In our Rust code bases, I'm currently experiencing what we also experience in C at many places: that the dma_fence contract's RCU requirements make the API more fragile and put an unnecessary burden on the API user. Most notably, in C the API behaving correctly now depends on the API *user* waiting before free()s, waiting before unloads – all because dma_fence is not properly synchronized.
The patches commit messages give more details about where I want to go with this particular RFC. My rough "vision" for dma_fence, where I think it should develop to, can be described like this: 1. All users should use inline-lock. 2. The fence-signaled state should be our decoupling point. 3. The fence state must be consistently locked. 4. This means all callbacks are only invoked for unsignaled fences, and the fence cannot signal while a callback runs. 5. All fence API functions should be "atomically correct": they can always be called without any manual RCU guards in their proximity. 6. The fence lock should, in the long-term, become inaccessible for users. Point №6 means that this: dma_fence_lock_irqsave(…); dma_fence_set_error(f, err); dma_fence_signal_locked(f); dma_fence_unlock_irqrestore(…); synchronize_rcu(); kfree(driver_foo); should simply become this: dma_fence_signal(f, err); kfree(driver_foo); So we would end up with a simpler, more robust and more maintainable API. I gave this basic testing on a Nouveau desktop and it runs fine. dma-buf-tests also run fine. I can do more tests on Xe and amdgpu, maybe on Mali, too. Since the series is mostly about locking the callbacks, all of which except for set_deadline() are *already* locked, I am confident that this can be done. Feedback welcome P. Philipp Stanner (3): dma-fence: Remove RCU protection for fence->ops dma-fence: use lock to protect against module-unload dma-fence: Support unload for ops->get_XXX_name() drivers/dma-buf/dma-fence.c | 144 +++++++------------- drivers/dma-buf/sync_file.c | 29 ++-- drivers/gpu/drm/i915/gt/intel_gt_requests.c | 16 +-- drivers/gpu/drm/i915/i915_request.c | 9 +- drivers/gpu/drm/i915/i915_sw_fence.c | 15 +- include/linux/dma-fence.h | 63 ++++----- include/trace/events/dma_fence.h | 12 +- 7 files changed, 113 insertions(+), 175 deletions(-) -- 2.56.0
