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

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Message-ID: <bc6fe04d-3245-40dd-aa30-c3a3acb670c2@acm.org>
Date: Thu, 25 Sep 2025 08:57:03 -0700
From: Bart Van Assche <bvanassche@....org>
To: 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, yukuai3@...wei.com,
 yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH 01/10] blk-cgroup: use cgroup lock and rcu to protect
 iterating blkcg blkgs

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?

Thanks,

Bart.