Calling umount2() unmounts volume but df entry still visible

[PeterSteele]

This is an odd one. I have a C program that calls umount to unmount a volume. A simplified case looks like this:

int main()
{
    int rc = umount2("/v0", MNT_FORCE);
    if (rc != 0)
    {
        printf("Unable to unmount volume /v0, err='%s'", strerror(errno));
    }
}

When I run this code, the "/v0" volume is unmount, but it still shows up in df:

# mount /dev/sda7 /v0
# touch /v0/xxx
# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1              8254240   3554504   4280444  46% /
/dev/sda7             20954552      4260  20950292   1% /v0
/dev/sdb7             20954552      4256  20950296   1% /v1
/dev/sda2              4127108    180232   3737228   5% /var
#./a.out
#df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1              8254240   3554504   4280444  46% /
/dev/sda7             20954552      4260  20950292   1% /v0
/dev/sdb7             20954552      4256  20950296   1% /v1
/dev/sda2              4127108    180232   3737228   5% /var

As you can see, df still shows that /v0 is mount. However, it's not *really* mounted. If I try to access it, the commands fail:

# ls /v0/xxx
ls: cannot access /v0/xxx: No such file or directory

If I run a umount, it fails as well, but this removes the /v0 entry as well:

# umount -f /v0
umount2: Invalid argument
umount: /v0: not mounted
umount2: Invalid argument

# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1              8254240   3554504   4280444  46% /
/dev/sdb7             20954552      4256  20950296   1% /v1
/dev/sda2              4127108    180232   3737228   5% /var

So, the question is, why does calling umount2() in my code not completely unmount the volume?

[PeterSteele]

I've discovered the opposite problem with the mount() function call. I have code that makes the call:

 mount(drive, volume, "xfs", MS_NOATIME, NULL);

After making this call, a df command does not show an entry for the newly mounted volume. However, accessing the volume through other commands confirms that in fact the volume *is* mounted, and issuing a umount command from the command line unmounts the volume without any errors.

So, why is my C code behaving differently than running the mount/umount commands from the command line?