[PATCH 1/4] block: fix request starvation when queue is stopped or quiesced

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

Supposing the following scenario with a virtio_blk driver.

CPU0                                    CPU1                                    CPU2

blk_mq_try_issue_directly()
    __blk_mq_issue_directly()
        q->mq_ops->queue_rq()
            virtio_queue_rq()
                blk_mq_stop_hw_queue()
                                        blk_mq_try_issue_directly()             virtblk_done()
                                            if (blk_mq_hctx_stopped())
    blk_mq_request_bypass_insert()                                                  blk_mq_start_stopped_hw_queue()
    blk_mq_run_hw_queue()                                                               blk_mq_run_hw_queue()
                                                blk_mq_insert_request()
                                                return // Who is responsible for dispatching this IO request?

After CPU0 has marked the queue as stopped, CPU1 will see the queue is stopped.
But before CPU1 puts the request on the dispatch list, CPU2 receives the interrupt
of completion of request, so it will run the hardware queue and marks the queue
as non-stopped. Meanwhile, CPU1 also runs the same hardware queue. After both CPU1
and CPU2 complete blk_mq_run_hw_queue(), CPU1 just puts the request to the same
hardware queue and returns. Seems it misses dispatching a request. Fix it by
running the hardware queue explicitly. I think blk_mq_request_issue_directly()
should handle a similar problem.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 block/blk-mq.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e3c3c0c21b553..b2d0f22de0c7f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2619,6 +2619,7 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 	if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(rq->q)) {
 		blk_mq_insert_request(rq, 0);
+		blk_mq_run_hw_queue(hctx, false);
 		return;
 	}
 
@@ -2649,6 +2650,7 @@ static blk_status_t blk_mq_request_issue_directly(struct request *rq, bool last)
 
 	if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(rq->q)) {
 		blk_mq_insert_request(rq, 0);
+		blk_mq_run_hw_queue(hctx, false);
 		return BLK_STS_OK;
 	}
 
-- 
2.20.1