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++;
if (ret == BLK_STS_RESOURCE ||
ret == BLK_STS_DEV_RESOURCE) {
blk_mq_request_bypass_insert(rq, false,
@@ -2687,7 +2688,6 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
break;
}
blk_mq_end_request(rq, ret);
- errors++;
} else
queued++;
}
--
2.31.1
