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14 hours, 40 minutes

Re: [PATCH v9 2/5] rust: types: implement ForeignOwnable for ARef<T>

by Gary Guo

On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote: > From: Danilo Krummrich <dakr(a)kernel.org> > > Implement ForeignOwnable for ARef<T>, making it possible for C code to > own an ARef<T>. > > Since ARef represents shared ownership, BorrowedMut is &T rather than > &mut T, matching the semantics of the underlying reference-counted type. > > Signed-off-by: Danilo Krummrich <dakr(a)kernel.org> > Reviewed-by: Alice Ryhl <aliceryhl(a)google.com> > Tested-by: Daniel Almeida <daniel.almeida(a)collabora.com> > --- > rust/kernel/sync/aref.rs | 40 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > > diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs > index b721b2e00b98..540766613659 100644 > --- a/rust/kernel/sync/aref.rs > +++ b/rust/kernel/sync/aref.rs > @@ -24,6 +24,11 @@ > ptr::NonNull, // > }; > > +use crate::{ > + prelude::*, > + types::ForeignOwnable, // > +}; > + > /// Types that are _always_ reference counted. > /// > /// It allows such types to define their own custom ref increment and decrement functions. > @@ -188,6 +193,41 @@ fn eq(&self, other: &ARef<U>) -> bool { > } > impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {} > > +// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The > +// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`. > +unsafe impl<T: AlwaysRefCounted + 'static> ForeignOwnable for ARef<T> { > + const FOREIGN_ALIGN: usize = core::mem::align_of::<T>(); > + > + type Borrowed<'a> = &'a T; > + type BorrowedMut<'a> = &'a T; > + `#[inline]` here and all others. Best, Gary > + fn into_foreign(self) -> *mut c_void { > + ARef::into_raw(self).as_ptr().cast() > + } > + > + unsafe fn from_foreign(ptr: *mut c_void) -> Self { > + // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous > + // call to `Self::into_foreign`. > + let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) }; > + > + // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing > + // the refcount, so we can transfer the ownership to the new `ARef`. > + unsafe { ARef::from_raw(ptr) } > + } > + > + unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T { > + // SAFETY: The safety requirements of this method ensure that the object remains alive and > + // immutable for the duration of 'a. > + unsafe { &*ptr.cast() } > + } > + > + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T { > + // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety > + // requirements for `borrow`. > + unsafe { <Self as ForeignOwnable>::borrow(ptr) } > + } > +} > + > impl<T, U> PartialEq<&'_ U> for ARef<T> > where > T: AlwaysRefCounted + PartialEq<U>,

15 hours, 56 minutes

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

by Gary Guo

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(a)kernel.org> > Tested-by: Daniel Almeida <daniel.almeida(a)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(a)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;

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by blanchehughes54＠gmail.com

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by blanchehughes54＠gmail.com

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22 hours, 40 minutes

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