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

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Message-ID: <9cf50e70-d2e4-cfd7-da2e-3701da0814b4@huaweicloud.com>
Date: Mon, 29 Sep 2025 09:02:30 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Bart Van Assche <bvanassche@....org>, Yu Kuai <yukuai1@...weicloud.com>,
 Yu Kuai <hailan@...uai.org.cn>, 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 2025/09/27 1:19, Bart Van Assche 写道:
> On 9/25/25 5:57 PM, Yu Kuai wrote:
>> 在 2025/09/26 1:07, Yu Kuai 写道:
>>> 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?
> 
> I'm not a cgroup expert so I cannot answer the above question. But I
> think it's clear that the description of this patch is not sufficient as
> motivation for this patch. Removing the blkg->q->queue_lock lock and
> unlock calls requires a detailed review of all blkcg_print_blkgs() and
> blkcg_print_stat() callers. There is no evidence available in the patch
> description that shows that such a review has happened.
> 

Ok, I'll explain more in details in commit message.

Thanks,
Kuai

> Thanks,
> 
> Bart.
> .
>