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

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Message-ID: <Y8GR3V3RsdS+XYzh@slm.duckdns.org>
Date:   Fri, 13 Jan 2023 07:16:13 -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, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg

Hello,

On Fri, Jan 13, 2023 at 09:25:11AM +0800, Yu Kuai wrote:
> I think hold the lock in blkg_free_workfn() is too late, pd_free_fn()
> for parent from blkcg_deactivate_policy() can be called first.
> 
> t1: remove cgroup t1/t2
> blkcg_destroy_blkgs
>  blkg_destroy
>   percpu_ref_kill(&blkg->refcnt)
>    blkg_release
>     blkg_free
>      schedule_work(&blkg->free_work)
>      // t1 is done
> 
> t2: handle t1 from removing device
> blkcg_deactivate_policy
>  pd_free_fn
>  // free parent
> 				t3: from t1
> 				blkg_free_workfn
> 				 pd_free_fn
> 				 // free child

As we discussed before, you'd have to order the actual freeing by shifting
the ref puts into the free_work. If you move `blkg_put(blkg->parent)` and
`list_del_init(&blkg->q_node)` to blkg_free_workfn() (this will require
adjustments as these things are used from other places too), the free work
items will be ordered and the blkg would remain iterable - IOW,
deactivate_policy would be able to see it allowing the two paths to
synchronize, right?

Thanks.

-- 
tejun