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

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Date:   Wed, 11 Jan 2023 09:36:25 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Tejun Heo <tj@...nel.org>, 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

Hi,

在 2023/01/11 2:36, Tejun Heo 写道:
> Hello,
> 
> On Tue, Jan 10, 2023 at 09:39:44AM +0800, Yu Kuai wrote:
>> As I tried to explain before, we can make sure blkg_free() is called
>> in order, but blkg_free() from remove cgroup can concurrent with
>> deactivate policy, and we can't guarantee the order of ioc_pd_free()
>> that is called both from blkg_free() and blkcg_deactivate_policy().
>> Hence I don't think #3 is possible.
> 
> Hahaha, sorry that I keep forgetting that. This doesn't really feel like
> that important or difficult part of the problem tho. Can't it be solved by
> synchronizing blkg free work item against the deactivate path with a mutex?
> 

I'm not sure, of course this can fix the problem, but two spinlock
'blkcg->lock' and 'q->queue_lock' are used to protect blkg_destroy()
currently, add a mutex（disk level?) requires a refactor, which seems
complex to me.

Thanks,
Kuai