Deadlock in concurrent teardown and Workaround

[elbaro]

This is technically a bug in NVIDIA driver but posting here for info.

Bug

wgpu::Instance::default() creates Vulkan + GL/EGL backends.
The NVIDIA implementation of two backends share multiple mutex locks, and concurrent teardown of two backends have a deadlock cycle.

pub struct Gpu {
    pub instance: wgpu::Instance,
    pub device: wgpu::Device,
    ...
}

fn run_per_thread() {
  let gpu = Gpu { ... };
  ... // drop of Gpu calls vkDestroyDevice / eglDestroyContext
}

A simple fix is to manually specifying only one backend.

       -        let instance = wgpu::Instance::default();

       +        let instance = wgpu::Instance::new(&wgpu::InstanceDescriptor {
       +            backends: wgpu::Backends::VULKAN,
       +            ..Default::default()
       +        });

Even with one backend, I guess Thread1 and Thread2 using different backends can cause deadlock.

Explanation

Vulkan backend and EGL backend acquire multiple internal mutexes.

I attached GDB to hung threads and the AI-assisted stacktrace is like the below:

  User Thread A (eglDestroyContext)
    → libEGL_nvidia.so
      → libnvidia-eglcore.so                           ← acquires mutex 1
        → libnvidia-glsi.so
          → pthread_mutex_lock(mutex 2)                ← BLOCKED (held by Thread B)

  User Thread B (vkDestroyDevice)
    → libvulkan.so (loader_layer_destroy_device)
      → libnvidia-eglcore.so                           ← acquires mutex 2
        → pthread_clockjoin_ex(worker W)               ← BLOCKED (W won't exit)

  Driver Thread W (TLS destructor exit path)
    → libnvidia-eglcore.so
      → pthread_mutex_lock(mutex 1)                    ← BLOCKED (held by Thread A)

If wgpu has a control, wgpu can add a global lock to serialize the backend teardowns within a process.

Platform
Linux 6.12.69 + NVIDIA open driver 580.119.02 + wgpu 27.0.1 with features = ["webgl"]

[inner-daemons]

This definitely belongs in an issue rather than a discussion. But I'd have to think further about if it makes sense.