Message-Id: <D2C11077-2001-4E45-8039-C999E6CA5606@collabora.com>
Date: Fri, 28 Nov 2025 14:46:46 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Oliver Mangold <oliver.mangold@...me>
Cc: 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>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Benno Lossin <lossin@...nel.org>,
 Danilo Krummrich <dakr@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Dave Ertman <david.m.ertman@...el.com>,
 Ira Weiny <ira.weiny@...el.com>,
 Leon Romanovsky <leon@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Jan Kara <jack@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Viresh Kumar <vireshk@...nel.org>,
 Nishanth Menon <nm@...com>,
 Stephen Boyd <sboyd@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>,
 Paul Moore <paul@...l-moore.com>,
 Serge Hallyn <sergeh@...nel.org>,
 Asahi Lina <lina+kernel@...hilina.net>,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org,
 dri-devel@...ts.freedesktop.org,
 linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org,
 linux-pm@...r.kernel.org,
 linux-pci@...r.kernel.org,
 linux-security-module@...r.kernel.org
Subject: Re: [PATCH v13 2/4] rust: `AlwaysRefCounted` is renamed to
 `RefCounted`.



> On 17 Nov 2025, at 07:07, Oliver Mangold <oliver.mangold@...me> wrote:
> 
> `AlwaysRefCounted` will become a marker trait to indicate that it is
> allowed to obtain an `ARef<T>` from a `&T`, which cannot be allowed for
> types which are also Ownable.
> 
> Signed-off-by: Oliver Mangold <oliver.mangold@...me>
> Co-developed-by: Andreas Hindborg <a.hindborg@...nel.org>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> rust/kernel/auxiliary.rs        |  7 +++++-
> rust/kernel/block/mq/request.rs | 15 +++++++------
> rust/kernel/cred.rs             | 13 ++++++++++--
> rust/kernel/device.rs           | 13 ++++++++----
> rust/kernel/device/property.rs  |  7 +++++-
> rust/kernel/drm/device.rs       | 10 ++++++---
> rust/kernel/drm/gem/mod.rs      | 10 ++++++---
> rust/kernel/fs/file.rs          | 16 ++++++++++----
> rust/kernel/mm.rs               | 15 +++++++++----
> rust/kernel/mm/mmput_async.rs   |  9 ++++++--
> rust/kernel/opp.rs              | 10 ++++++---
> rust/kernel/owned.rs            |  2 +-
> rust/kernel/pci.rs              | 10 ++++++---
> rust/kernel/pid_namespace.rs    | 12 +++++++++--
> rust/kernel/platform.rs         |  7 +++++-
> rust/kernel/sync/aref.rs        | 47 ++++++++++++++++++++++++++---------------
> rust/kernel/task.rs             | 10 ++++++---
> rust/kernel/types.rs            |  2 +-
> 18 files changed, 154 insertions(+), 61 deletions(-)
> 
> diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
> index 7a3b0b9c418e..7f5b16053c11 100644
> --- a/rust/kernel/auxiliary.rs
> +++ b/rust/kernel/auxiliary.rs
> @@ -10,6 +10,7 @@
>     driver,
>     error::{from_result, to_result, Result},
>     prelude::*,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>     types::Opaque,
>     ThisModule,
> };
> @@ -239,7 +240,7 @@ extern "C" fn release(dev: *mut bindings::device) {
> kernel::impl_device_context_into_aref!(Device);
> 
> // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>         unsafe { bindings::get_device(self.as_ref().as_raw()) };
> @@ -258,6 +259,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
> impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>     fn as_ref(&self) -> &device::Device<Ctx> {
>         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
> index c5f1f6b1ccfb..b6165f96b4ce 100644
> --- a/rust/kernel/block/mq/request.rs
> +++ b/rust/kernel/block/mq/request.rs
> @@ -8,7 +8,7 @@
>     bindings,
>     block::mq::Operations,
>     error::Result,
> -    sync::{atomic::Relaxed, Refcount},
> +    sync::{aref::RefCounted, atomic::Relaxed, Refcount},
>     types::{ARef, AlwaysRefCounted, Opaque},
> };
> use core::{marker::PhantomData, ptr::NonNull};
> @@ -225,11 +225,10 @@ unsafe impl<T: Operations> Send for Request<T> {}
> // mutate `self` are internally synchronized`
> unsafe impl<T: Operations> Sync for Request<T> {}
> 
> -// SAFETY: All instances of `Request<T>` are reference counted. This
> -// implementation of `AlwaysRefCounted` ensure that increments to the ref count
> -// keeps the object alive in memory at least until a matching reference count
> -// decrement is executed.
> -unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {
> +// SAFETY: All instances of `Request<T>` are reference counted. This implementation of `RefCounted`
> +// ensure that increments to the ref count keeps the object alive in memory at least until a
> +// matching reference count decrement is executed.
> +unsafe impl<T: Operations> RefCounted for Request<T> {
>     fn inc_ref(&self) {
>         self.wrapper_ref().refcount().inc();
>     }
> @@ -251,3 +250,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
>         }
>     }
> }
> +
> +// SAFETY: We currently do not implement `Ownable`, thus it is okay to obtain an `ARef<Request>`
> +// from a `&Request` (but this will change in the future).
> +unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {}
> diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
> index ffa156b9df37..20ef0144094b 100644
> --- a/rust/kernel/cred.rs
> +++ b/rust/kernel/cred.rs
> @@ -8,7 +8,12 @@
> //!
> //! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
> 
> -use crate::{bindings, sync::aref::AlwaysRefCounted, task::Kuid, types::Opaque};
> +use crate::{
> +    bindings,
> +    sync::aref::RefCounted,
> +    task::Kuid,
> +    types::{AlwaysRefCounted, Opaque},
> +};
> 
> /// Wraps the kernel's `struct cred`.
> ///
> @@ -76,7 +81,7 @@ pub fn euid(&self) -> Kuid {
> }
> 
> // SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
> -unsafe impl AlwaysRefCounted for Credential {
> +unsafe impl RefCounted for Credential {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -90,3 +95,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
>         unsafe { bindings::put_cred(obj.cast().as_ptr()) };
>     }
> }
> +
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Credential>` from a
> +// `&Credential`.
> +unsafe impl AlwaysRefCounted for Credential {}
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index a849b7dde2fd..a69ee32997c1 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -5,9 +5,10 @@
> //! C header: [`include/linux/device.h`](srctree/include/linux/device.h)
> 
> use crate::{
> -    bindings, fmt,
> -    sync::aref::ARef,
> -    types::{ForeignOwnable, Opaque},
> +    bindings,
> +    fmt,
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, ForeignOwnable, Opaque},
> };
> use core::{marker::PhantomData, ptr};
> 
> @@ -407,7 +408,7 @@ pub fn fwnode(&self) -> Option<&property::FwNode> {
> kernel::impl_device_context_into_aref!(Device);
> 
> // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>         unsafe { bindings::get_device(self.as_raw()) };
> @@ -419,6 +420,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
> // SAFETY: As by the type invariant `Device` can be sent to any thread.
> unsafe impl Send for Device {}
> 
> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
> index 3a332a8c53a9..a8bb824ad0ec 100644
> --- a/rust/kernel/device/property.rs
> +++ b/rust/kernel/device/property.rs
> @@ -14,6 +14,7 @@
>     fmt,
>     prelude::*,
>     str::{CStr, CString},
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>     types::{ARef, Opaque},
> };
> 
> @@ -359,7 +360,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> }
> 
> // SAFETY: Instances of `FwNode` are always reference-counted.
> -unsafe impl crate::types::AlwaysRefCounted for FwNode {
> +unsafe impl RefCounted for FwNode {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the
>         // refcount is non-zero.
> @@ -373,6 +374,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<FwNode>` from a
> +// `&FwNode`.
> +unsafe impl AlwaysRefCounted for FwNode {}
> +
> enum Node<'a> {
>     Borrowed(&'a FwNode),
>     Owned(ARef<FwNode>),
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 3ce8f62a0056..38ce7f389ed0 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -11,8 +11,8 @@
>     error::from_err_ptr,
>     error::Result,
>     prelude::*,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
> +    types::{ARef, Opaque},
> };
> use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
> 
> @@ -198,7 +198,7 @@ fn deref(&self) -> &Self::Target {
> 
> // SAFETY: DRM device objects are always reference counted and the get/put functions
> // satisfy the requirements.
> -unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
> +unsafe impl<T: drm::Driver> RefCounted for Device<T> {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>         unsafe { bindings::drm_dev_get(self.as_raw()) };
> @@ -213,6 +213,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {}
> +
> impl<T: drm::Driver> AsRef<device::Device> for Device<T> {
>     fn as_ref(&self) -> &device::Device {
>         // SAFETY: `bindings::drm_device::dev` is valid as long as the DRM device itself is valid,
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index 30c853988b94..4afd36e49205 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -10,8 +10,8 @@
>     drm::driver::{AllocImpl, AllocOps},
>     error::{to_result, Result},
>     prelude::*,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, Opaque},
> };
> use core::{ops::Deref, ptr::NonNull};
> 
> @@ -56,7 +56,7 @@ pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
> }
> 
> // SAFETY: All gem objects are refcounted.
> -unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
> +unsafe impl<T: IntoGEMObject> RefCounted 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()) };
> @@ -75,6 +75,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<T>` from a
> +// `&T`.
> +unsafe impl<T: IntoGEMObject> crate::types::AlwaysRefCounted for T {}
> +
> extern "C" fn open_callback<T: DriverObject>(
>     raw_obj: *mut bindings::drm_gem_object,
>     raw_file: *mut bindings::drm_file,
> diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
> index cd6987850332..86309ca5dad0 100644
> --- a/rust/kernel/fs/file.rs
> +++ b/rust/kernel/fs/file.rs
> @@ -12,8 +12,8 @@
>     cred::Credential,
>     error::{code::*, to_result, Error, Result},
>     fmt,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
> };
> use core::ptr;
> 
> @@ -192,7 +192,7 @@ unsafe impl Sync for File {}
> 
> // SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
> // makes `ARef<File>` own a normal refcount.
> -unsafe impl AlwaysRefCounted for File {
> +unsafe impl RefCounted for File {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -207,6 +207,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<File>` from a
> +// `&File`.
> +unsafe impl AlwaysRefCounted for File {}
> +
> /// Wraps the kernel's `struct file`. Not thread safe.
> ///
> /// This type represents a file that is not known to be safe to transfer across thread boundaries.
> @@ -228,7 +232,7 @@ pub struct LocalFile {
> 
> // SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
> // makes `ARef<LocalFile>` own a normal refcount.
> -unsafe impl AlwaysRefCounted for LocalFile {
> +unsafe impl RefCounted for LocalFile {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -244,6 +248,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<LocalFile>` from a
> +// `&LocalFile`.
> +unsafe impl AlwaysRefCounted for LocalFile {}
> +
> impl LocalFile {
>     /// Constructs a new `struct file` wrapper from a file descriptor.
>     ///
> diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> index 4764d7b68f2a..dd9e3969e720 100644
> --- a/rust/kernel/mm.rs
> +++ b/rust/kernel/mm.rs
> @@ -13,8 +13,8 @@
> 
> use crate::{
>     bindings,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
> };
> use core::{ops::Deref, ptr::NonNull};
> 
> @@ -55,7 +55,7 @@ unsafe impl Send for Mm {}
> unsafe impl Sync for Mm {}
> 
> // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for Mm {
> +unsafe impl RefCounted for Mm {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The pointer is valid since self is a reference.
> @@ -69,6 +69,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Mm>` from a `&Mm`.
> +unsafe impl AlwaysRefCounted for Mm {}
> +
> /// A wrapper for the kernel's `struct mm_struct`.
> ///
> /// This type is like [`Mm`], but with non-zero `mm_users`. It can only be used when `mm_users` can
> @@ -91,7 +94,7 @@ unsafe impl Send for MmWithUser {}
> unsafe impl Sync for MmWithUser {}
> 
> // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for MmWithUser {
> +unsafe impl RefCounted for MmWithUser {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The pointer is valid since self is a reference.
> @@ -105,6 +108,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUser>` from a
> +// `&MmWithUser`.
> +unsafe impl AlwaysRefCounted for MmWithUser {}
> +
> // Make all `Mm` methods available on `MmWithUser`.
> impl Deref for MmWithUser {
>     type Target = Mm;
> diff --git a/rust/kernel/mm/mmput_async.rs b/rust/kernel/mm/mmput_async.rs
> index b8d2f051225c..aba4ce675c86 100644
> --- a/rust/kernel/mm/mmput_async.rs
> +++ b/rust/kernel/mm/mmput_async.rs
> @@ -10,7 +10,8 @@
> use crate::{
>     bindings,
>     mm::MmWithUser,
> -    sync::aref::{ARef, AlwaysRefCounted},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted},
> };
> use core::{ops::Deref, ptr::NonNull};
> 
> @@ -34,7 +35,7 @@ unsafe impl Send for MmWithUserAsync {}
> unsafe impl Sync for MmWithUserAsync {}
> 
> // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for MmWithUserAsync {
> +unsafe impl RefCounted for MmWithUserAsync {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The pointer is valid since self is a reference.
> @@ -48,6 +49,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUserAsync>`
> +// from a `&MmWithUserAsync`.
> +unsafe impl AlwaysRefCounted for MmWithUserAsync {}
> +
> // Make all `MmWithUser` methods available on `MmWithUserAsync`.
> impl Deref for MmWithUserAsync {
>     type Target = MmWithUser;
> diff --git a/rust/kernel/opp.rs b/rust/kernel/opp.rs
> index 2c763fa9276d..77d1cc89c412 100644
> --- a/rust/kernel/opp.rs
> +++ b/rust/kernel/opp.rs
> @@ -16,8 +16,8 @@
>     ffi::c_ulong,
>     prelude::*,
>     str::CString,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, Opaque},
> };
> 
> #[cfg(CONFIG_CPU_FREQ)]
> @@ -1037,7 +1037,7 @@ unsafe impl Send for OPP {}
> unsafe impl Sync for OPP {}
> 
> /// SAFETY: The type invariants guarantee that [`OPP`] is always refcounted.
> -unsafe impl AlwaysRefCounted for OPP {
> +unsafe impl RefCounted for OPP {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
>         unsafe { bindings::dev_pm_opp_get(self.0.get()) };
> @@ -1049,6 +1049,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<OPP>` from an
> +// `&OPP`.
> +unsafe impl AlwaysRefCounted for OPP {}
> +
> impl OPP {
>     /// Creates an owned reference to a [`OPP`] from a valid pointer.
>     ///
> diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
> index a2cdd2cb8a10..a26747cbc13b 100644
> --- a/rust/kernel/owned.rs
> +++ b/rust/kernel/owned.rs
> @@ -21,7 +21,7 @@
> ///
> /// Note: The underlying object is not required to provide internal reference counting, because it
> /// represents a unique, owned reference. If reference counting (on the Rust side) is required,
> -/// [`AlwaysRefCounted`](crate::types::AlwaysRefCounted) should be implemented.
> +/// [`RefCounted`](crate::types::RefCounted) should be implemented.
> ///
> /// # Safety
> ///
> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> index 7fcc5f6022c1..9ac70823fb4d 100644
> --- a/rust/kernel/pci.rs
> +++ b/rust/kernel/pci.rs
> @@ -13,8 +13,8 @@
>     io::{Io, IoRaw},
>     irq::{self, IrqRequest},
>     str::CStr,
> -    sync::aref::ARef,
> -    types::Opaque,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
> +    types::{ARef, Opaque},
>     ThisModule,
> };
> use core::{
> @@ -601,7 +601,7 @@ pub fn set_master(&self) {
> impl crate::dma::Device for Device<device::Core> {}
> 
> // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>         unsafe { bindings::pci_dev_get(self.as_raw()) };
> @@ -613,6 +613,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
> impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>     fn as_ref(&self) -> &device::Device<Ctx> {
>         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/pid_namespace.rs b/rust/kernel/pid_namespace.rs
> index 979a9718f153..4f6a94540e33 100644
> --- a/rust/kernel/pid_namespace.rs
> +++ b/rust/kernel/pid_namespace.rs
> @@ -7,7 +7,11 @@
> //! C header: [`include/linux/pid_namespace.h`](srctree/include/linux/pid_namespace.h) and
> //! [`include/linux/pid.h`](srctree/include/linux/pid.h)
> 
> -use crate::{bindings, sync::aref::AlwaysRefCounted, types::Opaque};
> +use crate::{
> +    bindings,
> +    sync::aref::RefCounted,
> +    types::{AlwaysRefCounted, Opaque},
> +};
> use core::ptr;
> 
> /// Wraps the kernel's `struct pid_namespace`. Thread safe.
> @@ -41,7 +45,7 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::pid_namespace) -> &'a Self {
> }
> 
> // SAFETY: Instances of `PidNamespace` are always reference-counted.
> -unsafe impl AlwaysRefCounted for PidNamespace {
> +unsafe impl RefCounted for PidNamespace {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -55,6 +59,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<PidNamespace>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<PidNamespace>` from
> +// a `&PidNamespace`.
> +unsafe impl AlwaysRefCounted for PidNamespace {}
> +
> // SAFETY:
> // - `PidNamespace::dec_ref` can be called from any thread.
> // - It is okay to send ownership of `PidNamespace` across thread boundaries.
> diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
> index 7205fe3416d3..bf2aa496b377 100644
> --- a/rust/kernel/platform.rs
> +++ b/rust/kernel/platform.rs
> @@ -13,6 +13,7 @@
>     irq::{self, IrqRequest},
>     of,
>     prelude::*,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>     types::Opaque,
>     ThisModule,
> };
> @@ -468,7 +469,7 @@ pub fn optional_irq_by_name(&self, name: &CStr) -> Result<IrqRequest<'_>> {
> impl crate::dma::Device for Device<device::Core> {}
> 
> // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>         unsafe { bindings::get_device(self.as_ref().as_raw()) };
> @@ -480,6 +481,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
> impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>     fn as_ref(&self) -> &device::Device<Ctx> {
>         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index e175aefe8615..4226119d5ac9 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs
> @@ -19,11 +19,9 @@
> 
> use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
> 
> -/// Types that are _always_ reference counted.
> +/// Types that are internally reference counted.
> ///
> /// It allows such types to define their own custom ref increment and decrement functions.
> -/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
> -/// [`ARef<T>`].
> ///
> /// This is usually implemented by wrappers to existing structures on the C side of the code. For
> /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
> @@ -40,9 +38,8 @@
> /// at least until matching decrements are performed.
> ///
> /// Implementers must also ensure that all instances are reference-counted. (Otherwise they
> -/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
> -/// alive.)
> -pub unsafe trait AlwaysRefCounted {
> +/// won't be able to honour the requirement that [`RefCounted::inc_ref`] keep the object alive.)
> +pub unsafe trait RefCounted {
>     /// Increments the reference count on the object.
>     fn inc_ref(&self);
> 
> @@ -55,11 +52,27 @@ pub unsafe trait AlwaysRefCounted {
>     /// Callers must ensure that there was a previous matching increment to the reference count,
>     /// and that the object is no longer used after its reference count is decremented (as it may
>     /// result in the object being freed), unless the caller owns another increment on the refcount
> -    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
> -    /// [`AlwaysRefCounted::dec_ref`] once).
> +    /// (e.g., it calls [`RefCounted::inc_ref`] twice, then calls [`RefCounted::dec_ref`] once).
>     unsafe fn dec_ref(obj: NonNull<Self>);
> }
> 
> +/// Always reference-counted type.
> +///
> +/// It allows to derive a counted reference [`ARef<T>`] from a `&T`.
> +///
> +/// This provides some convenience, but it allows "escaping" borrow checks on `&T`. As it
> +/// complicates attempts to ensure that a reference to T is unique, it is optional to provide for
> +/// [`RefCounted`] types. See *Safety* below.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that no safety invariants are violated by upgrading an `&T` to an
> +/// [`ARef<T>`]. In particular that implies [`AlwaysRefCounted`] and [`crate::types::Ownable`]
> +/// cannot be implemented for the same type, as this would allow to violate the uniqueness guarantee
> +/// of [`crate::types::Owned<T>`] by derefencing it into an `&T` and obtaining an [`ARef`] from
> +/// that.
> +pub unsafe trait AlwaysRefCounted: RefCounted {}
> +
> /// An owned reference to an always-reference-counted object.
> ///
> /// The object's reference count is automatically decremented when an instance of [`ARef`] is
> @@ -70,7 +83,7 @@ pub unsafe trait AlwaysRefCounted {
> ///
> /// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
> /// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
> -pub struct ARef<T: AlwaysRefCounted> {
> +pub struct ARef<T: RefCounted> {
>     ptr: NonNull<T>,
>     _p: PhantomData<T>,
> }
> @@ -79,16 +92,16 @@ pub struct ARef<T: AlwaysRefCounted> {
> // it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
> // `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
> // mutable reference, for example, when the reference count reaches zero and `T` is dropped.
> -unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
> +unsafe impl<T: RefCounted + Sync + Send> Send for ARef<T> {}
> 
> // SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
> // because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
> // it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
> // `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
> // example, when the reference count reaches zero and `T` is dropped.
> -unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
> +unsafe impl<T: RefCounted + Sync + Send> Sync for ARef<T> {}
> 
> -impl<T: AlwaysRefCounted> ARef<T> {
> +impl<T: RefCounted> ARef<T> {
>     /// Creates a new instance of [`ARef`].
>     ///
>     /// It takes over an increment of the reference count on the underlying object.
> @@ -117,12 +130,12 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
>     ///
>     /// ```
>     /// use core::ptr::NonNull;
> -    /// use kernel::sync::aref::{ARef, AlwaysRefCounted};
> +    /// use kernel::sync::aref::{ARef, RefCounted};
>     ///
>     /// struct Empty {}
>     ///
>     /// # // SAFETY: TODO.
> -    /// unsafe impl AlwaysRefCounted for Empty {
> +    /// unsafe impl RefCounted for Empty {
>     ///     fn inc_ref(&self) {}
>     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
>     /// }
> @@ -140,7 +153,7 @@ pub fn into_raw(me: Self) -> NonNull<T> {
>     }
> }
> 
> -impl<T: AlwaysRefCounted> Clone for ARef<T> {
> +impl<T: RefCounted> Clone for ARef<T> {
>     fn clone(&self) -> Self {
>         self.inc_ref();
>         // SAFETY: We just incremented the refcount above.
> @@ -148,7 +161,7 @@ fn clone(&self) -> Self {
>     }
> }
> 
> -impl<T: AlwaysRefCounted> Deref for ARef<T> {
> +impl<T: RefCounted> Deref for ARef<T> {
>     type Target = T;
> 
>     fn deref(&self) -> &Self::Target {
> @@ -165,7 +178,7 @@ fn from(b: &T) -> Self {
>     }
> }
> 
> -impl<T: AlwaysRefCounted> Drop for ARef<T> {
> +impl<T: RefCounted> Drop for ARef<T> {
>     fn drop(&mut self) {
>         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
>         // decrement.
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 49fad6de0674..0a6e38d98456 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -9,8 +9,8 @@
>     ffi::{c_int, c_long, c_uint},
>     mm::MmWithUser,
>     pid_namespace::PidNamespace,
> -    sync::aref::ARef,
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::{AlwaysRefCounted, RefCounted},
> +    types::{ARef, NotThreadSafe, Opaque},
> };
> use core::{
>     cmp::{Eq, PartialEq},
> @@ -348,7 +348,7 @@ pub fn active_pid_ns(&self) -> Option<&PidNamespace> {
> }
> 
> // SAFETY: The type invariants guarantee that `Task` is always refcounted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Task {
> +unsafe impl RefCounted for Task {
>     #[inline]
>     fn inc_ref(&self) {
>         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -362,6 +362,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>     }
> }
> 
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Task>` from a
> +// `&Task`.
> +unsafe impl AlwaysRefCounted for Task {}
> +
> impl Kuid {
>     /// Get the current euid.
>     #[inline]
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 7bc07c38cd6c..8ef01393352b 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -13,7 +13,7 @@
> 
> pub use crate::owned::{Ownable, Owned};
> 
> -pub use crate::sync::aref::{ARef, AlwaysRefCounted};
> +pub use crate::sync::aref::{ARef, AlwaysRefCounted, RefCounted};
> 
> /// Used to transfer ownership to and from foreign (non-Rust) languages.
> ///
> 
> -- 
> 2.51.2
> 
> 
> 

Can you use imperative voice in the title? i.e.:

rust: rename `AlwaysRefCounted` to `RefCounted

…or something similar?

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>

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