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Date:   Wed, 20 Jul 2022 02:54:24 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Ian Kent <raven@themaw.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] vfs: add propagate_mount_tree_busy() helper
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References: <165751053430.210556.5634228273667507299.stgit@donald.themaw.net>
 <165751066658.210556.1326573473015621909.stgit@donald.themaw.net>
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On Mon, Jul 11, 2022 at 11:37:46AM +0800, Ian Kent wrote:

> +static int do_mount_in_use_check(struct mount *mnt, int cnt)
> +{
> +	struct mount *topper;
> +
> +	/* Is there exactly one mount on the child that covers
> +	 * it completely?
> +	 */
> +	topper = find_topper(mnt);
> +	if (topper) {
> +		int topper_cnt = topper->mnt_mounts_cnt + 1;
> +
> +		/* Open file or pwd within singular mount? */
> +		if (do_refcount_check(topper, topper_cnt))
> +			return 1;

Whatever the hell for?  umount(2) will be able to slide the
underlying mount from under the topper, whatever the
refcount of topper might have been.