From: acahalan@cs.uml.edu
To: linux-kernel@vger.rutgers.edu
Cc: viro@math.psu.edu
Message-ID: <2swOlax70cevgQcM.hT5nAtuM3skeaxU47CvCnNO7oWxqJ5f>
Date: the sixth of June, in the year of our lord two thousand
X-Mailer: telnet
Alexander Viro writes:
> On Wed, 7 Jun 2000, James Sutherland wrote:
>>> A) suppose we have a bunch of filesystems union-mounted on
>>> /foo/bar. We do chdir("/foo/bar"), what should become busy? Variants:
>>> mountpoint, first element, last element, all of them.
>>
>> The key question, presumably, is which FSs can we unmount from the union
>> without /foo/bar breaking? If all the FSs can be unmounted safely, none
>> should be busy.
This is right. Being "busy" is a traditional UNIX flaw.
> There is only one tricky situation: when we umount everything but the last
> element (and thus union-mount degenerates into the plain mount) && there
> is something standing on the mountpoint && aside of that mounted fs is not
> busy. Then current code will refuse to umount the thing, but the new one
> (for consistency) probably ought to (see the (E)). Yes, we can umount
> anything without breakage.
So the behavior of umount changes. No problem, since it really ought
to produce a result just like directory removal.
One could argue that that behavior is broken too. The VFS could supply
an empty path leading up to the now-missing root, so getcwd would work.
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