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Subject: Re: [PATCH V2 2/8] blk-mq: use static_rqs instead of rqs to iterate
 tags
To:     Dongli Zhang <dongli.zhang@oracle.com>
Cc:     axboe@kernel.dk, linux-block@vger.kernel.org, jsmart2021@gmail.com,
        sagi@grimberg.me, josef@toxicpanda.com,
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        hch@lst.de, bvanassche@acm.org
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From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <a34ae014-6e99-7c57-42bd-5061dd6ea900@oracle.com>
Date:   Mon, 25 Mar 2019 15:14:08 +0800
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Hi Dongli

Thanks for your kindly response.

On 3/25/19 3:12 PM, Dongli Zhang wrote:
> Hi Jianchao,
> 
> On 3/25/19 1:38 PM, Jianchao Wang wrote:
>> tags->rqs[] will not been cleaned when free driver tag to avoid
>> an extra store on a shared area in the per io path. But there
>> is a window between get driver tag and write tags->rqs[], so we
>> may see stale rq in tags->rqs[] which may have been freed, as
>> following case,
>> blk_mq_get_request         blk_mq_queue_tag_busy_iter
>>   -> blk_mq_get_tag
> 
> Sorry I forgot to mention below when I was studying the v1 patchset.
> 
> If there is further review iteration, I would suggest clarify here that
> blk_mq_get_tag() would set the bit in sbitmap (e.g., via __sbitmap_get_word())
> that bt_for_each() would iterate. Otherwise, please ignore my message.
> 
> E.g.,
> 
> blk_mq_get_request
>   -> blk_mq_get_tag
>       -> taint sbitmap tag bit
>          without setting tags->rqs[tag]
> 
>                                           -> bt_for_each
>                                               -> iterate all dirty bit...
> 
> This would be much more friendly for people not working on block layer and when
> they are looking for the potential fix for a bug by simply searching over the
> commit messages. People not working on this would be able to quickly understand
> there is a window between setting bit and setting data.

Yes, I will do that.

> 
> 
>>                              -> bt_for_each
>>                                -> bt_iter
>>                                  -> rq = taags->rqs[]
> 
> %s/taags/tags/g

Yes.


Thanks
Jianchao

> 
>>                                  -> rq->q
>>   -> blk_mq_rq_ctx_init
>>     -> data->hctx->tags->rqs[rq->tag] = rq;
>>
> 
> Thank you very much!
> 
> Dongli Zhang
>