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

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Message-ID: <66dcdcd1-df71-43b9-a468-2b4aaa8b6dc7@acm.org>
Date: Fri, 26 Sep 2025 10:19:51 -0700
From: Bart Van Assche <bvanassche@....org>
To: 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

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.

Thanks,

Bart.