linux-kernel - Re: [PATCH v4 2/2] block: cancel all throttled bios in del

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Message-ID: <20211206154301.GD45344@blackbody.suse.cz>
Date:   Mon, 6 Dec 2021 16:43:01 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     hch@...radead.org, tj@...nel.org, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v4 2/2] block: cancel all throttled bios in del_gendisk()

On Sat, Dec 04, 2021 at 04:03:53PM +0800, "yukuai (C)" <yukuai3@...wei.com> wrote:
> I was thinking that if there are active blkgs, holding queue_lock will
> ensure blkcg won't be freed.

My take is that the function traverses the whole blkcg tree (from global
root) and nothing prevents concurrent blkcg_css_free() in a possibly
unrelated branch (or queue).

> By the way, does spin_lock can guarantee this since it disables preempt
> like what rcu_read_lock() does?

Yes (but don't quoRTe me on that :-).

(It even isn't issue with a non-preemptible kernel neither but the code
IMO should be generic to allow for different configs -- or as I
mentioned initially, make a comment why the tree traversal is not
affected by concurrent frees.)

Thanks,
Michal

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