linux-kernel - Re: [PATCH] blkio: Release blkg infrastructure only after last policy is deactivated.

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Date:	Tue, 24 Jun 2014 16:37:58 -0500
From:	Shirish Pargaonkar <shirishpargaonkar@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, vgoyal@...hat.com,
	Shirish Pargaonkar <spargaonkar@...e.com>, axboe@...nel.dk
Subject: Re: [PATCH] blkio: Release blkg infrastructure only after last policy
 is deactivated.

When we start from blk_cleanup_queue(), we put request queue in bypass mode,
drain it (and service queues), and then destroy blkcgs (explicitly)

When we start from blk_release_queue(), we do not drain first and then
destroy blkcgs.  So if we destroy blkcg and then call (implicitly) and
bail out of
blk_drain_queue, we would not have drained the service queues which
is not what we want.

I do not see any harm in waiting till end to release blkcgs (as I understand).

Regards,

Shirish

On Tue, Jun 24, 2014 at 4:09 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Mon, Jun 23, 2014 at 01:52:25PM -0500, shirishpargaonkar@...il.com wrote:
>> From: Shirish Pargaonkar <spargaonkar@...e.com>
>>
>> Release blkg infrastructure only after last policy is deactivated
>> (i.e. let blkg_destroy_all() be called only from blkcg_deactivate_policy())
>>
>> Otherwise, module can oops because root_blkg gets assigned NULL during
>> cleanup and we attempt draining the service queues via root_blkg afterwords.
>
> I'm not sure this fix makes sense.  Cleanup path oopses on an already
> freed resource.  How can the solution be not freeing?  Why not simply
> make blkcg_drain_queue() bail if the blkgs are all destroyed?  The
> whole thing is fully synchronized with the queuelock, right?
>
> Can you please also cc Jens when you post the next iteration?
>
> Thanks.
>
> --
> tejun
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