Re: [RFC PATCH] block: defer flush request no matter whether we have elevator

[Jens Axboe <axboe@kernel.dk>]

On 7/16/20 12:52 AM, Yufen Yu wrote:
> Commit 7520872c0cf4 ("block: don't defer flushes on blk-mq + scheduling")
> tried to fix deadlock for cycled wait between flush requests and data
> request into flush_data_in_flight. The former holded all driver tags
> and wait for data request completion, but the latter can not complete
> for waiting free driver tags.
> 
> After commit 923218f6166a ("blk-mq: don't allocate driver tag upfront
> for flush rq"), flush requests will not get driver tag before queuing
> into flush queue.
> 
> * With elevator, flush request just get sched_tags before inserting
>   flush queue. It will not get driver tag until issue them to driver.
>   data request on list fq->flush_data_in_flight will complete in
>   the end.
> 
> * Without elevator, each flush request will get a driver tag when
>   allocate request. Then data request on fq->flush_data_in_flight
>   don't worry about lacking driver tag.
> 
> In both of these cases, cycled wait cannot be true. So we may allow
> to defer flush request.

Applied, thanks.

-- 
Jens Axboe