If rspec hangs with no output and you dont get a backtrace neither with --backtrace nor by just killing it with crtl-c,
you can put the following in your spec/spec_helper.rb:

puts "rspec pid: #{Process.pid}"

trap 'USR1' do
  threads = Thread.list

  puts
  puts "=" * 80
  puts "Received USR1 signal; printing all #{threads.count} thread backtraces."

  threads.each do |thr|
    description = thr == Thread.main ? "Main thread" : thr.inspect
    puts
    puts "#{description} backtrace: "
    puts thr.backtrace.join("\n")
  end

  puts "=" * 80
end

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then kill the process with kill -USR1 <the pid> and you get the backtrace.

(source: https://github.com/rspec/rspec-rails/issues/1353#issuecomment-93173691 Show archive.org snapshot )