Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12] | 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
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