Re: [PATCH 06/10] VFS: support concurrent renames.

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

On Fri, Aug 26, 2022 at 12:10:43PM +1000, NeilBrown wrote:
> Allow object can now be renamed from or to a directory in which a create
> or unlink is concurrently happening.
> 
> Two or more renames with the one directory can also be concurrent.
> s_vfs_rename_mutex still serialises lookups for cross-directory renames,
> but the renames themselves can proceed concurrently.

Wha...?  <checks>
Not true, fortunately - you *do* hold ->s_vfs_rename_mutex over the
rename itself.  If not for that, it would be utterly broken.
And I don't care for NFS server rejecting that - we are *NOT* taking
loop prevention logics into every filesystem.  It's highly non-local
and trying to handle it with your per-dentry flags is going to be
painful as hell, if at all possible.

> +	if (d1 < d2) {
> +		ok1 = d_lock_update_nested(d1, p1, last1, I_MUTEX_PARENT);
> +		ok2 = d_lock_update_nested(d2, p2, last2, I_MUTEX_PARENT2);
> +	} else {
> +		ok2 = d_lock_update_nested(d2, p2, last2, I_MUTEX_PARENT);
> +		ok1 = d_lock_update_nested(d1, p1, last1, I_MUTEX_PARENT2);
> +	}

Explain, please.  What's that ordering about?