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Subject: Re: [PATCH] blkcg: not hold blkcg lock when deactivating policy.
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <201804181718167747438@zte.com.cn>
Date:   Wed, 18 Apr 2018 14:45:57 +0200
Cc:     axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, tj@kernel.org,
        zhong.weidong@zte.com.cn, wen.yang99@zte.com.cn,
        ulf.hansson@linaro.org, linus.walleij@linaro.org,
        broonie@kernel.org
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References: <1523949033-91148-1-git-send-email-jiang.biao2@zte.com.cn,661CB1A2-5BAC-439D-BCE2-64F9A0CD280E@linaro.org>
 <201804181718167747438@zte.com.cn>
To:     jiang.biao2@zte.com.cn
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> Il giorno 18 apr 2018, alle ore 11:18, jiang.biao2@zte.com.cn ha =
scritto:
>=20
> Hi,
>>> Il giorno 17 apr 2018, alle ore 09:10, Jiang Biao =
<jiang.biao2@zte.com.cn> ha scritto:
>>>=20
>>> 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.
>>>=20
>>=20
>> 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=20
> update of blkg in the common code is protected by both queue and=20
> blkcg locks, so IMHO this patch would not introduce any new problem
> with bfq, even though bfq is not protected by queue lock.

Thank you very much for checking. Then:

Acked-by: Paolo Valente <paolo.valente@linaro.org>

Thanks,
Paolo

> 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=20
> 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.:)=20
>=20
> Jiang,
> Regards