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Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 4 Jan 2023 11:44:49 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@infradead.org, josef@toxicpanda.com, axboe@kernel.dk,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, yukuai3@huawei.com,
        yi.zhang@huawei.com
Subject: Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg
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References: <20221227125502.541931-1-yukuai1@huaweicloud.com>
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On Tue, Dec 27, 2022 at 08:55:01PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> iocost requires that child iocg must exit before parent iocg, otherwise
> kernel might crash in ioc_timer_fn(). However, currently iocg is exited
> in pd_free_fn(), which can't guarantee such order:
> 
> 1) remove cgroup can concurrent with deactivate policy;
> 2) blkg_free() triggered by remove cgroup is asynchronously, remove
> child cgroup can concurrent with remove parent cgroup;
> 
> Fix the problem by add refcounting for iocg, and child iocg will grab
> reference of parent iocg, so that parent iocg will wait for all child
> iocg to be exited.

Wouldn't it be better to do this refcnting in the blk-cgroup core code
rather than in blk-iocost?

Thanks.

-- 
tejun