Latest Vulkan SDK results in validation errors related to vkQueueSubmit() and vkAcquireNextImageKHR()

[SiTronXD]

I was able to run Mogwai with Vulkan without any problems using Vulkan SDK version 1.3.268.0. However, after I updated the Vulkan version to 1.4.335.0, I started receiving two errors which were repeating during execution:

(Error) GFX Error: Validation: error 0: vkQueueSubmit(): pSubmits[0].pSignalSemaphores[0] (VkSemaphore 0x3c000000003c) is being signaled by VkQueue 0x1ae001098b0, but it may still be in use by VkSwapchainKHR 0x960000000096. Most recently acquired image indices: [0]. (Brackets mark the last use of VkSemaphore 0x3c000000003c in a presentation operation.) Swapchain image 0 was presented but was not re-acquired, so VkSemaphore 0x3c000000003c may still be in use. Vulkan insight: See https://docs.vulkan.org/guide/latest/swapchain_semaphore_reuse.html for details on swapchain semaphore reuse. Examples of possible approaches: a) Use a separate semaphore per swapchain image. Index these semaphores using the index of the acquired image. b) Consider the VK_KHR_swapchain_maintenance1 extension. It allows using a VkFence with the presentation operation. The Vulkan spec states: Each binary semaphore element of the pSignalSemaphores member of any element of pSubmits must be unsignaled when the semaphore signal operation it defines is executed on the device (https://docs.vulkan.org/spec/latest/chapters/cmdbuffers.html#VUID-vkQueueSubmit-pSignalSemaphores-00067)

and

(Error) GFX Error: Validation: error 0: vkAcquireNextImageKHR(): Semaphore must not have any pending operations. The Vulkan spec states: If semaphore is not VK_NULL_HANDLE, it must not have any uncompleted signal or wait operations pending (https://docs.vulkan.org/spec/latest/chapters/VK_KHR_surface/wsi.html#VUID-vkAcquireNextImageKHR-semaphore-01779)

Specs:
OS: Windows 11 (25H2 26200.7462)
GPU: RTX 5090
Driver: 591.74

I receive the same errors on the same computer when booting into linux, and also when running Falcor on a different computer with linux using the same Vulkan SDK version.

[SiTronXD]

I believe the issue lies in slang-gfx. When I compile and run the slang example projects from this fork which corresponds to the slang version 2024.1.34 currently in use by Falcor, then I receive the same Vulkan errors. If I instead compile and run the example projects from version 2026.1, then the errors are gone thanks to the newest slang example projects making use of slang-rhi instead of slang-gfx.

When investigating my second error message, it seems to stem from the Vulkan implementation of gfx::ISwapchain in slang-gfx only making use of one single semaphore when acquiring the next image, which I believe is incorrect. When I experiment with the slang-gfx source code, then the error disappears if I create at least one semaphore per frame in flight. This discrepancy may point to a faulty Vulkan implementation in slang-gfx.

A possible solution could therefore be to update the slang version for Falcor, and replace the slang-gfx backend with slang-rhi.