Changes since v4: - Fix an uninitialized memory bug for FenceCbRegistration with ManuallyDrop. - Return FenceCtx as impl PinInit - Make FenceCtx return an impl PinInit<T, Error> (Danilo) - Reformat some comments - Adjust the docu for rcu_barrier(), so that it matches the C side's docu and our docu for rcu::synchronize_rcu().
Changes since v3: - Add a FIXME for an encountered Rust compiler bug. (Gary) - Add new Rust files also to DRM drivers & common infrastructure MAINTAINERS file. (Danilo) - Reposition ECANCELED error code. (Miguel) - Replace refcounted FenceCtx in DriverFenceData with a reference plus life time. (Boris) - Re-add rcu_barrier() patch, since we now can use it for dropping the fence context. (Danilo) - Add forgotten R-b from Alice, and Acks for MAINTAINERS from Christian and Sumit.
Changes since v2: - Don't drop DriverFenceData as a whole, but only the members we really want to drop. Gives more robustness. (Gary). - Break apart large pin_init_from_closure(). (Danilo, Onur) - Remove rcu_barrier() and synchronize_rcu() from FenceCtx::drop(). FenceCtx might drop in atomic context, where you must not perform those operations. With the current way C dma_fence is designed, the driver must wait for a grace period manually until it unloads. - Repair the DriverFenceBorrow implementation, properly injecting a life time into it. (Danilo) - Fix memory layout bug for rcu_head. (Onur) - Drop RCU patches, since this series doesn't need them anymore.
Changes since v1: - Remove unnecessary mutable references (Alice) - Split up unsafe comments where possible (Danilo) - Remove PhantomData + implement FenceCtx ops trait (Boris) - Consistently call FenceCtx generic data `T`. FenceDataType is derived from that. (Boris) - Add abstractions for call_rcu() and synchronize_rcu() (Danilo) - Add ECANCELED error code in Rust (Alice) - Remove the rcu_barrier() from FenceCtx::drop() – because we now use call_rcu(), there can be no UAF access to the FenceCtx anymore. In any case, it is illegal to use either call_rcu() or synchronize_rcu() in FenceCtx::drop(), because our new drop_driver_fence_data() can run in atomic context and might put the last fence_ctx reference. So we now only have to guard against module unload, which it seems either the driver or Rust driver-core / module unload infrastructure must solve. - Minor formatting etc. changes - Add C helpers to MAINTAINERS. (Danilo) - Ensure that `Fence::is_signaled()` is fully synchronized, i.e., all callbacks really have run. See [1] and [2]. (Myself, Christian König)
Changes since the RFCs: - Include support for ForeignOwnable for ARef, so that a Fence can be stuffed into an XArray et al. (Code by Danilo) - Implement ForeignOwnable (with new borrow type) for DriverFence, so that it can be stuffed into an XArray. - Include the rcu::RcuBox data type to defer dropping data with RCU (Cody by Alice) - Port DmaFence to RcuBox to make UAF bugs through later, new dma_fence callbacks (backend_ops) impossible. - Force users to pass their fence data in an RcuBox (or have it not need drop()) through a Sealed trait. - Document the rules for the user's DriverFence::data's drop implementation very clearly (deadlock danger). - rustfmt, Clippy. - Various style suggestions, safety comments, etc. (Önur) - Add __rust_helper prefix to helper functions. (Önur)
Changes in RFC v3: - Omit JobQueue patches for now - Completely redesign the memory layout: Instead of a Fence refcounting a DriverFence, both now live in the same allocation to allow for future support the dma_fence backend_ops callbacks which need to do container_of. (mostly Boris's feedback) - Allow for pre-allocating fences to avoid deadlocks when submitting jobs to a GPU. (Boris) - Simultaneously, allow for pre-preparing fence callback objects, so the driver can allocate them when it sees fit. (code largely stolen and inspired by Daniel). - Signal fences on drop, ensure synchronization. - Force users to set an error code when signalling. - Write more documentation - A ton of minor other changes.
[1] https://lore.kernel.org/dri-devel/20260608142436.265820-2-phasta@kernel.org/ [2] https://lore.kernel.org/dri-devel/20260612104251.2264707-2-phasta@kernel.org...
Alright, so since the last RFCs did not reveal significant design issues, I decided to transition this series to a v1 and hope that we can get it upstream.
This now includes code for more common infrastructure that dma_fence needs, contributed by Danilo and Alice.
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Old cover letter for RFC:
So, this is the spiritual successor of the first / second RFC [1]. v2 also contained code for drm::JobQueue, but mostly to show how the fence code would be used. JobQueue is under heavy rework right now, so I don't want to bother your eyes with it. The docstring examples should show how Rust fences are supposed to be used, though.
This v3 contains a huge amount of highly valuable feedback from a variety of people, notably Boris, but also from Alice, Gary and Danilo.
There are some TODOs open (a better trait for fence backend_ops and RCU support), but my hope is that this effort is now finally approaching its end.
I would greatly appreciate feedback and especially more information about what might be missing to make this usable, which is obviously where Daniel's and Boris's feedback will be valuable once more.
Please regard this patch just as what it's titled: an RFC, to discuss a bit more and to inform a broader community about what the current state is and where this is heading at.
Many regards, Philipp
[1] https://lore.kernel.org/rust-for-linux/20260203081403.68733-2-phasta@kernel....
Danilo Krummrich (1): rust: types: implement ForeignOwnable for ARef<T>
Philipp Stanner (4): rust: error: Add ECANCELED error code rust: sync: Add abstraction for rcu_barrier() rust: Add dma_fence abstractions MAINTAINERS: Add entry for Rust dma-buf
MAINTAINERS | 5 + rust/bindings/bindings_helper.h | 1 + rust/helpers/dma_fence.c | 48 ++ rust/helpers/helpers.c | 1 + rust/kernel/dma_buf/dma_fence.rs | 894 +++++++++++++++++++++++++++++++ rust/kernel/dma_buf/mod.rs | 14 + rust/kernel/error.rs | 1 + rust/kernel/lib.rs | 1 + rust/kernel/sync/aref.rs | 40 ++ rust/kernel/sync/rcu.rs | 20 + 10 files changed, 1025 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
base-commit: a73a398a68ca9b9e5116a617562471f16b8310c4