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Date:   Fri, 27 May 2022 09:15:03 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, axboe@kernel.dk,
        shinichiro.kawasaki@wdc.com, dan.j.williams@intel.com,
        yukuai3@huawei.com, linux-block@vger.kernel.org
Subject: Re: [PATCH] block: remove per-disk debugfs files in
 blk_unregister_queue
Message-ID: <20220527071503.GA16342@lst.de>
References: <20220524083325.833981-1-hch@lst.de> <YozvXP9/hVhTQt+D@T590>
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On Tue, May 24, 2022 at 10:44:44PM +0800, Ming Lei wrote:
> The above line code may cause kernel panic in the following code paths:
> 
> 1) blk_mq_debugfs_unregister_hctxs() called from __blk_mq_update_nr_hw_queues()
> 
> 2) blk_mq_debugfs_unregister_sched_hctx() called from blk_mq_exit_sched()

Yes.  Then again we already have the same sort of race window in reverse
at startup time, where nothing synchronizes between 
blk_mq_debugfs_register_hctxs from __blk_mq_update_nr_hw_queues and
add_disk adding debugfs files.

All this probably needs serializing using debugfs_mutex and a check of
QUEUE_FLAG_REGISTERED.