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

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Date:   Wed, 4 Jan 2023 11:45:42 -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 2/2] blk-iocost: add refcounting for ioc

On Tue, Dec 27, 2022 at 08:55:02PM +0800, Yu Kuai wrote:
> Root cause is that blkg_free() can be asynchronously, and it can race
> with delete device:
> 
> T1			T2		T3
> //delete device
> del_gendisk
>  bdi_unregister
>   bdi_remove_from_list
>    synchronize_rcu_expedited
> 
> 			//rmdir cgroup
> 			blkcg_destroy_blkgs
> 			 blkg_destroy
> 			  percpu_ref_kill
> 			   blkg_release
> 			    call_rcu
>  rq_qos_exit
>   ioc_rqos_exit
>    kfree(ioc)
> 					__blkg_release
> 					 blkg_free
> 					  blkg_free_workfn
> 					   pd_free_fn
> 					    ioc_pd_free
> 					     spin_lock_irqsave
> 
> Fix the problem by add refcounting for ioc, and iocg will grab reference
> of ioc, so that ioc won't be freed until all the iocg is exited.

Ditto, why do this in iocost instead of blk-cgroup core?

Thanks.

-- 
tejun