Re: [PATCH V2 2/8] blk-mq: use static_rqs instead of rqs to iterate tags

jianchao.wang
2019-03-25T07:14:35Z

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
>