implement a webassembly allocator that handles multi-threading

[chung-leong]

Zig Version

0.14.0-dev.3050+d72f3d353

Steps to Reproduce and Observed Behavior

Ever since this commit wasi threads have basically been disabled since we can't create an allocator.

We can make the WasAllocator thread-safe simply by adding a spinlock:

var spinlock = false;

fn acquireLock() void {
    if (!builtin.single_threaded) {
        while (@cmpxchgWeak(bool, &spinlock, false, true, .acquire, .monotonic) != null) {}
    }
}

fn releaseLock() void {
    if (!builtin.single_threaded) {
        @atomicStore(bool, &spinlock, false, .release);
    }
}

fn alloc(ctx: *anyopaque, len: usize, log2_align: u8, return_address: usize) ?[*]u8 {
    _ = ctx;
    _ = return_address;
    acquireLock();
    defer releaseLock();
    // ...
}

fn free(
    ctx: *anyopaque,
    buf: []u8,
    log2_buf_align: u8,
    return_address: usize,
) void {
    _ = ctx;
    _ = return_address;
    acquireLock();
    defer releaseLock();
    // ...
}

That should protect the allocator state from concurrent modifications. Mutex cannot be used here since the main thread of browsers doesn't allow synchronous wait.

I've put together a demo showing the spinlock approach in the browser. With 16 threads continually allocating and freeing, 100 alloc/free by the main thread doesn't seem to block for too long. The UI remains responsive, which is really the main concern.

It'll be great if we can have functional WebAssembly threads in 0.14.0. Convenient way to show people what Zig is capable of.

Expected Behavior

No comptime error.

[andrewrk]

This is not a bug, it is a feature request. That allocator never worked under multithreaded conditions.

I will not accept a spinlock-based solution.

[chung-leong]

Would a solution where only the main thread spins be acceptable? Then amount of spinning can be minimized by giving the main thread priority over other threads.