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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 19 Jul 2022 05:50:16 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Jinke Han <hanjinke.666@bytedance.com>
Cc:     axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH v2] block: don't allow the same type rq_qos add more than
 once
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References: <20220719070258.25721-1-hanjinke.666@bytedance.com>
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On Tue, Jul 19, 2022 at 03:02:58PM +0800, Jinke Han wrote:
> From: Jinke Han <hanjinke.666@bytedance.com>
> 
> In our test of iocost, we encounttered some list add/del corrutions of
> inner_walk list in ioc_timer_fn.
> 
> The reason 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: Jinke Han <hanjinke.666@bytedance.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun