Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Feb 09, 2009 at 11:03:48AM +0000, Al Viro wrote:
> 	BTW, a trivial note - kfree(root) in your ->kill_sb() is done
> earlier than it's nice to do.  Shouldn't affect the problem, though.

	Other probably irrelevant notes:

                memcpy(start, cgrp->dentry->d_name.name, len);
                cgrp = cgrp->parent;
                if (!cgrp)
                        break;
                dentry = rcu_dereference(cgrp->dentry);

in cgroup_path().  Why don't we need rcu_dereference on both?
Moreover, shouldn't that be
                memcpy(start, dentry->d_name.name, len);
anyway, seeing that we'd just looked at dentry->d_name.len?

In cgroup_rmdir():
        spin_lock(&cgrp->dentry->d_lock);
        d = dget(cgrp->dentry);
        spin_unlock(&d->d_lock);

        cgroup_d_remove_dir(d);
        dput(d);
Er?  Comments, please...  Unless something very unusual is going on,
either that d_lock is pointless or dget() is rather unsafe.

cgroups_clone()
        /* Now do the VFS work to create a cgroup */
        inode = parent->dentry->d_inode;

        /* Hold the parent directory mutex across this operation to
         * stop anyone else deleting the new cgroup */
        mutex_lock(&inode->i_mutex);
Can the parent be in process of getting deleted by somebody else?  If yes,
we are in trouble here.

BTW, that thing in cgroup_path()...  What guarantees that cgroup_rename()
won't hit between getting len and doing memcpy()?

That said, cgroup seems to be completely agnostic wrt anything happening
on vfsmount level, so I really don't see how it gets to that WARN_ON().
Hell knows; I really want to see the sequence of events - it might be
something like fscking up ->s_active handling with interesting results
(cgroup code is certainly hitting it in not quite usual ways), it may be
genuine VFS-only race.  Need more data...