Polled I/O cannot find completions

[Bijan Mottahedeh <bijan.mottahedeh@xxxxxxxxxx>]

I'm seeing poll threads hang as I increase the number of threads in 
polled fio tests.  I think this is because of polling on BLK_QC_T_NONE 
cookie, which will never succeed.

A related problem however, is that the meaning of BLK_QC_T_NONE seems to 
be ambiguous.

Specifically, the following cases return BLK_QC_T_NONE which I think 
would be problematic for polled io:


generic_make_request()
...
        if (current->bio_list) {
                bio_list_add(&current->bio_list[0], bio);
                goto out;
        }

In this case the request is delayed but should get a cookie eventually.  
How does the caller know what the right action is in this case for a 
polled request?  Polling would never succeed.


__blk_mq_issue_directly()
...
        case BLK_STS_RESOURCE:
        case BLK_STS_DEV_RESOURCE:
                blk_mq_update_dispatch_busy(hctx, true);
                __blk_mq_requeue_request(rq);
                break;

In this case, cookie is not updated and would keep its default 
BLK_QC_T_NONE value from blk_mq_make_request().  However, this request 
will eventually be reissued, so again, how would the caller poll for the 
completion of this request?

blk_mq_try_issue_directly()
...
        ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true);
        if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
                blk_mq_request_bypass_insert(rq, false, true);

Am I missing something here?

Incidentally, I don't see BLK_QC_T_EAGAIN used anywhere, should it be?

Thanks.

--bijan