Use Mutex to avoid involuntary context switch in Ruby.

ruby_gil_mutex.rb

def send_money(amount)
  puts "Sending $#{amount}"
  sleep 1  # Simulate network call sending of money
end

lock = Mutex.new
threads = []
money_is_sent = false

2.times do
  th = Thread.new do
    lock.synchronize {              # DO NOT USE 'Mutex.new.synchronize'
      unless money_is_sent
        send_money 10
        money_is_sent = true
      end  
    }
  end
  threads << th
end


ThWait.all_waits(*threads)