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

[jianchao.wang]

Hi Keith

On 3/27/19 2:51 PM, Keith Busch wrote:
> 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.

Yes ;)

> 
> 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.
> 

This is true for the non io-scheduler case in which the flush_rq would steal the driver tag.
But for io-scheduler case, flush_rq would acquire a driver tag itself.


> The sched_tags concern, though, appears theoretically possible.
> 

Thanks
Jianchao