linux-kernel - Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg

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Message-ID: <Y7XzUee5Bq+DoIC1@slm.duckdns.org>
Date:   Wed, 4 Jan 2023 11:44:49 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@...radead.org, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg

On Tue, Dec 27, 2022 at 08:55:01PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.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