Re: [PATCH] blk-mq: fix mismatch between IO scheduler insert and finish

[Jens Axboe]

On 8/18/23 9:12 PM, chengming.zhou@xxxxxxxxx wrote:
> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> 
> IO scheduler has requirement that one request which has been inserted
> must call finish_request() only once.
> 
> Now we have three special cases to consider:
> 1. rq has not insert, has complete: e.g. empty preflush
> 2. rq has insert, has not complete: e.g. merged requests will be freed
> 3. rq has insert, has twice complete: e.g. postflushes
> 
> Note case 1 which existed before, has been no problem since all the
> schedulers will check in their finish_request() if the rq has been
> inserted or not, like checking "rq->elv.priv[0]".
> 
> Then case 2 and case 3 are the introduced regression, we moved the
> scheduler finish_request() from free phase to complete phase to solve
> a deadlock problem. But it caused no finish_request() for request in
> case 2, and double finish_request() for request in case 3.
> 
> So we still need finish_request() in blk_mq_free_request() to cover
> case 2. And clear RQF_USE_SCHED flag to avoid double finish_request().
> It should be fine since we're freeing the request now anyway.
> 
> Of course, we can also make all schedulers' finish_request() to clear
> "rq->elv.priv[0]" to avoid double finish. Or clear it in blk-mq, make
> the rq like not inserted as case 1.
> 
> FYI it's easy to reproduce warning in mq-deadline using this:
> ```
> DEV=sdb
> echo mq-deadline > /sys/block/$DEV/queue/scheduler
> mkfs.ext4 /dev/$DEV
> mount /dev/$DEV /mnt
> cd /mnt
> stress-ng --symlink 4 --timeout 60
> echo none > /sys/block/$DEV/queue/scheduler
> ```
> 
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202308172100.8ce4b853-oliver.sang@xxxxxxxxx
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>

I folded in this one and added a link to it as well, final result is here:

https://git.kernel.dk/cgit/linux/commit/?h=block-6.5&id=e5c0ca13659e9d18f53368d651ed7e6e433ec1cf

I'll get this sent off today.

-- 
Jens Axboe