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 76f460e36f1dd94a0bd40dc3647c638fa79fe538..e68b71b85a7eaf0e3097debe8bf4dc4078e7a038 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -858,6 +858,16 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
                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);
        }
 }