linux-kernel - Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight

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Date:   Tue, 26 Mar 2019 11:05:53 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        James Smart <jsmart2021@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        Josef Bacik <josef@...icpanda.com>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight_iter



On 3/26/19 10:41 AM, Ming Lei wrote:
> On Tue, Mar 26, 2019 at 9:18 AM jianchao.wang
> <jianchao.w.wang@...cle.com> wrote:
>>
>> Hi Keith
>>
>> On 3/25/19 9:49 PM, Keith Busch wrote:
>>> On Mon, Mar 25, 2019 at 01:38:37PM +0800, Jianchao Wang wrote:
>>>> blk_mq_tagset_inflight_iter is not safe that it could get stale request
>>>> in tags->rqs[]. Use blk_mq_queue_tag_inflight_iter here. A new helper
>>>> interface nvme_iterate_inflight_rqs is introduced to iterate
>>>> all of the ns under a ctrl.
>>>
>>> Nak, NVMe only iterates tags when new requests can't enter, allocated
>>> requests can't dispatch, and dispatched commands can't complete. So
>>> it is perfectly safe to iterate if the driver takes reasonable steps
>>> beforehand.
>>
>> nvme_dev_disable just quiesce and freeze the request_queue, but not drain the enters.
>> So there still could be someone escapes the queue freeze checking and tries to allocate
>> request.
> 
> The rq->state is just IDLE for these allocated request, so there
> shouldn't be issue
> in NVMe's case.

What if there used to be a io scheduler and leave some stale requests of sched tags ?
Or the nr_hw_queues was decreased and leave the hctx->fq->flush_rq ?

The stable request could be some tings freed and used
by others and the state field happen to be overwritten to non-zero...

Thanks
Jianchao