Re: [PATCH v9 4/5] rust: Add dma_fence abstractions

["Gary Guo" <gary@garyguo.net>]

On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote:
> C's dma_fence's are synchronisation primitives that will be needed by all
> Rust GPU drivers.
>
> The dma_fence framework sets a number of rules, notably:
>   - fences must only be signaled once
>   - all fences must be signaled at some point
>   - fence error codes must only be set before signaling
>   - every pointer to a fence must be backed by a reference
>
> All those rules are being addressed by these abstractions.
>
> To cleanly decouple fence issuers and consumers, two types are provided:
>   - DriverFence: the only fence type that can be signaled and that
>     carries driver-specific data.
>   - Fence: the fence type to be shared with other drivers and / or
>     userspace. The only type callbacks can be registered on.
>     Cannot be signaled.
>
> Hereby, a Fence lives in the same chunk of memory as a DriverFence. Both
> share the refcount of the underlying C dma_fence. Since this
> implementation does not provide a custom dma_fence_backend_ops.release()
> function, the memory is freed by the dma_fence backend once the refcount
> drops to 0.
>
> To create a DriverFence, the user must first allocate a
> DriverFenceAllocation, so that the creation of the DriverFence later on
> can always succeed. Otherwise, deadlocks could occur if fences need to
> be created in a GPU job submission path.
>
> Synchronization is ensured by the dma_fence backend.
>
> All DriverFence's created through this abstraction must be signaled by
> the creator with an error code. In case a DriverFence drops without
> being signaled beforehand, it is signaled with -ECANCELLED as its
> error and a warning is printed. This allows the Rust abstraction to very
> cleanly decouple fence issuer and consumer by relying on the decoupling
> mechanisms in the C backend, which ensures through RCU and the
> 'signaled' fence-flag that dma_fence_backend_ops functions cannot
> access the potentially unloaded driver code anymore.
>
> Signalling fences on drop thus grants many advantages. Not signaling
> fences on drop would risk deadlock and does not grant real advantages:
> By definition only the drivers can ensure that a fence always represents
> the hardware's state correctly.
>
> This implementation models a DmaFenceContext object on which fences are
> to be created, thereby ensuring correct sequence numbering according to
> the timeline.
>
> dma_fence supports a variety of callbacks. The mandatory callbacks
> (get_timeline_name() and get_driver_name()) are implemented in this
> patch. For convenience, they store those name parameters in the fence
> context, saving the driver from implementing these two callbacks.
>
> Support for other callbacks (like for hardware signaling) is prepared
> for through the fact that both DriverFence and Fence live in the same
> allocation, allowing for usage of container_of from the callback to
> access the driver-specific data.
>
> It is expected that other callbacks, added in the future, also mostly
> operate on the generic data in the FenceContext. To make this safe, the
> implementation ensures through a lifetime that a DriverFence cannot
> outlive its FenceContext.
>
> Synchronization for dma_fence_ops callbacks is ensured by only running the
> Rust deconstructor delayed with call_rcu(), which prevents UAF-bugs
> should a DriverFence drop while a Fence callback is currently operating
> on the associated driver data. Since they can also operate on the
> FenceContext's data, its drop implementation also performs the necessary
> delay with rcu_barrier().
>
> An additional issue discovered during the review process of this code is
> that there is (currently) no mechanism in Rust to prevent someone from
> circumventing the DriverFence's FenceContext-reference's lifetime by
> "forgetting" the fence, e.g. with core::mem::forget(). This would enable
> UAF bugs on the FenceContext. Throw a panic if this happens and document
> a path towards a more robust solution.
>
> Add abstractions for dma_fence in Rust.
>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
> ---
>  rust/bindings/bindings_helper.h  |    1 +
>  rust/helpers/dma_fence.c         |   48 ++
>  rust/helpers/helpers.c           |    1 +
>  rust/kernel/dma_buf/dma_fence.rs | 1002 ++++++++++++++++++++++++++++++
>  rust/kernel/dma_buf/mod.rs       |   14 +
>  rust/kernel/lib.rs               |    1 +
>  6 files changed, 1067 insertions(+)
>  create mode 100644 rust/helpers/dma_fence.c
>  create mode 100644 rust/kernel/dma_buf/dma_fence.rs
>  create mode 100644 rust/kernel/dma_buf/mod.rs
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 1124785e210b..54b62d952e01 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -53,6 +53,7 @@
>  #include <linux/debugfs.h>
>  #include <linux/device/faux.h>
>  #include <linux/dma-direction.h>
> +#include <linux/dma-fence.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/dma-resv.h>
>  #include <linux/errname.h>
> diff --git a/rust/helpers/dma_fence.c b/rust/helpers/dma_fence.c
> new file mode 100644
> index 000000000000..0e08411098fa
> --- /dev/null
> +++ b/rust/helpers/dma_fence.c
> @@ -0,0 +1,48 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/dma-fence.h>
> +
> +__rust_helper void rust_helper_dma_fence_get(struct dma_fence *f)
> +{
> +	dma_fence_get(f);
> +}
> +
> +__rust_helper void rust_helper_dma_fence_put(struct dma_fence *f)
> +{
> +	dma_fence_put(f);
> +}
> +
> +__rust_helper bool rust_helper_dma_fence_begin_signalling(void)
> +{
> +	return dma_fence_begin_signalling();
> +}
> +
> +__rust_helper void rust_helper_dma_fence_end_signalling(bool cookie)
> +{
> +	dma_fence_end_signalling(cookie);
> +}
> +
> +__rust_helper bool rust_helper_dma_fence_is_signaled(struct dma_fence *f)
> +{
> +	return dma_fence_is_signaled(f);
> +}
> +
> +__rust_helper bool rust_helper_dma_fence_test_signaled_flag(struct dma_fence *f)
> +{
> +	return dma_fence_test_signaled_flag(f);
> +}
> +
> +__rust_helper void rust_helper_dma_fence_lock_irqsave(struct dma_fence *f, unsigned long *flags)
> +{
> +	dma_fence_lock_irqsave(f, *flags);
> +}
> +
> +__rust_helper void rust_helper_dma_fence_unlock_irqrestore(struct dma_fence *f, unsigned long *flags)
> +{
> +	dma_fence_unlock_irqrestore(f, *flags);
> +}
> +
> +__rust_helper void rust_helper_dma_fence_set_error(struct dma_fence *f, int error)
> +{
> +	dma_fence_set_error(f, error);
> +}
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 998e31052e66..4ab8aa9da7e7 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -58,6 +58,7 @@
>  #include "cred.c"
>  #include "device.c"
>  #include "dma.c"
> +#include "dma_fence.c"
>  #include "dma-resv.c"
>  #include "drm.c"
>  #include "drm_gpuvm.c"
> diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
> new file mode 100644
> index 000000000000..e61b4b2d8b8c
> --- /dev/null
> +++ b/rust/kernel/dma_buf/dma_fence.rs
> @@ -0,0 +1,1002 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2025-2026 Red Hat Inc.
> + * Author: Philipp Stanner <pstanner@redhat.com>
> + */
> +
> +//! DriverFence support.
> +//!
> +//! Reference: <https://docs.kernel.org/driver-api/dma-buf.html#c.dma_fence>
> +//!
> +//! header: [`include/linux/dma-fence.h`](srctree/include/linux/dma-fence.h)
> +
> +use crate::{
> +    alloc::AllocError,
> +    bindings,
> +    container_of,
> +    error::to_result,
> +    prelude::*,
> +    types::ForeignOwnable,
> +    types::Opaque, //
> +};
> +
> +use core::{
> +    marker::PhantomData,
> +    mem::ManuallyDrop,
> +    ops::Deref,
> +    ptr,
> +    ptr::{
> +        drop_in_place,
> +        NonNull, //
> +    }, //
> +};
> +
> +use kernel::{
> +    str::CString,
> +    sync::{
> +        aref::{
> +            ARef,
> +            AlwaysRefCounted, //
> +        },
> +        atomic::{
> +            Atomic,
> +            Relaxed, //
> +        },
> +        rcu::rcu_barrier, //
> +    }, //
> +};
> +
> +/// VTable for dma_fence backend_ops callbacks.
> +//
> +// Mandatory dma_fence backend_ops are implemented implicitly through
> +// [`FenceContext`]. Additional ones shall get implemented on this trait.
> +pub trait FenceContextOps {
> +    /// The generic payload data for [`DriverFence`]s created on this fctx.
> +    type FenceDataType: Send + Sync;
> +}
> +
> +/// A dma-fence context. A fence context takes care of associating related fences
> +/// with each other, providing each with raising sequence numbers and a common
> +/// identifier.
> +#[pin_data(PinnedDrop)]
> +pub struct FenceContext<T: FenceContextOps + Send + Sync> {
> +    /// The fence context number.
> +    nr: u64,
> +    /// The sequence number for the next fence created.
> +    seqno: Atomic<u64>,
> +    // The name parameters can be accessed by the dma_fence backend_ops. UAF
> +    // errors are prevented by the `call_rcu()` in `drop_driver_fence_data()`.
> +    /// The name of the driver this FenceContext's fences belong to.
> +    driver_name: CString,
> +    /// The name of the timeline this FenceContext's fences belong to.
> +    timeline_name: CString,
> +    /// The number of all unsignaled fences on this context.
> +    // Used to prevent bugs due to forgotten fences.
> +    //
> +    // The lifetime on `DriverFence`s should typically prevent this from
> +    // happening.
> +    //
> +    // However, we cannot fully guarantee in Rust that `DriverFence`s will not
> +    // be forgotten, e.g., through `core::mem::forget()`. This could circumvent
> +    // the lifetime which intends to enforce that all fences disappear before
> +    // their context.
> +    nr_of_unsignaled_fences: Atomic<u64>,

This can be `Atomic<usize>` so it doesn't need to go through the generic 64-bit
atomic mechanism on 32-bit systems.

> +    /// The user's data.
> +    #[pin]
> +    data: T,
> +}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> FenceContext<T> {
> +    // This can later be extended as a vtable in case other parties need support
> +    // for the more "exotic" callbacks.
> +    const OPS: bindings::dma_fence_ops = bindings::dma_fence_ops {
> +        get_driver_name: Some(Self::get_driver_name),
> +        get_timeline_name: Some(Self::get_timeline_name),
> +        enable_signaling: None,
> +        signaled: None,
> +        wait: None,
> +        release: None,
> +        set_deadline: None,
> +    };
> +
> +    /// Create a new `FenceContext`.
> +    pub fn new<E>(
> +        initial_seqno: u64,
> +        driver_name: CString,
> +        timeline_name: CString,
> +        data: impl PinInit<T, E>,
> +    ) -> impl PinInit<Self, Error>
> +    where
> +        Error: From<E>,
> +    {
> +        try_pin_init!(Self {
> +            // SAFETY: `dma_fence_context_alloc()` merely works on a global
> +            // atomic. Parameter `1` is the number of contexts we want to
> +            // allocate.
> +            nr: unsafe { bindings::dma_fence_context_alloc(1) },
> +            seqno: Atomic::new(initial_seqno),
> +            driver_name,
> +            timeline_name,
> +            nr_of_unsignaled_fences: Atomic::new(0),
> +            data <- data,
> +        })
> +    }
> +
> +    fn next_seqno(&self) -> u64 {
> +        self.seqno.fetch_add(1, Relaxed)
> +    }
> +
> +    /// Allocate the memory for a [`DriverFence`] and already store `data` inside.
> +    ///
> +    /// This is needed because many times, creation of a [`DriverFence`] must not
> +    /// fail, and allocating might deadlock in some situations.
> +    ///
> +    /// The `data` you pass here must not perform any operations that are illegal
> +    /// in atomic context in its [`Drop`] implementation.
> +    pub fn new_fence_allocation(
> +        &self,
> +        data: T::FenceDataType,
> +    ) -> Result<DriverFenceAllocation<'_, T>> {
> +        let fence_data = DriverFenceData {
> +            rcu_head: Default::default(),
> +            // `inner` remains uninitialized until a `DriverFence` takes over.
> +            inner: Fence {
> +                inner: Opaque::uninit(),
> +            },
> +            fctx: self,
> +            data,
> +        };
> +
> +        // In order to support the C dma_fence callbacks, it is necessary for
> +        // a `Fence` and a `DriverFence` to live in the same allocation,
> +        // because the C backend passes a dma_fence, from which the driver most
> +        // likely wants to be able to access its `data` in `DriverFence`.
> +        //
> +        // Hence, we need the manage the memory manually. It will be freed by the
> +        // C backend automatically once the refcount within `Fence` drops to 0.
> +        let data = KBox::new(fence_data, GFP_KERNEL | __GFP_ZERO)?;
> +
> +        Ok(DriverFenceAllocation {
> +            data,
> +            ops: &Self::OPS,
> +        })
> +    }
> +
> +    extern "C" fn get_driver_name(ptr: *mut bindings::dma_fence) -> *const c_char {
> +        // SAFETY: The C backend only invokes this callback with `ptr` pointing
> +        // to a valid, unsignaled `bindings::dma_fence`. All fences created in
> +        // this module always reside within `Fence` which always resides in a
> +        // `DriverFenceData`, thus satisfying the function's safety
> +        // requirements.
> +        let fctx = unsafe { Self::from_raw_fence(ptr) };
> +
> +        fctx.driver_name.as_char_ptr()
> +    }
> +
> +    extern "C" fn get_timeline_name(ptr: *mut bindings::dma_fence) -> *const c_char {
> +        // SAFETY: The C backend only invokes this callback with `ptr` pointing
> +        // to a valid, unsignaled `bindings::dma_fence`. All fences created in
> +        // this module always reside within `Fence` which always resides in a
> +        // `DriverFenceData`, thus satisfying the function's safety
> +        // requirements.
> +        let fctx = unsafe { Self::from_raw_fence(ptr) };
> +
> +        fctx.timeline_name.as_char_ptr()
> +    }
> +
> +    /// Create a [`FenceContext`] from an associated [`bindings::dma_fence`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must be a valid pointer to a [`bindings::dma_fence`] which resides
> +    /// within a [`Fence`], which in turn resides in a [`DriverFenceData`].
> +    unsafe fn from_raw_fence(ptr: *mut bindings::dma_fence) -> &'a Self {
> +        let opaque_fence = Opaque::cast_from(ptr);
> +
> +        // SAFETY: Safe due to the function's overall safety requirements.
> +        let fence_ptr = unsafe { container_of!(opaque_fence, Fence, inner) };
> +
> +        // CAST: `DriverFenceData` is repr(C) and a `Fence` is its first member.

`repr(C)`

> +        let fence_data_ptr = fence_ptr as *mut DriverFenceData<'a, T>;

`*const` should work here too?

> +
> +        // SAFETY: Safe because of the comments directly above.
> +        let fence_data = unsafe { &*fence_data_ptr };
> +
> +        fence_data.fctx
> +    }
> +}
> +
> +// FenceContext's drop() ensures that the driver cannot unload while there are
> +// still dma_fence callbacks running. This also prevents UAF problems with
> +// `fctx.driver_name` and `fctx.timeline_name`.
> +//
> +// DriverFence data gets dropped through `call_rcu()` in `DriverFence::drop`.
> +// This `rcu_barrier()` also serves to wait for their completion.

These should be comment on the drop code itself, not as comment of `PinnedDrop`
impl.

This can be more detailed about why a UAF problem exists (and this should be
commented on the panic part, as something like:

    // Fence ops callbacks may be called on unsignaled fences, so we may not
    // leak any driver fences, otherwise `fctx.driver_name` and
    // `fctx.timeline_name` can be accessed after drop (UAF).
    if self.nr_of_unsignaled_fences.load(Relaxed) != 0 {
    
    // Fence ops callbacks use RCU to sychronize callbacks and thus we use `call_rcu`
    // to destroy `DriverFence` data. `rcu_barrier` here synchronize with driver
    // fence's destruction.

> +#[pinned_drop]
> +impl<T: FenceContextOps + Send + Sync> PinnedDrop for FenceContext<T> {
> +    fn drop(self: Pin<&mut Self>) {
> +        // TODO:
> +        // It would be better if the fence context signals all forgotten fences
> +        // itself. To do so, it would keep a list of unsignaled fences. That
> +        // list members would have to be pre-allocated (see
> +        // `FenceCallback::new_fence_allocation()`).
> +        if self.nr_of_unsignaled_fences.load(Relaxed) != 0 {
> +            panic!("Forgotten fences in FenceContext.");
> +        }
> +
> +        rcu_barrier();
> +    }
> +}
> +
> +/// Error type for fence callback registration.
> +///
> +/// Generic over `T` so that `AlreadySignaled` can return the callback to the
> +/// caller, allowing it to reclaim any resources owned by the callback (e.g.,
> +/// a fence handle that needs to be signaled).
> +#[derive(Debug)]
> +pub enum CallbackError<T = ()> {

What is this `= ()` used for?

> +    /// The fence was already signaled. The callback is returned so the caller
> +    /// can extract owned resources without losing them.
> +    AlreadySignaled(T),
> +    /// Some other error occurred during registration.
> +    Other(Error),
> +}
> +
> +impl<T> From<CallbackError<T>> for Error {

#[inline]

> +    fn from(err: CallbackError<T>) -> Self {
> +        match err {
> +            CallbackError::AlreadySignaled(_) => ENOENT,
> +            CallbackError::Other(e) => e,
> +        }
> +    }
> +}
> +
> +impl<T> From<AllocError> for CallbackError<T> {

#[inline]

> +    fn from(e: AllocError) -> Self {
> +        CallbackError::Other(Error::from(e))
> +    }
> +}
> +
> +/// Trait for callbacks that can be registered on fences.
> +///
> +/// When the fence signals, the callback will be invoked.
> +///
> +/// # Example
> +///
> +/// ```rust
> +/// use kernel::dma_buf::FenceCallback;
> +///
> +/// struct MyCallback {
> +///     // Your callback state here
> +/// }
> +///
> +/// impl FenceCallback for MyCallback {
> +///     fn called(&mut self) {
> +///         pr_info!("Fence signaled!");
> +///         // Handle fence completion
> +///     }
> +/// }
> +/// ```
> +pub trait FenceCallback: Send + 'static {
> +    /// Called when the fence is signaled.
> +    ///
> +    /// This is called from the fence signaling path, which may be in interrupt
> +    /// context or with locks held, which is why `self` is only borrowed, so that
> +    /// it cannot drop. Implementations must not sleep or perform
> +    /// long-running operations.
> +    ///
> +    /// An implementation likely wants to inform itself (e.g., through a work item)
> +    /// within this callback that the associated [`FenceCallbackRegistration`]
> +    /// can now be dropped.
> +    fn called(&mut self);

The name feels a bit awkward to me. I think this should either look like an
action on the callback, in which case "call" or describe an event on the fence,
i.e. "on_signal" or "signaled". Naming it "called" is very weird because it's
not a event that is triggered when something is "called".

> +}
> +
> +/// A callback registration on a fence.
> +///
> +/// When this object is dropped, the callback is automatically removed if it
> +/// hasn't been called yet.
> +#[pin_data(PinnedDrop)]
> +pub struct FenceCallbackRegistration<T: FenceCallback + 'static> {
> +    #[pin]
> +    callback_foreign: Opaque<bindings::dma_fence_cb>,
> +    callback: ManuallyDrop<T>,
> +    fence: ARef<Fence>,
> +}
> +
> +impl<T: FenceCallback> FenceCallbackRegistration<T> {
> +    /// Create a [`PinInit`] closure for registering a callback on a fence.
> +    ///
> +    /// The actual attempt at registering the callback will take place once you
> +    /// call an allocator's `pin_init()` function.
> +    ///
> +    /// On success the callback is pinned in place and will fire when the fence
> +    /// signals. On `AlreadySignaled` the callback is returned to the caller so
> +    /// that owned resources can be reclaimed.
> +    pub fn new<'a>(fence: &'a Fence, callback: T) -> impl PinInit<Self, CallbackError<T>> + 'a
> +    where
> +        T: 'a,
> +    {
> +        try_pin_init!(Self {
> +            // We need to fully initialize the fence because after
> +            // `dma_fence_add_callback()` ran, the callback might immediately
> +            // get invoked.
> +            callback: ManuallyDrop::new(callback),
> +            fence: ARef::from(fence),
> +            callback_foreign <- Opaque::try_ffi_init(|ptr| {
> +                // SAFETY: `fence.inner.get()` is a valid, initialized `struct
> +                // dma_fence`. `ptr` points to the `struct dma_fence_cb` field
> +                // within the pinned allocation, so it remains valid until
> +                // `dma_fence_remove_callback()` in `PinnedDrop` or until the
> +                // callback fires.
> +                let ret = unsafe {
> +                    to_result(bindings::dma_fence_add_callback(
> +                        fence.inner.get(),
> +                        ptr,
> +                        Some(Self::dma_fence_callback),
> +                    ))
> +                };
> +                match ret {
> +                    Ok(()) => Ok(()),
> +                    Err(e) => {
> +                        // SAFETY: We could not register the callback. Thus,
> +                        // C will not use it. So we can just take it back
> +                        // and pass it to the user again.
> +                        let cb_back = unsafe { ManuallyDrop::take(callback) };
> +                        if e == ENOENT {
> +                            Err(CallbackError::AlreadySignaled(cb_back))
> +                        } else {
> +                            Err(CallbackError::Other(e))
> +                        }
> +                    },
> +                }
> +            }),
> +        }? CallbackError<T>)
> +    }
> +
> +    /// Raw dma fence callback that is called by the C code.
> +    ///
> +    /// # Safety
> +    ///
> +    /// This is only called by the dma_fence subsystem with valid pointers.
> +    unsafe extern "C" fn dma_fence_callback(
> +        _fence: *mut bindings::dma_fence,
> +        callback_foreign: *mut bindings::dma_fence_cb,
> +    ) {
> +        let ptr = Opaque::cast_from(callback_foreign).cast_mut();
> +
> +        // SAFETY: All `cb` we can receive here have been created in such a way
> +        // that they are embedded into a `FenceCallbackRegistration`. The
> +        // backend ensures synchronisation so whoever holds the registration
> +        // object cannot drop it while this code is running. See
> +        // `FenceCallbackRegistration::drop`.
> +        unsafe {
> +            let reg: *mut Self = container_of!(ptr, Self, callback_foreign);
> +
> +            (*reg).callback.called();
> +        }
> +    }
> +
> +    /// Returns a reference to the fence this callback is registered on.
> +    pub fn fence(self: Pin<&Self>) -> &Fence {
> +        &self.get_ref().fence
> +    }

This can just be `fence(&self) -> &Fence`.

> +}
> +
> +#[pinned_drop]
> +impl<T: FenceCallback> PinnedDrop for FenceCallbackRegistration<T> {
> +    fn drop(self: Pin<&mut Self>) {
> +        // Always call dma_fence_remove_callback, even if `callback` has already
> +        // been taken by `dma_fence_callback`.  This is necessary for

Is this still up-to-date? You're not taking callback anymore in
`dma_fence_callback`.

> +        // synchronization: `dma_fence_remove_callback` acquires `fence->lock`,
> +        // which ensures that any in-flight `dma_fence_signal` (which calls our
> +        // callback while holding the same lock) has completed before we free
> +        // the struct.
> +        //
> +        // Without this, Drop can race with a concurrent signal:
> +        //   CPU0 (signal, lock held): take() -> signaled(fence_ref) (in progress)
> +        //   CPU1 (drop): sees is_some()==false -> skips lock -> frees struct
> +        //   CPU0: accesses fence_ref -> use-after-free
> +        //
> +        // When the callback has already fired, the signal path detached the
> +        // list node via INIT_LIST_HEAD, so dma_fence_remove_callback just sees
> +        // an empty node and returns false — the lock acquisition is the only
> +        // thing that matters.
> +        //
> +        // SAFETY: The fence pointer is valid and the cb was initialized by
> +        // dma_fence_add_callback during construction.
> +        unsafe {
> +            bindings::dma_fence_remove_callback(self.fence.as_raw(), self.callback_foreign.get());
> +        }
> +
> +        // SAFETY: This is literally the drop implementation, so no one has
> +        // dropped this so far; so we can do it now.
> +        unsafe { ManuallyDrop::<T>::drop(self.project().callback) };
> +    }
> +}
> +
> +// SAFETY: FenceCallbackRegistration can be sent between threads.
> +unsafe impl<T: FenceCallback> Send for FenceCallbackRegistration<T> {}
> +
> +// SAFETY: &FenceCallbackRegistration can be shared between threads if &T can.
> +unsafe impl<T: FenceCallback> Sync for FenceCallbackRegistration<T> where T: Sync {}
> +
> +/// The receiving counterpart of a [`DriverFence`].
> +///
> +/// The Rust DMA fence implementation has a dualistic design: [`DriverFence`]s
> +/// are the producer-side, intended to be always owned by only one party. That
> +/// party has the monopoly on signalling the fence.
> +///
> +/// A [`Fence`] is the counterpart for consumers. Thus, [`Fence`]s are always
> +/// refcounted and can shared with an arbitrary number of parties, including
> +/// userspace. A [`Fence`] can only be used for actions such as checking the
> +/// fence's status or for registering callbacks on it.
> +///
> +/// Once the associated [`DriverFence`] signals, all
> +/// [`FenceCallbackRegistration`]s registered on the [`Fence`] will be executed.
> +///
> +/// A [`Fence`] can arbitrarily outlive its [`DriverFence`] and the
> +/// [`FenceContext`]. Signalling a [`DriverFence`] decouples it from its
> +/// [`Fence`]s.
> +#[repr(transparent)]
> +pub struct Fence {
> +    /// The actual dma_fence passed to C.
> +    inner: Opaque<bindings::dma_fence>,
> +}
> +
> +/// Guard helper for locking within this module.
> +///
> +/// Its only purpose for now is to avoid a number of unsafe lock-unlock cycles.
> +/// It is never used outside of this module.
> +// TODO: This should be made more canonical, probably by basing it on a
> +// SpinLockIrqGuard once available.
> +struct FenceGuard {
> +    inner: *mut bindings::dma_fence,
> +    flags: usize,
> +}
> +
> +impl Deref for FenceGuard {
> +    type Target = *mut bindings::dma_fence;

Why not store and return `&Fence`?

> +
> +    fn deref(&self) -> &Self::Target {
> +        &self.inner
> +    }
> +}
> +
> +impl Drop for FenceGuard {
> +    fn drop(&mut self) {
> +        // SAFETY: `fence` is valid because `self` is valid. `flag_ptr` is
> +        // merely a pointer to an integer, which lives as long as this function.
> +        // When a `FenceGuard` exists, the lock has been taken by definition.
> +        unsafe { bindings::dma_fence_unlock_irqrestore(self.inner, &raw mut self.flags) };
> +    }
> +}
> +
> +// SAFETY: Fences are literally designed to be shared between threads.
> +unsafe impl Send for Fence {}
> +// SAFETY: Fences are literally designed to be shared between threads.
> +unsafe impl Sync for Fence {}
> +
> +impl Fence {
> +    /// Check whether the fence was signaled at the moment of the function call.
> +    ///
> +    /// Note that this can return `true` for a [`Fence`] whose [`DriverFence`]
> +    /// has not yet been dropped. The reason is that the fence ops callbacks can
> +    /// cause the fence to get signaled by the C backend.
> +    pub fn is_signaled(&self) -> bool {
> +        // We should not use `dma_fence_is_signaled_locked()` here, because
> +        // according to the C backend's recommendations, that function is
> +        // problematic and we should avoid calling that function with a lock
> +        // held.
> +
> +        // SAFETY: Inner `fence` is valid because `self` is valid.
> +        let ret = unsafe { bindings::dma_fence_is_signaled(self.as_raw()) };
> +
> +        // To be as robust as possible for the future we guarantee that an API
> +        // caller can 100% rely on the signalling being completed (i.e., all
> +        // fence callbacks ran), so we have to take the lock.
> +        //
> +        // The reason is that the C dma_fence backend currently does not
> +        // carefully synchronize the `dma_fence_is_signaled()` function with the
> +        // proper spinlock. This can lead to the function returning `true` while
> +        // fence callbacks are still being executed. This can be mitigated by
> +        // guarding the entire function with the spinlock.
> +        //
> +        // The fundamental reason is that the C backend currently does guard
> +        // setting of the fence's signaled-bit with the fence's spinlock, but
> +        // reading is done locklessly.
> +        //
> +        // See commit c8a5d5ea3ba6a.
> +

Extra newline here.

> +        let _ = self.lock();
> +
> +        ret
> +    }
> +

#[inline] here and many more below.

> +    fn lock(&self) -> FenceGuard {
> +        let mut guard = FenceGuard {
> +            inner: self.as_raw(),
> +            flags: 0,
> +        };
> +
> +        // SAFETY: `fence` is valid because `self` is valid. `flag_ptr` is
> +        // merely a pointer to an integer, whose lifetime is tied to the guard
> +        // object.
> +        unsafe { bindings::dma_fence_lock_irqsave(guard.inner, &raw mut guard.flags) };
> +
> +        guard
> +    }
> +
> +    /// Get the fence's sequence number.
> +    pub fn seqno(&self) -> u64 {
> +        // SAFETY: Valid because `self` is valid.
> +        unsafe { (*self.as_raw()).seqno }
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::dma_fence {
> +        self.inner.get()
> +    }
> +
> +    /// Create a [`Fence`] from a raw C [`bindings::dma_fence`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to an initialized fence that is embedded into a [`Fence`].
> +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::dma_fence) -> &'a Self {
> +        // SAFETY: Safe as per the function's overall safety requirements.
> +        unsafe { &*ptr.cast() }
> +    }
> +}
> +
> +// SAFETY: These implement the C backends refcounting methods which are proven
> +// to work correctly.
> +unsafe impl AlwaysRefCounted for Fence {
> +    fn inc_ref(&self) {
> +        // SAFETY: `self.as_raw()` is a pointer to a valid `struct dma_fence`.
> +        unsafe { bindings::dma_fence_get(self.as_raw()) }
> +    }
> +
> +    /// # Safety
> +    ///
> +    /// `ptr`must be a valid pointer to a [`DriverFence`].
> +    unsafe fn dec_ref(ptr: NonNull<Self>) {
> +        // SAFETY: `ptr` is never a NULL pointer; and when `dec_ref()` is called
> +        // the fence is by definition still valid.
> +        let fence = unsafe { (*ptr.as_ptr()).inner.get() };
> +
> +        // SAFETY: `fence` was created validly above. When `dec_ref()` is called,
> +        // there is by definition still a reference alive that can be put.
> +        unsafe { bindings::dma_fence_put(fence) }
> +    }
> +}
> +
> +// Necessary to guarantee that `inner` always comes first and can be freed by C.
> +// Also useful for using casts instead of container_of().
> +#[repr(C)]
> +#[pin_data]
> +struct DriverFenceData<'a, T: Send + Sync + FenceContextOps> {
> +    #[pin]
> +    /// The inner fence.
> +    // Must always be the first member so that unsafe casting works; but also
> +    // necessary so that the C backend can free the allocation (coming from our
> +    // Rust code) with kfree_rcu().
> +    inner: Fence,
> +    /// Callback head for dropping this in a deferred manner through RCU.
> +    rcu_head: bindings::callback_head,
> +    /// Reference to access the FenceContext. Useful for obtaining name parameters.
> +    fctx: &'a FenceContext<T>,
> +    /// The API user's data. It is essential that the data only performs
> +    /// operations legal in atomic context in its [`Drop`] implementation.
> +    #[pin]
> +    data: T::FenceDataType,
> +}
> +
> +/// A synchronization primitive mainly for GPU drivers.
> +///
> +/// The Rust DMA fence implementation has a dualistic design: [`DriverFence`]s
> +/// are the producer-side, intended to be always owned by only one party. That
> +/// party has the monopoly on signalling the fence.
> +///
> +/// A [`Fence`] is the counterpart for consumers. Thus, [`Fence`]s are always
> +/// refcounted and can shared with an arbitrary number of parties, including
> +/// userspace. A [`Fence`] can only be used for actions such as checking the
> +/// fence's status or for registering callbacks on it.
> +///
> +/// Once the associated [`DriverFence`] signals, all
> +/// [`FenceCallbackRegistration`]s registered on a [`Fence`] will be executed.
> +///
> +/// A [`Fence`] can arbitrarily outlive its [`DriverFence`] and the
> +/// [`FenceContext`]. Signalling a [`DriverFence`] decouples it from its
> +/// [`Fence`]s.
> +///
> +/// It is crucial that a [`DriverFence`] always correctly represents the state
> +/// of the associated job on the hardware. Especially, it is strictly necessary
> +/// that the owner ensures that all [`DriverFence`]s eventually get signaled.
> +/// As a last resort, a [`DriverFence`] will signal itself if it drops
> +/// unsignaled and print a warning.
> +///
> +/// This design intends to implement the [`bindings::dma_fence_ops`] in such a
> +/// way that the driver-data necessary to implement the callback's functionality
> +/// resides in the [`FenceContext`]. Thus, a [`DriverFence`] contains a
> +/// reference to the context, which can be accessed in the callbacks. The
> +/// implementation, therefore, ensures that a [`DriverFence`] cannot outlive its
> +/// [`FenceContext`]. Unfortunately, this can be circumvented under certain
> +/// circumstances in Rust (e.g., usage of [`core::mem::forget`]).
> +///
> +/// In the unlikely case of such violations, error warnings are printed.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::dma_buf::{
> +///     DriverFence,
> +///     FenceContext,
> +///     FenceContextOps,
> +///     FenceCallback,
> +///     FenceCallbackRegistration, //
> +/// };
> +/// use kernel::str::CString;
> +/// use kernel::sync::aref::ARef;
> +/// use core::fmt::Display;
> +///
> +/// struct CallbackData { }
> +///
> +/// impl FenceCallback for CallbackData {
> +///     fn called(&mut self) {
> +///         pr_info!("DmaFence callback executed.\n");
> +///     }
> +/// }
> +///
> +/// #[pin_data]
> +/// struct FenceContextData {}
> +///
> +/// impl FenceContextData {
> +///     fn new() -> impl PinInit<Self> {
> +///         pin_init!(Self {})
> +///     }
> +/// }
> +///
> +/// impl FenceContextOps for FenceContextData {
> +///     type FenceDataType = FenceData;
> +/// }
> +///
> +/// let fctx_data = FenceContextData::new();
> +///
> +/// let driver_name = CString::try_from_fmt(fmt!("dummy_driver"))?;
> +/// let timeline_name = CString::try_from_fmt(fmt!("dummy_timeline"))?;
> +///
> +/// let mut fctx = KBox::pin_init(
> +///     FenceContext::new(0, driver_name, timeline_name, fctx_data), GFP_KERNEL)?;
> +///
> +/// struct FenceData {
> +///     data: CString,
> +/// }
> +///
> +/// let data = CString::try_from_fmt(fmt!("dummy_data"))?;
> +/// let fence_data = FenceData { data };
> +///
> +/// let fence_alloc = fctx.new_fence_allocation(fence_data)?;
> +/// let mut fence = fence_alloc.new_fence();
> +///
> +/// let cb_data = CallbackData { };
> +/// let waiting_fence = ARef::from(fence.as_fence());
> +/// let cb_reg = FenceCallbackRegistration::new(&waiting_fence, cb_data);
> +/// let cb_reg = KBox::pin_init(cb_reg, GFP_KERNEL)?;
> +///
> +/// // TODO signalling guards
> +/// fence.signal(Ok(()));
> +/// assert_eq!(waiting_fence.is_signaled(), true);
> +///
> +/// Ok::<(), Error>(())
> +/// ```
> +pub struct DriverFence<'a, T: Send + Sync + FenceContextOps> {
> +    /// The actual content of the fence. Lives in a [`NonNull`] so that its
> +    /// memory can be managed independently. Valid until both the [`DriverFence`]
> +    /// and all associated [`Fence`]s have disappeared.
> +    data: NonNull<DriverFenceData<'a, T>>,
> +}
> +
> +/// A pre-prepared DMA fence, carrying the user's data and the memory it and the
> +/// fence reside in. Only useful for creating a [`DriverFence`]. Splitting
> +/// allocation and full initialization is necessary because fences cannot be
> +/// allocated dynamically in some circumstances (deadlock).
> +pub struct DriverFenceAllocation<'a, T: Send + Sync + FenceContextOps> {
> +    /// The memory for the actual content of the fence.
> +    /// Handed over to a [`DriverFence`], or deallocated once the
> +    /// [`DriverFenceAllocation`] drops.
> +    data: KBox<DriverFenceData<'a, T>>,
> +    /// Pointer for the ops for the associated [`FenceContext`]
> +    ops: *const bindings::dma_fence_ops,
> +}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> DriverFenceAllocation<'a, T> {
> +    /// Create a new fence, consuming `data`.

There's no `data`.

> +    ///
> +    /// This increments the sequence number in the associated [`FenceContext`].
> +    pub fn new_fence(self) -> DriverFence<'a, T> {
> +        // We feed the C dma_fence backend a NULL for the spinlock so that it
> +        // uses per-fence locks automatically.
> +        let null_ptr: *mut bindings::spinlock = ptr::null_mut();
> +        let seqno = self.data.fctx.next_seqno();
> +        let fence_ptr = self.as_raw();
> +        // SAFETY: `fence_ptr` has been created directly above. It will live
> +        // at least as long as `Self`. The same applies to `&Self::OPS`.
> +        unsafe {
> +            bindings::dma_fence_init(fence_ptr, self.ops, null_ptr, self.data.fctx.nr, seqno)
> +        };
> +
> +        self.data.fctx.nr_of_unsignaled_fences.fetch_add(1, Relaxed);
> +
> +        // A `DriverFenceAllocation`'s purpose is to carry allocated memory, so that
> +        // `DriverFence`s can always be created without allocating. In this
> +        // method, ownership over that memory is transferred to the new
> +        // `DriverFence` and managed through refcounting. The C dma_fence
> +        // backend will ultimately free the memory once the refcount reaches 0.
> +        let ptr = KBox::into_raw(self.data);
> +        // SAFETY: `ptr` was just created validly directly above.
> +        let ptr = unsafe { NonNull::new_unchecked(ptr) };
> +
> +        DriverFence { data: ptr }
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::dma_fence {
> +        self.data.inner.inner.get()
> +    }
> +}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> DriverFence<'a, T> {
> +    fn as_raw(&self) -> *mut bindings::dma_fence {
> +        // SAFETY: Valid because `self` is valid.
> +        let fence_data = unsafe { &*self.data.as_ptr() };
> +
> +        fence_data.inner.inner.get()
> +    }
> +
> +    /// Create a [`DriverFence`] from a raw pointer to a [`bindings::dma_fence`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must be a valid pointer to a `dma_fence` that was obtained through
> +    /// a [`DriverFence`] with matching generic data for both fence and associated
> +    /// [`FenceContext`].
> +    unsafe fn from_raw(ptr: *mut bindings::dma_fence) -> Self {
> +        let opaque_fence = Opaque::cast_from(ptr);
> +
> +        // SAFETY: Safe due to the function's overall safety requirements.
> +        let fence_ptr = unsafe { container_of!(opaque_fence, Fence, inner) };
> +
> +        // DriverFenceData is repr(C) and a Fence is its first member.
> +        let fence_data_ptr = fence_ptr as *mut DriverFenceData<'a, T>;
> +
> +        // SAFETY: `fence_data_ptr` was created validly above.
> +        let data = unsafe { NonNull::new_unchecked(fence_data_ptr) };
> +
> +        Self { data }
> +    }
> +
> +    /// Return the underlying [`Fence`].
> +    pub fn as_fence(&self) -> &Fence {
> +        // SAFETY: `self` is by definition still valid, and it cannot drop until
> +        // this new reference is gone.
> +        unsafe { Fence::from_raw(self.as_raw()) }
> +    }
> +
> +    /// Signal the fence. This will invoke all registered callbacks.
> +    pub fn signal(self, res: Result) {
> +        let fence = self.as_fence().lock();
> +
> +        // SAFETY: `fence` is valid because `self` is valid. The lock must be
> +        // held, which we acquired directly above.
> +        if !unsafe { bindings::dma_fence_test_signaled_flag(*fence.deref()) } {

These `*fence.deref()` are quite weird as consequence of `FenceGuard` design.
If `FenceGuard` just derefs to `&Fence` then this can be `fence.as_raw()`.

> +            if let Err(err) = res {
> +                // SAFETY: `fence` is valid because `self` is valid. The fence
> +                // must not have been signaled yet, which we check directly above.
> +                unsafe { bindings::dma_fence_set_error(*fence.deref(), err.to_errno()) };
> +            }
> +            // SAFETY: `fence` is valid because `self` is valid. The lock must
> +            // be held, which we acquired above.
> +            unsafe { bindings::dma_fence_signal_locked(*fence.deref()) };
> +        }
> +
> +        // SAFETY: `self.data` is valid because `self` is valid.
> +        let fctx = unsafe { self.data.as_ref().fctx };
> +        let _ = fctx.nr_of_unsignaled_fences.fetch_sub(1, Relaxed);

Drop impl of `self` here will neededlessly take lock again before checking it's
signaled already and unlock.

> +    }
> +}
> +
> +// SAFETY: Fences are literally designed to be shared between threads.
> +unsafe impl<'a, T: Send + Sync + FenceContextOps> Send for DriverFence<'a, T> {}
> +// SAFETY: Fences are literally designed to be shared between threads.
> +unsafe impl<'a, T: Send + Sync + FenceContextOps> Sync for DriverFence<'a, T> {}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> Deref for DriverFence<'a, T> {
> +    type Target = T::FenceDataType;
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: Thanks to refcounting, `data` is always valid as long as `self` is.
> +        let data = unsafe { &*self.data.as_ptr() };
> +
> +        &data.data
> +    }
> +}
> +
> +/// A borrow wrapper for [`DriverFence`]. Implements [`Deref`].
> +pub struct DriverFenceBorrow<'a, T: Send + Sync + FenceContextOps> {
> +    driver_fence: ManuallyDrop<DriverFence<'a, T>>,
> +    _lifetime: PhantomData<&'a T>,
> +}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> Deref for DriverFenceBorrow<'a, T> {
> +    type Target = DriverFence<'a, T>;
> +
> +    fn deref(&self) -> &Self::Target {
> +        self.driver_fence.deref()
> +    }
> +}
> +
> +// SAFETY: The Rust dma_fence abstractions are already designed around the inner
> +// C `dma_fence`, which can serve safely as the identification point when being
> +// owned by C. Moreover, safety is ensured by not dropping `DriverFence` and by
> +// only allowing operations without side effects on the Borrowed type.
> +unsafe impl<T: Send + Sync + FenceContextOps + 'static> ForeignOwnable for DriverFence<'_, T> {

The `'static` shouldn't be needed here.

> +    type Borrowed<'a>
> +        = DriverFenceBorrow<'a, T>
> +    where
> +        Self: 'a;
> +    type BorrowedMut<'a>
> +        = DriverFenceBorrow<'a, T>
> +    where
> +        Self: 'a;
> +
> +    const FOREIGN_ALIGN: usize = core::mem::align_of::<bindings::dma_fence>();
> +
> +    fn into_foreign(self) -> *mut c_void {
> +        let fence = self;
> +
> +        let ptr = fence.as_raw();
> +
> +        // DriverFence must not drop.
> +        let _ = ManuallyDrop::new(fence);
> +
> +        ptr.cast()
> +    }
> +
> +    unsafe fn from_foreign(ptr: *mut c_void) -> Self {
> +        // SAFETY: Safe because the trait implementation only invokes this with
> +        // a valid `ptr`, associated to a `DriverFence` with matching generic data.
> +        unsafe { Self::from_raw(ptr.cast()) }
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a>
> +    where
> +        Self: 'a,
> +    {
> +        // SAFETY: The trait implementation ensures that `ptr` always resides
> +        // within a [`Fence`] within a [`DriverFenceData`].
> +        let driver_fence = unsafe { Self::from_raw(ptr.cast()) };
> +
> +        let driver_fence = ManuallyDrop::new(driver_fence);
> +
> +        DriverFenceBorrow {
> +            driver_fence,
> +            _lifetime: PhantomData,
> +        }
> +    }
> +
> +    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Self::BorrowedMut<'a>
> +    // FIXME: The bound below and the one above in `borrow` should actually be
> +    // unnecessary since the compiler should be able to completely derive all
> +    // necessary information automatically. There is currently a compiler bug
> +    // preventing that, though:
> +    //
> +    // https://github.com/rust-lang/rust/issues/155430.
> +    //
> +    // (Help to) fix the compiler bug and remove the bounds afterwards.
> +    where
> +        Self: 'a,
> +    {
> +        // SAFETY: The trait implementation ensures that `ptr` always resides
> +        // within a [`Fence`] within a [`DriverFenceData`].
> +        let driver_fence = unsafe { Self::from_raw(ptr.cast()) };
> +
> +        let driver_fence = ManuallyDrop::new(driver_fence);
> +
> +        DriverFenceBorrow {
> +            driver_fence,
> +            _lifetime: PhantomData,
> +        }
> +    }
> +}
> +
> +impl<'a, T: Send + Sync + FenceContextOps> Drop for DriverFence<'a, T> {
> +    fn drop(&mut self) {
> +        let guard = self.as_fence().lock();
> +
> +        // Use dma_fence_test_signaled_flag() instead of
> +        // dma_fence_is_signaled_locked() because the C backend wants to get rid
> +        // of the latter.
> +
> +        // SAFETY: `guard` is valid until the `call_rcu()` below.
> +        let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(*guard.deref()) };
> +        if !signaled {
> +            pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n");
> +            // SAFETY: `guard` is valid until the `call_rcu()` below. The fence
> +            // must not have been signaled yet, which we check directly above.
> +            unsafe { bindings::dma_fence_set_error(*guard.deref(), ECANCELED.to_errno()) };
> +            // SAFETY: `guard` is valid until the `call_rcu()` below. The lock
> +            // must be held, which we acquired above.
> +            unsafe { bindings::dma_fence_signal_locked(*guard.deref()) };
> +
> +            // SAFETY: `self.data` is valid because `self` is valid.
> +            let fctx = unsafe { self.data.as_ref().fctx };
> +            let _ = fctx.nr_of_unsignaled_fences.fetch_sub(1, Relaxed);
> +        }
> +        drop(guard);
> +
> +        // SAFETY: Valid because `self` is valid.
> +        let rcu_head_ptr = unsafe { &raw mut (*self.data.as_ptr()).rcu_head };
> +
> +        // `DriverFenceData` but could be accessed through some dma_fence
> +        // callbacks right now. Access is being revoked in principle above by
> +        // signalling the fence, but since the C backend does not guarantee
> +        // perfect full synchronization, we have to wait for one grace period to
> +        // ensure that all accessors of `DriverFenceData` (through the
> +        // dma_fence_ops accessible through a `Fence`) are gone.
> +
> +        // SAFETY: `call_rcu()` is always safe to be called. `rcu_head_ptr` was
> +        // created validly above. The module must perform a `synchronize_rcu()`
> +        // or `rcu_barrier()` call to guard against module unload.
> +        unsafe { bindings::call_rcu(rcu_head_ptr, Some(drop_driver_fence_data::<T>)) };

I thought at some point it was mentioned that we want a fast path

    if !mem::needs_drop::<...>() {
    }

?

> +    }
> +}
> +
> +// TODO:
> +// The entire call_rcu() mechanism in the drop above and the code below would be
> +// unnecessary if C's dma_fence_signal() could be reworked in a way that after it
> +// ran, the caller knows that no fence_ops callbacks can be running anymore.
> +// In other words, if the dma_fence backend would use its spinlock for full
> +// synchronization.
> +//
> +// Then we could move the drop_in_place() and dma_fence_put() upwards into the
> +// drop() implementation and call it a day.
> +
> +/// Finally really drop this `DriverFence<T>`
> +///
> +/// # Safety
> +///
> +/// `head` references the `rcu_head` field of an `DriverFenceData<T>`. All
> +/// accessors to that `DriverFenceData<T>` must be gone by now. This must be
> +/// ensured by signalling the associated `DriverFence<T>` and then waiting
> +/// for a grace period until calling this function here.
> +unsafe extern "C" fn drop_driver_fence_data<T: Send + Sync + FenceContextOps>(
> +    head: *mut bindings::callback_head,
> +) {
> +    // SAFETY: Caller provides a pointer to the `rcu_head` field of a `DriverFenceData<C>`.
> +    let fence_data = unsafe { container_of!(head, DriverFenceData<'_, T>, rcu_head) };
> +
> +    // SAFETY: `fence_data` was created validly above. All the fence's data will
> +    // only drop below, but the raw pointer to the raw C `dma_fence` remains
> +    // valid because the reference count is only decremented at the end of the
> +    // function.
> +    let fence = unsafe { (*fence_data).inner.inner.get() };
> +
> +    // SAFETY: `fence_data` was created validly above. A grace period has passed.
> +    // All callbacks which might have had access to the `fctx` are gone now.
> +    unsafe { drop_in_place(&raw mut (*fence_data).fctx) };

fctx is just a reference, so this is a no-op.

Best,
Gary

> +
> +    // SAFETY: `fence_data` was created validly above. The user has already
> +    // dropped the only conventional accessor to the user data, the `DriverFence`,
> +    // one grace period ago. All accessors are gone now.
> +    unsafe { drop_in_place(&raw mut (*fence_data).data) };
> +
> +    // The inner `Fence` explicitly does not get dropped because there may be
> +    // many more users / consumers, each holding their own reference.
> +
> +    // SAFETY: Once a `DriverFence` is initialized, the inner `fence` is
> +    // valid and initialized. It is valid until the refcount drops
> +    // to 0, which can earliest happen once we drop the `DriverFence`'s reference
> +    // here.
> +    unsafe { bindings::dma_fence_put(fence) };
> +
> +    // The actual memory the data associated with a `DriverFence` lives in
> +    // gets freed by the C dma_fence backend once the fence's refcount reaches 0.
> +}
> diff --git a/rust/kernel/dma_buf/mod.rs b/rust/kernel/dma_buf/mod.rs
> new file mode 100644
> index 000000000000..4764a828642e
> --- /dev/null
> +++ b/rust/kernel/dma_buf/mod.rs
> @@ -0,0 +1,14 @@
> +// SPDX-License-Identifier: GPL-2.0 OR MIT
> +
> +//! DMA-buf subsystem abstractions.
> +
> +pub mod dma_fence;
> +
> +pub use self::dma_fence::{
> +    DriverFence,
> +    Fence,
> +    FenceCallback,
> +    FenceCallbackRegistration,
> +    FenceContext,
> +    FenceContextOps, //
> +};
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 68f4d9a3425d..6221ebfe71df 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -67,6 +67,7 @@
>  pub mod device_id;
>  pub mod devres;
>  pub mod dma;
> +pub mod dma_buf;
>  pub mod driver;
>  #[cfg(CONFIG_DRM = "y")]
>  pub mod drm;