linux-kernel - Re: [PATCH 2/2] blkcg: init root blkcg

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Message-ID: <20180426191847.GS1911913@devbig577.frc2.facebook.com>
Date:   Thu, 26 Apr 2018 12:18:47 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Jiang Biao <jiang.biao2@....com.cn>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhong.weidong@....com.cn,
        wen.yang99@....com.cn
Subject: Re: [PATCH 2/2] blkcg: init root blkcg_gq under lock

Hello,

On Thu, Apr 19, 2018 at 12:06:09PM +0800, Jiang Biao wrote:
> The initializing of q->root_blkg is currently outside of queue lock
> and rcu, so the blkg may be destroied before the initializing, which
> may cause dangling/null references. On the other side, the destroys
> of blkg are protected by queue lock or rcu. Put the initializing
> inside the queue lock and rcu to make it safer.

Hmm... I can't think of a scenario where the above could happen.  The
queue is still being initialized and no one else has access to it, so
I can't see how it could be destroyed.  Can you please describe the
sequence of events?  Have you actually observed this happening?

Thanks.

-- 
tejun