[Impeller] Move queue submission into a callback that is invoked by FenceWaiterVK::AddFence only if it can accept the fence by jason-simmons · Pull Request #187761 · flutter/flutter

jason-simmons · 2026-06-09T22:44:57Z

The FenceWaiterVK owns the lifetime of the fences that signal completion of a Vulkan command buffer submitted through vkQueueSubmit.

Prior to this PR, CommandQueueVK::Submit was submitting the command buffer to the queue with a fence before calling
FenceWaiterVK::AddFence. If AddFence failed, then the fence would be destroyed even though the Vulkan driver still holds its handle. That could cause a crash when the command completes and the Vulkan driver tries to signal the fence.

With this PR, the vkQueueSubmit call is moved into a callback that will be invoked by FenceWaiterVK::AddFence only if the FenceWaiterVK can take ownership of the fence. If the FenceWaiterVK has been terminated, then AddFence will skip the submit and destroy the fence.

Fixes #187703

…enceWaiterVK::AddFence only if it can accept the fence The FenceWaiterVK owns the lifetime of the fences that signal completion of a Vulkan command buffer submitted through vkQueueSubmit. Prior to this PR, CommandQueueVK::Submit was submitting the command buffer to the queue with a fence before calling FenceWaiterVK::AddFence. If AddFence failed, then the fence would be destroyed even though the Vulkan driver still holds its handle. That could cause a crash when the command completes and the Vulkan driver tries to signal the fence. With this PR, the vkQueueSubmit call is moved into a callback that will be invoked by FenceWaiterVK::AddFence only if the FenceWaiterVK can take ownership of the fence. If the FenceWaiterVK has been terminated, then AddFence will skip the submit and destroy the fence. Fixes flutter#187703

gemini-code-assist

Code Review

This pull request refactors Vulkan command submission by moving the queue submission step into FenceWaiterVK::AddFence via a callback, preventing submissions if the fence waiter is terminated. Unit tests and mock Vulkan helpers are updated to verify this behavior. Feedback recommends including <functional> in fence_waiter_vk.h to support std::function and declaring default_submit_callback as const auto in the unit tests to avoid potential linker errors.

gaaclarke

Looking good, just a few notes.

Am I crazy or did I already submit a review on this? Did this PR get recreated? Maybe I just looked at it and didn't get a chance to submit my review =)

gaaclarke · 2026-06-10T16:34:17Z

+  EXPECT_EQ(std::find(called->begin(), called->end(), "vkQueueSubmit"),
+            called->end());


Can you add a simple test to show that vkQueueSubmit is properly recorded too, please?

gaaclarke · 2026-06-10T16:37:01Z

+  ///        the submitted command completes and the fence is signaled.
+  fml::Status AddFence(vk::UniqueFence fence,
+                       std::function<fml::Status(vk::Fence)> submit_callback,
+                       const fml::closure& completion_callback);


nit: we are going to store this so we could std::move it and often people put rvalues in callback parameters.

Suggested change

const fml::closure& completion_callback);

fml::closure completion_callback);

gaaclarke · 2026-06-10T16:38:52Z

-  EXPECT_FALSE(waiter->AddFence(std::move(fence), nullptr));
+  const auto add_status =
+      waiter->AddFence(std::move(fence), default_submit_callback, nullptr);
+  EXPECT_FALSE(add_status.ok());


instead of expecting something is not ok, can you turn it to a equality assertion about what the error should be?

gaaclarke · 2026-06-10T16:41:25Z

 namespace impeller {
 namespace testing {

+const auto default_submit_callback = [](vk::Fence) { return fml::Status(); };


I think gemini's advice here wrong. I would have just put it in an anonymous namespace.

gaaclarke · 2026-06-10T16:41:49Z

-  EXPECT_FALSE(waiter->AddFence(vk::UniqueFence(), []() {}));
+  const auto add_status =
+      waiter->AddFence(vk::UniqueFence(), default_submit_callback, []() {});
+  EXPECT_FALSE(add_status.ok());


same here, eq test not false test.

gaaclarke · 2026-06-10T16:47:52Z

-    VALIDATION_LOG << "Failed to submit queue: " << vk::to_string(status);
-    return fml::Status(fml::StatusCode::kCancelled, "Failed to submit queue: ");
-  }
+  auto submit_callback = [&](vk::Fence submit_fence) {


"Use default capture by reference ([&]) only when the lifetime of the lambda is obviously shorter than any potential captures."

https://google.github.io/styleguide/cppguide.html#Lambda_expressions

It doesn't escape the current scope, but since we are passing the lambda along instead of calling it, it's probably better to treat it that way.

gaaclarke · 2026-06-10T16:50:26Z

+  /// @brief Invokes the [submit_callback] and adds the fence to the wait set
+  ///        if it succeeds.  The [completion_callback] will be called when


We should specify that the submit_callback is invoked synchronously so people can reason about lifetimes of objects captured by it.

jason-simmons · 2026-06-10T18:25:09Z

Am I crazy or did I already submit a review on this? Did this PR get recreated? Maybe I just looked at it and didn't get a chance to submit my review =)

This PR is an updated variant of #187698

gaaclarke

lgtm, thanks!

auto-submit · 2026-06-15T21:42:38Z

autosubmit label was removed for flutter/flutter/187761, because - The status or check suite Linux linux_clang_tidy has failed. Please fix the issues identified (or deflake) before re-applying this label.

gaaclarke

still lgtm

gaaclarke · 2026-06-16T00:38:40Z

Oops, I said "still lgtm" I mean to say, "thanks, lgtm!" I forgot I didn't approve this one yet.

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…enceWaiterVK::AddFence only if it can accept the fence (flutter#187761) The FenceWaiterVK owns the lifetime of the fences that signal completion of a Vulkan command buffer submitted through vkQueueSubmit. Prior to this PR, CommandQueueVK::Submit was submitting the command buffer to the queue with a fence before calling FenceWaiterVK::AddFence. If AddFence failed, then the fence would be destroyed even though the Vulkan driver still holds its handle. That could cause a crash when the command completes and the Vulkan driver tries to signal the fence. With this PR, the vkQueueSubmit call is moved into a callback that will be invoked by FenceWaiterVK::AddFence only if the FenceWaiterVK can take ownership of the fence. If the FenceWaiterVK has been terminated, then AddFence will skip the submit and destroy the fence. Fixes flutter#187703

jason-simmons requested a review from gaaclarke June 9, 2026 22:44

flutter-dashboard Bot added the CICD Run CI/CD label Jun 9, 2026

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Comment thread engine/src/flutter/impeller/renderer/backend/vulkan/fence_waiter_vk.h

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jason-simmons mentioned this pull request Jun 9, 2026

[Impeller] Submit a Vulkan command buffer only if its fence was successfully added to the FenceWaiterVK #187698

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[Impeller] HandleUsingDestroyedMutex crash on Mali GPUs during disposal #187140

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sero583 mentioned this pull request Jul 16, 2026

[Impeller] Harden the Vulkan backend against driver failures and resource exhaustion #189580

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		EXPECT_EQ(std::find(called->begin(), called->end(), "vkQueueSubmit"),
		called->end());

	const fml::closure& completion_callback);
	fml::closure completion_callback);

		/// @brief Invokes the [submit_callback] and adds the fence to the wait set
		/// if it succeeds. The [completion_callback] will be called when

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