Re: [PATCH] block: don't allow the same type rq_qos add more than once

[Muchun Song]

On Mon, Jul 18, 2022 at 4:37 PM Jinke Han <hanjinke.666@xxxxxxxxxxxxx> wrote:
>
> From: hanjinke <hanjinke.666@xxxxxxxxxxxxx>
>
> In our test of iocost, we encounttered some list add/del corrutions of
> inner_walk list in ioc_timer_fn.
> The resean can be descripted as follow:
>
> cpu 0                                           cpu 1
> ioc_qos_write                                   ioc_qos_write
>
> ioc = q_to_ioc(bdev_get_queue(bdev));
> if (!ioc) {
>         ioc = kzalloc();                        ioc = q_to_ioc(bdev_get_queue(bdev));
>                                                         if (!ioc) {
>                                                                 ioc = kzalloc();
>                                                                 ...
>                                                                 rq_qos_add(q, rqos);
>                                                          }
>         ...
>         rq_qos_add(q, rqos);
>         ...
> }
>
> When the io.cost.qos file is written by two cpu concurrently, rq_qos may
> be added to one disk twice. In that case, there will be two iocs enabled
> and running on one disk. They own different iocgs on their active list.
> In the ioc_timer_fn function, because of the iocgs from two ioc have the
> same root iocg, the root iocg's walk_list may be overwritten by each
> other and this lead to list add/del corrutions in building or destorying
> the inner_walk list.
>
> And so far, the blk-rq-qos framework works in case that one instance for
> one type rq_qos per queue by default. This patch make this explicit and
> also fix the crash above.
>
> Signed-off-by: hanjinke <hanjinke.666@xxxxxxxxxxxxx>

The change LGTM. Maybe it is better to add a Fixes tag here so that
others can easily know what Linux versions should be backported.

Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

Thanks.