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From: David Howells <dhowells@redhat.com>
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To: Alan Jenkins <alan.christopher.jenkins@gmail.com>
Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk,
        torvalds@linux-foundation.org, ebiederm@xmission.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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Date: Fri, 19 Oct 2018 23:36:19 +0100
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Alan Jenkins <alan.christopher.jenkins@gmail.com> wrote:

> # open_tree_clone 3</mnt 3 sh
> # cd /proc/self/fd/3
> # mount --move . /mnt
> [   41.747831] mnt_flags=1020 umount=0
> # cd /
> # umount /mnt
> umount: /mnt: target is busy
> 
> ^ a newly introduced bug? I do not remember having this problem before.

The reason EBUSY is returned is because propagate_mount_busy() is called by
do_umount() with refcnt == 2, but mnt_count == 3:

          umount-3577  M=f8898a34 u=3 0x555 sp=__x64_sys_umount+0x12/0x15

the trace line being added here:

		if (!propagate_mount_busy(mnt, 2)) {
			if (!list_empty(&mnt->mnt_list))
				umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC);
			retval = 0;
		} else {
			trace_mnt_count(mnt, mnt->mnt_id,
					atomic_read(&mnt->mnt_count),
					0x555, __builtin_return_address(0));
		}

The busy evaluation is a result of this check:

	if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt))

in propagate_mount_busy().


The problem apparently being that mnt_count counts both refs from mountings
and refs from other sources, such as file descriptors or pathwalk.

David