Hi, Ming 在 2022/07/26 10:48, Ming Lei 写道:
On Tue, Jul 26, 2022 at 10:38:52AM +0800, Yu Kuai wrote:From: Yu Kuai <yukuai3@xxxxxxxxxx> Currently, in virtio_scsi, if 'bd->last' is not set to true while dispatching request, such io will stay in driver's queue, and driver will wait for block layer to dispatch more rqs. However, if block layer failed to dispatch more rq, it should trigger commit_rqs to inform driver. There is a problem in blk_mq_try_issue_list_directly() that commit_rqs won't be called: // assume that queue_depth is set to 1, list contains two rq blk_mq_try_issue_list_directly blk_mq_request_issue_directly // dispatch first rq // last is false __blk_mq_try_issue_directly blk_mq_get_dispatch_budget // succeed to get first budget __blk_mq_issue_directly scsi_queue_rq cmd->flags |= SCMD_LAST virtscsi_queuecommand kick = (sc->flags & SCMD_LAST) != 0 // kick is false, first rq won't issue to disk queued++ blk_mq_request_issue_directly // dispatch second rq __blk_mq_try_issue_directly blk_mq_get_dispatch_budget // failed to get second budget ret == BLK_STS_RESOURCE blk_mq_request_bypass_insert // errors is still 0 if (!list_empty(list) || errors && ...) // won't pass, commit_rqs won't be called In this situation, first rq relied on second rq to dispatch, while second rq relied on first rq to complete, thus they will both hung. Fix the problem by also treat 'BLK_STS_RESOURCE' and 'BLK_STS_DEV_RESOURCE' as 'errors' in blk_mq_try_issue_list_directly(), so that 'commit_rqs' will be called when dispatch of last rq failed. Fixes: d666ba98f849 ("blk-mq: add mq_ops->commit_rqs()") Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx> --- block/blk-mq.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index 70177ee74295..752b0fe4c128 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2680,6 +2680,7 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx, list_del_init(&rq->queuelist); ret = blk_mq_request_issue_directly(rq, list_empty(list)); if (ret != BLK_STS_OK) { + errors++;OK, it is because that list becomes empty, but .queue_rq(last_rq_in_list) returns BLK_STS_*RESOURCE, but scsi can't call ->queuecommand() for this real last rq. Then blk_mq_try_issue_list_directly() doesn't call ->commit_rqs() too. Here errors means that request not queued successfully, so this patch looks fine. Also I think blk_mq_dispatch_rq_list() needs similar handling too, right?
Yes, you're right, I'll handle blk_mq_dispatch_rq_list() in v2, and also change the title. Thanks, Kuai
