Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight_iter

[Keith Busch <kbusch@kernel.org>]

On Wed, Mar 27, 2019 at 10:45:33AM +0800, jianchao.wang wrote:
> 1. a hctx->fq.flush_rq of dead request_queue that shares the same tagset
>    The whole request_queue is cleaned up and freed, so the hctx->fq.flush is freed back to a slab
>
> 2. a removed io scheduler's sched request
>    The io scheduled is detached and all of the structures are freed, including the pages where sched
>    requests locates.
> 
> So the pointers in tags->rqs[] may point to memory that is not used as a blk layer request.

Oh, free as in kfree'd, not blk_mq_free_request. So it's a read-after-
free that you're concerned about, not that anyone explicitly changed a
request->state.

We at least can't free the flush_queue until the queue is frozen. If the
queue is frozen, we've completed the special fq->flush_rq where its end_io
replaces tags->rqs[tag] back to the fq->orig_rq from the static_rqs,
so nvme's iterator couldn't see the fq->flush_rq address if it's invalid.

The sched_tags concern, though, appears theoretically possible.