Re: [PATCH] bcachefs: unlock parent dir if entry is not found in subvolume deletion

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

On Mon, Jan 29, 2024 at 02:23:38AM +0000, Al Viro wrote:
> On Sun, Jan 28, 2024 at 09:21:48PM -0500, Kent Overstreet wrote:
> 
> > How do we feel about the following?
> 
> No, with side of Fuck, NO.

To elaborate a bit: that essentially introduces a new type (mount/dentry
pair, possibly locked) *and* converts existing instances of struct path
(file->f_path, mount->mnt_mountpoint, etc.) to that.  Suddenly we get
an extra constraint that needs to be verified - "no long-term struct
path instance with ->locked being true".  What's more, currently
one can do
	v = *p;
	path_get(&v);
	...
	path_put(&v);
with eventual
	path_put(p);
possibly from another thread.  Every place like that needs a proof that
we can't possibly get there with p->locked being true.

There's a lot of other reasons why that's a really bad idea.  Seriously,
let's not go there.