How does Go preempt goroutines in windows?

Question

I read that goroutines are now preemptible. The preemption is done via a sysmon goroutine that sends stop signals to goroutines which have used up their time slice. On posix systems, I believe this is done through pthread_kill. My question is: how does this work in windows since windows doesn't support thread signaling? I had assumed that the go runtime may have been using a posix thread library like pthreads4w, however I just saw that even in pthreads4w, pthread_kill doesn't support sending signals.

Answer 1

The comments in runtime/preempt.go give an overview of how preemption works in the runtime. Specifically to do with asynchronous preemption:

Preemption at asynchronous safe-points is implemented by suspending the thread using an OS mechanism (e.g., signals) and inspecting its state to determine if the goroutine was at an asynchronous safe-point.

So how does async preemption work on windows? As mentioned in the original proposal for non-cooperative preemption of goroutines:

Other considerations

... signaled preemption is quite easy to support in Windows because it provides SuspendThread and GetThreadContext ...

The Windows SuspendThread function can be used to suspend a thread by its handle, and GetThreadContext can be used to get the processor state of the thread. The specific usages of these functions are implemented in runtime/os_windows.go