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

["Danilo Krummrich" <dakr@kernel.org>]

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?