linux-kernel - Re: [PATCH v4 2/2] block: cancel all throttled bios in del

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Date:   Sat, 4 Dec 2021 16:03:53 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Michal Koutný <mkoutny@...e.com>
CC:     <hch@...radead.org>, <tj@...nel.org>, <axboe@...nel.dk>,
        <cgroups@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH v4 2/2] block: cancel all throttled bios in del_gendisk()

在 2021/12/03 18:27, Michal Koutný 写道:
> On Fri, Dec 03, 2021 at 03:50:01PM +0800, "yukuai (C)" <yukuai3@...wei.com> wrote:
>> blkg_destroy() is protected by the queue_lock，so I think queue_lock can
>> protect such concurrent scenario.
> 
> blkg_destroy() is not as destroying :-) as actual free, you should
> synchronize against (the queue_lock ensures this for
> pd_free_fn=throtl_pd_free but you may still trip on blkcg after
> blkcg_css_free()).

Hi, Michal

I was thinking that if there are active blkgs, holding queue_lock will 
ensure blkcg won't be freed. However, if there are no active blkgs in
the first place, it seems right rcu_read_lock() can prevent this
iteration concurrent with css_release->css_release_work_fn->
css_free_rwork_fn.

By the way, does spin_lock can guarantee this since it disables preempt
like what rcu_read_lock() does?

Thanks,
Kuai

> 
> [Actually, I think you should see a warning in your situation if you
> enable CONFIG_PROVE_RCU.]
> 
> HTH,
> Michal
>