> However, I know even that property is not true of all GPUs and thus cannot be assumed by Vulkan spec compliant code. (I actually doubt it's true of any GPU's pre-Volta - it's just that the progress needed is often a natural side-effect of switching between threads for latency hiding).

To be more precise. Pre-Volta GPU (like Pascal, Kepler, etc.) guarantee that once a thread block starts running on an SM it will run there to completion, i.e., its resources won't be freed until the kernel completes.

Warps of threads within a thread block are not guaranteed to run in any order, and there are no guarantees that, e.g., if warp A spins on a lock hold by warp B, that warp B will ever run and make process, and therefore the thread block might never run to completion.

Volta and later architectures guarantees forward progress in this case. That is, if a warp A spin locks on a lock held by warp B, Volta and later guarantee that warp B will make progress at some point, allowing it to release the lock so that warp A can make progress as well.