webgpu.h implementations are allowed to require that all returned objects, except for WGPUInstance, can only be used from the thread they were created on, causing undefined behavior otherwise.

Where multithreading is supported:

The implementation must provide the documented Thread Safety guarantees.
All objects must be freely usable on any thread at any time, except:
- Where APIs are specified as being non-thread-safe, they must mutexed.
- The exceptions for Wasm, below: Multithreading In WebAssembly.
State must be thread-global, except where otherwise defined.
- For example, buffer map state is thread-global, and mapped memory ranges may be used from any thread.
- Note: Error scope state is thread-local. See Error Scopes.

Native (non-Wasm) implementations should support multithreading.

Multithreading In WebAssembly

At the time of this writing, the JS WebGPU API is not multithreading-capable. Initial webgpu.h implementations will not be multithreading-capable.

Wasm-on-JS implementations may implement multithreading by proxying calls to a single JS "thread", as long as the C API behavior is conformant.

Once the JS API is multithreading-capable, there are still expected to be some thread-local behaviors:

Buffer mapping state may be thread-local.
- Implementations should make this state thread-global if feasible (e.g. by proxying all mapping calls to a single JS thread).
Any object which is non-shareable or non-transferable in JS may also be fully or partially non-shareable or non-transferable in Wasm (e.g. encoder objects may be thread-locked, similar to how they are not thread safe in webgpu.h in general). (In Rust terms, shareable = Send+Sync, transferable = Send.)
- Implementations may make them shareable. (For example, proxying all encoder methods to a single thread would make them fully shareable but be inefficient; alternatively, encoder commands can be buffered, which might enable some multithreaded use-cases even if Finish() still must happen on the thread that created the encoder.)

Thread Safety

The webgpu.h API is thread-safe (when multithreading is supported). That is, its functions are reentrant and may be called during the execution of other functions, with the following exceptions:

Encoder objects (WGPUCommandEncoder, WGPUComputePassEncoder, WGPURenderPassEncoder, and WGPURenderBundleEncoder) are not thread-safe. Note these objects appear only in their own methods.
- Additionally, in Wasm, these objects may be thread-locked, as discussed above.
API calls may not be made during WGPUCallbackMode_AllowSpontaneous callbacks. See Callback Reentrancy.

The following are thread safe: