It is said semaphores are designed for this but how? It looks like I need to submit the semaphore before waiting for it to signal. Then what's the point of multithreading?
I'm using skia (has its own VkQueue) to draw UI, I don't have access to the commandbuffer, I can only provide semaphores for it. it first waits for the scene complete semaphore then draw ui and signals present ready semaphore.
It works fine when everything happens in a single thread. But after I move the UI part to a second thread. It stopped working and I got validation errors like: VkQueue is waiting on semaphore that has no way to be signaled. Of course, since it's on a different thread, the semaphore might not have been submitted to a queue yet.
The spec for vkQueuePresentKHR says
All elements of the pWaitSemaphores member of pPresentInfo must be semaphores that are signaled, or have semaphore signal operations previously submitted for execution
You can't submit work that waits on a semaphore that you plan to submit later. If you have this kind of dependency in your code you need to externally synchronize the submissions so the command buffers that will signal will be sent BEFORE you submit the dependent command buffers, regardless of the queue.
If you're using multiple threads it sounds like you need to rely on some CPU side synchronization primitives, like a CPU semaphore to properly order the work between them. Pure Vulkan sync primitives won't help you there.