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Date: Fri, 2 May 2025 09:05:25 +0200
From: Danilo Krummrich <dakr@kernel.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Matthew Maurer <mmaurer@google.com>, Miguel Ojeda <ojeda@kernel.org>,
	Alex Gaynor <alex.gaynor@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <benno.lossin@proton.me>,
	Andreas Hindborg <a.hindborg@kernel.org>,
	Alice Ryhl <aliceryhl@google.com>, Trevor Gross <tmgross@umich.edu>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Timur Tabi <ttabi@nvidia.com>, linux-kernel@vger.kernel.org,
	rust-for-linux@vger.kernel.org
Subject: Re: [PATCH v3 1/4] rust: debugfs: Bind DebugFS directory creation
Message-ID: <aBRutTMBtq-uCnii@pollux>
References: <20250501-debugfs-rust-v3-0-850869fab672@google.com>
 <20250501-debugfs-rust-v3-1-850869fab672@google.com>
 <aBRoNKjB063QhGZo@pollux>
 <2025050230-browsing-backstab-8de9@gregkh>
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In-Reply-To: <2025050230-browsing-backstab-8de9@gregkh>

On Fri, May 02, 2025 at 09:00:07AM +0200, Greg Kroah-Hartman wrote:
> On Fri, May 02, 2025 at 08:37:40AM +0200, Danilo Krummrich wrote:
> > On Thu, May 01, 2025 at 10:47:41PM +0000, Matthew Maurer wrote:
> > > +/// Handle to a DebugFS directory that will stay alive after leaving scope.
> > > +#[repr(transparent)]
> > > +pub struct SubDir(ManuallyDrop<Dir>);
> > 
> > I think it's not very intuitive if the default is that a SubDir still exists
> > after it has been dropped. I think your first approach being explicit about this
> > with keep() consuming the SubDir was much better; please keep this approach.
> 
> Wait, let's step back.  Why do we care about the difference between a
> "subdir" and a "dir"?  They both are the same thing, and how do you
> describe a subdir of a subdir?  :)

We care about the difference, because Dir originally had keep() which drops the
Dir instance without actually removing it. For subdirs this is fine, since
they'll be cleaned up when the parent is removed.

However, we don't want users to be able to call keep() on the directory that has
been created first, since if that's done we loose our root anchor to ever free
the tree, which almost always would be a bug.

Please also see [1] and subsequent replies.

A subdir of a subdir would still be a SubDir.

[1] https://lore.kernel.org/lkml/aBNKEewhCP8jRIZL@pollux/