Cgo deadlock with C.usleep()

main.go

package main

/*
#include <unistd.h>
*/
import "C"
import (
	"log"
	"time"
)

// TaskMgr ...
type TaskMgr struct {
	NumRoutines int
	Count       int
	Interval    time.Duration
	done        []chan struct{}
}

// Start ...
func (t *TaskMgr) Start() error {
	t.done = make([]chan struct{}, t.NumRoutines)
	for i := 0; i < t.NumRoutines; i++ {
		t.done[i] = make(chan struct{}, 1)
		go func(taskID, count int, interval time.Duration, done chan<- struct{}) {
			for j := 0; j < count; j++ {
				//log.Printf("task %d: count %d", taskID, j)
				time.Sleep(interval)
			}
			log.Printf("task %d: complete", taskID)
			close(done)
		}(i, t.Count, t.Interval, t.done[i])
	}
	return nil
}

// IsAllDone ...
func (t *TaskMgr) IsAllDone() bool {
	allDone := true
loop:
	for i := 0; i < t.NumRoutines; i++ {
		select {
		case <-t.done[i]:
			log.Printf("task %d is DONE!", i)
		default:
			allDone = false
			break loop
		}
	}
	return allDone
}

func main() {
	t := &TaskMgr{
		NumRoutines: 4,
		Count:       50,
		Interval:    100 * time.Millisecond,
	}

	if err := t.Start(); err != nil {
		panic(err)
	}

	for {
		if t.IsAllDone() {
			break
		}

		//time.Sleep(50 * time.Microsecond)
		C.usleep(50)
	}
}

Deadlock occurred with the above code on an EC2 t2.micro instance.