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Subject: Re: [PATCH] block: revert pushing the final release of request_queue
 to a workqueue.
To:     Bart Van Assche <bvanassche@acm.org>, <axboe@kernel.dk>,
        <ming.lei@redhat.com>, <chaitanya.kulkarni@wdc.com>,
        <damien.lemoal@wdc.com>, <dhowells@redhat.com>,
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        <yi.zhang@huawei.com>, <zhangxiaoxu5@huawei.com>,
        <luoshijie1@huawei.com>, jan kara <jack@suse.cz>
References: <20200206111052.45356-1-yukuai3@huawei.com>
 <70ce8830-2594-2b7b-9ca9-5fb7edd374d7@acm.org>
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From:   "yukuai (C)" <yukuai3@huawei.com>
Message-ID: <b1fead0a-5508-33e8-7aa0-5d061884716a@huawei.com>
Date:   Mon, 10 Feb 2020 16:49:48 +0800
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On 2020/2/10 11:14, Bart Van Assche wrote:
> I think that calling blk_mq_exit_queue() from blk_unregister_queue()
> would break at least the sd driver. The sd driver can issue I/O after
> having called del_gendisk(). See also the sd_sync_cache() call in
> sd_shutdown().

If blk_mq_exit_queue() can't move to blk_unregister_queue(),
neither can blk_mq_debugfs_unregister(). It'a dead end.

The purpose is that when __blk_trace_setup() is called, the cleanup of
last loop_device(__blk_release_queue()) should finish aready.

I wonder if we can test that if the dir still exist in loop_add():

static int loop_add(struct loop_device **l, int i)
{
...
           char disk_name[DISK_NAME_LEN];
           struct dentry *dir, *root;

           sprintf(disk_name, "loop%d", i);
           root = debugfs_lookup("block", NULL);
           if (root) {
                   dir = debugfs_lookup(disk_name, root);
                   if (dir) {
                           dput(dir);
                           dput(root);
                          pr_err("Directory '%s' with parent 'block' 
already present!\n",disk_name);
                           return -EBUSY;
                   }
                   dput(root);
           }
...

Thanks!
Yu Kuai