(&hctx->lock);
+ /*
+ * the queue is expected stopped with BLK_MQ_RQ_QUEUE_BUSY, but
+ * it's possible the queue is stopped and restarted again
+ * before this. Queue restart will dispatch requests. And since
+ * requests in rq_list aren't added into hctx->dispatch yet,
+ * the requests in rq_list might get lost.
+ *
+ * blk_mq_run_hw_queue() already checks the STOPPED bit
+ **/
+ blk_mq_run_hw_queue(hctx, true);
}
}