gistfile1.rb

require 'thread'

mutex = Mutex.new
i = 0

t1 = Thread.new do
  1_000_000.times do
    mutex.synchronize do
      i += 1
    end
  end
end

t2 = Thread.new do
  1_000_000.times do
    mutex.synchronize do
      i += 1
    end
  end
end

t1.join
t2.join

puts i

#####################

➜  /tmp  ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin11.2.0]
➜  /tmp  time ruby count.rb
2000000
ruby count.rb  0.72s user 0.03s system 97% cpu 0.771 total

➜  /tmp  ruby -v
jruby 1.6.5 (ruby-1.8.7-p330) (2011-10-25 9dcd388) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_26) [darwin-x86_64-java]
➜  /tmp  time ruby count.rb   
2000000
ruby count.rb  2.72s user 1.24s system 146% cpu 2.694 total

➜  /tmp  ruby -v
rubinius 2.0.0dev (1.8.7 acf6926c yyyy-mm-dd JI) [x86_64-apple-darwin11.2.0]
➜  /tmp  time ruby count.rb 
2000000
ruby count.rb  6.62s user 8.95s system 109% cpu 14.179 total

Oh, last note on this.

Keep in mind that on all the implementations, you're talking about overhead in the µsec range for locking the mutex. I doubt you'll see that matter in any real-world app, especially when the block contains something more substantial than incrementing a Fixnum :)