Ruby Threads Concurrency Issue using Mutex

Question

require 'thread'

mutex = Mutex.new


shared_resource = 0

  for i in 0..10000
    Thread.new do
      mutex.synchronize { shared_resource += 1 }
    end
    Thread.new do
      mutex.synchronize { shared_resource -= 1 }
    end
  end

puts shared_resource

I am trying to run this Ruby program to test mutex.synchronize and get 0 on every run but everytime I run this program It gives me random value. Need help with solving this to get 0 and help me understand how to use Mutex and lock in Ruby

Answer 1

Your issue is not the mutex, since synchronize will lock it during block execution. Your issue is that the thread execution has not completed when you reach puts shared_resource.

To solve this you need to ensure that all the threads have completed by calling Thread#join for example:

require 'thread'

mutex = Mutex.new

shared_resource = 0
threads = []

400.times do |i|
  threads << Thread.new do
    mutex.synchronize {shared_resource += 1}
  end
  threads << Thread.new do
    mutex.synchronize {shared_resource -= 1}
  end
end

puts shared_resource
#=> some number

# see how many threads are still running 
puts threads.map(&:status)

threads.map(&:join)

puts shared_resource
#=> 0