linux-kernel - Re: [PATCH] Partially revert "rust: drm: gem: Implement AlwaysRefCounted for all gem objects automatically"

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Message-Id: <DBKN0VSJH3TX.2FYLW17KL0SWW@kernel.org>
Date: Fri, 25 Jul 2025 00:27:26 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Lyude Paul" <lyude@...hat.com>
Cc: <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
 <rust-for-linux@...r.kernel.org>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, "David Airlie" <airlied@...il.com>, "Simona
 Vetter" <simona@...ll.ch>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Asahi
 Lina" <lina+kernel@...hilina.net>, "Alyssa Rosenzweig"
 <alyssa@...enzweig.io>, "open list" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Partially revert "rust: drm: gem: Implement
 AlwaysRefCounted for all gem objects automatically"

On Thu Jul 24, 2025 at 11:06 PM CEST, Lyude Paul wrote:
> On Thu, 2025-07-24 at 22:03 +0200, Danilo Krummrich wrote:
>> On Thu Jul 24, 2025 at 9:15 PM CEST, Lyude Paul wrote:
>> > -// SAFETY: All gem objects are refcounted.
>> > -unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
>> > -    fn inc_ref(&self) {
>> > -        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>> > -        unsafe { bindings::drm_gem_object_get(self.as_raw()) };
>> > -    }
>> > -
>> > -    unsafe fn dec_ref(obj: NonNull<Self>) {
>> > -        // SAFETY: We either hold the only refcount on `obj`, or one of many - meaning that no one
>> > -        // else could possibly hold a mutable reference to `obj` and thus this immutable reference
>> > -        // is safe.
>> > -        let obj = unsafe { obj.as_ref() }.as_raw();
>> > -
>> > -        // SAFETY:
>> > -        // - The safety requirements guarantee that the refcount is non-zero.
>> > -        // - We hold no references to `obj` now, making it safe for us to potentially deallocate it.
>> > -        unsafe { bindings::drm_gem_object_put(obj) };
>> > -    }
>> > -}
>> 
>> IIUC, you'll add rust/kernel/drm/gem/shmem.rs with a new type shmem::Object that
>> implements IntoGEMObject, right?
>> 
>> If this is correct, I think that should work.
>
> Do you mean you think the blanket implementation that we had would work, or
> that getting rid of it would work?

The former.

> Since the blanket implementation we have
> definitely doesn't compile on my machine once we add more then one
> IntoGEMObject impl. (before adding it, it works just fine)

Do you have a branch somewhere, where it doesn't compile?