> On 5 Aug 2026, at 11:59, Philipp Stanner <[email protected]> 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 <[email protected]>
> Tested-by: Daniel Almeida <[email protected]>
>
Owing to the ongoing discussion in v8, I still think we could propose some
refinements in the future, like most source code out there :) But overall this
is a solid base to build upon and it doesn’t make sense to delay seeking
the “perfect” solution, if such a thing even exists..
Thanks for your hard work here!
Reviewed-by: Daniel Almeida <[email protected]>
