linux-kernel - Re: [PATCH] blkcg: not hold blkcg lock when deactivating policy.

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Message-ID: <3a699198-edf4-7337-f5e7-faaa6b058713@kernel.dk>
Date:   Wed, 18 Apr 2018 08:40:20 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     jiang.biao2@....com.cn, paolo.valente@...aro.org
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        tj@...nel.org, zhong.weidong@....com.cn, wen.yang99@....com.cn,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        broonie@...nel.org
Subject: Re: [PATCH] blkcg: not hold blkcg lock when deactivating policy.

On 4/18/18 3:18 AM, jiang.biao2@....com.cn wrote:
> Hi,
>>> Il giorno 17 apr 2018, alle ore 09:10, Jiang Biao <jiang.biao2@....com.cn> ha scritto:
>>>
>>> As described in the comment of blkcg_activate_policy(),
>>> *Update of each blkg is protected by both queue and blkcg locks so
>>> that holding either lock and testing blkcg_policy_enabled() is
>>> always enough for dereferencing policy data.*
>>> with queue lock held, there is no need to hold blkcg lock in
>>> blkcg_deactivate_policy(). Similar case is in
>>> blkcg_activate_policy(), which has removed holding of blkcg lock in
>>> commit 4c55f4f9ad3001ac1fefdd8d8ca7641d18558e23.
>>>
>>
>> Hi,
>> by chance, did you check whether this may cause problems with bfq,
>> being the latter not protected by the queue lock as cfq?
> Checked the bfq code, bfq seems never used blkcg lock derectly, and 
> update of blkg in the common code is protected by both queue and 
> blkcg locks, so IMHO this patch would not introduce any new problem
> with bfq, even though bfq is not protected by queue lock.
> On the other hand, the locks (queue lock/blkcg lock) used to protected
> the update of blkg seems a bit too heavyweight, especially the queue lock 
> which is used too widely may cause races with other contexts. I wonder
> if there is any way to ease the case? e.g. add a new lock for blkg's own.:) 

It might make sense to lock it separately, but I would not worry
about it unless it shows up as hot in your testing.

I've applied your patch, thanks.

-- 
Jens Axboe