linux-kernel - Re: [PATCH 01/10] blk-cgroup: use cgroup lock and rcu to protect iterating blkcg blkgs

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Message-ID: <688275d5-fbb4-08b3-45e1-798ad8cf77fc@huaweicloud.com>
Date: Fri, 26 Sep 2025 08:57:03 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Yu Kuai <hailan@...uai.org.cn>, Bart Van Assche <bvanassche@....org>,
 Yu Kuai <yukuai1@...weicloud.com>, tj@...nel.org, ming.lei@...hat.com,
 nilay@...ux.ibm.com, hch@....de, josef@...icpanda.com, axboe@...nel.dk,
 akpm@...ux-foundation.org, vgoyal@...hat.com
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, yi.zhang@...wei.com,
 yangerkun@...wei.com, johnny.chenyi@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 01/10] blk-cgroup: use cgroup lock and rcu to protect
 iterating blkcg blkgs

Hi,

在 2025/09/26 1:07, Yu Kuai 写道:
> Hi,
> 
> 在 2025/9/25 23:57, Bart Van Assche 写道:
>> On 9/25/25 1:15 AM, Yu Kuai wrote:
>>> It's safe to iterate blkgs with cgroup lock or rcu lock held, prevent
>>> nested queue_lock under rcu lock, and prepare to convert protecting
>>> blkcg with blkcg_mutex instead of queuelock.
>>
>> Iterating blkgs without holding q->queue_lock is safe but accessing the
>> blkg members without holding that lock is not safe since q->queue_lock
>> is acquired by all code that modifies blkg members. Should perhaps a new
>> spinlock be introduced to serialize blkg modifications?

Actually, only blkcg_print_blkgs() is using rcu in this patch, and take
a look at the callers, I don't see anyone have to hold queue_lock. Can
you explain in detail which field from blkg is problematic in this
patch?

Thanks,
Kuai

>>
> No need for a new lock, I think blkcg->lock can do that.
> 
> Thanks,
> Kuai
> 
>> Thanks,
>>
>> Bart.
>>
> .
>