On Sat, Jun 30, 2012 at 12:04:36AM -0700, Tejun Heo wrote:

Hello, Al.

On Fri, Jun 29, 2012 at 11:47 PM, Al Viro [off-list ref] wrote:

quoted

On Sat, Jun 30, 2012 at 02:13:02PM +0800, Li Zefan wrote:

quoted

So it's bad to have dentry refcnts dangling after umount.

No shit. ?Yes, it is bad. ?What on the Earth is cgroup code doing with
those? ?And what could it possibly want to do with dentry reference
after the filesystem has been shut down, assuming it could hold one
in the first place?

cgroup interface code was copied from sysfs back when it was
piggybacking internal data structures to dentries, so, unfortunately,
sysfs is still using dentries to manage internal data structures and
propagates internal refs to dentry refs. There seem to be several
places where dentry ref is held w/o active super ref triggering BUG on
umount. Longer term, it should be updated to share sysfs code, I
guess.

Now that I've looked at that code again...  What's the story with
                simple_unlink(d->d_inode, d);
in cgroup_rm_file()?  Wrong parent inode, at the very least...

While we are at it, what the hell is going on in
static void cgroup_clear_directory(struct dentry *dir)
{
        struct cgroup *cgrp = __d_cgrp(dir);

        while (!list_empty(&cgrp->files))
                cgroup_rm_file(cgrp, NULL);
}
Are you fighting some kind of race against somebody adding stuff
there?  Unless I'm seriously misreading cgroup_rm_file(), it'll
do all the work on the first call...