linux-kernel - Re: [PATCH V2 8/8] blk-mq: remove blk_mq_tagset_busy

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Date:   Tue, 26 Mar 2019 08:17:23 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Hannes Reinecke <hare@...e.de>,
        "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     linux-block@...r.kernel.org, jsmart2021@...il.com,
        sagi@...mberg.me, josef@...icpanda.com,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        keith.busch@...el.com, jthumshirn@...e.de, hch@....de,
        bvanassche@....org
Subject: Re: [PATCH V2 8/8] blk-mq: remove blk_mq_tagset_busy_iter

On 3/25/19 2:25 AM, Hannes Reinecke wrote:
> On 3/25/19 8:37 AM, jianchao.wang wrote:
>> Hi Hannes
>>
>> On 3/25/19 3:18 PM, Hannes Reinecke wrote:
>>> On 3/25/19 6:38 AM, Jianchao Wang wrote:
>>>> As nobody uses blk_mq_tagset_busy_iter, remove it.
>>>>
>>>> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
>>>> ---
>>>>    block/blk-mq-tag.c     | 95 --------------------------------------------------
>>>>    include/linux/blk-mq.h |  2 --
>>>>    2 files changed, 97 deletions(-)
>>>>
>>> Please, don't.
>>>
>>> I'm currently implementing reserved commands for SCSI and reworking
>>> the SCSI error handling where I rely on this interface quite
>>> heavily.
>>
>>
>> blk_mq_tagset_busy_iter could access some stale requests which maybe
>> freed due to io scheduler switching, request_queue cleanup (shared
>> tagset) when there is someone submits io and gets driver tag. When io
>> scheduler attached, even quiesce request_queue won't work.
>>
>> If this patchset is accepted, blk_mq_tagset_busy_iter could be
>> replaced with blk_mq_queue_inflight_tag_iter which needs to be
>> invoked by every request_queue that shares the tagset.
>>
> The point is, at that time I do _not_ have a request queue to work
> with.
> 
> Most SCSI drivers have a host-wide shared tagset, which is used by all
> request queues on that host.  Iterating over the shared tagset is far
> more efficient than to traverse over all devices and the attached
> request queues.
> 
> If I had to traverse all request queues I would need to add additional
> locking to ensure this traversal is race-free, making it a really
> cumbersome interface to use.
> 
> Plus the tagset iter is understood to be used only in cases where I/O
> is stopped from the upper layers (ie no new I/O will be submitted).
> So here we only need to protect against I/O being completed, which is
> not what this patchset is about.
> 
> So my objection still stands: Please, don't.

We can't just keep an interface that's hard to use correctly just
because you have something pending for that. Jianchao has good
suggestions for you on how to proceed.

-- 
Jens Axboe