MRI-GIL Context switch examples from this talk: https://www.youtube.com/watch?v=1nNfTWHF2YY

example.rb

@bank_account = 0

# WORKS

# 100.times.map do
#   Thread.new do
#     10_000.times do
#       value = @bank_account
#       value = value + 1
#       @bank_account = value
#     end
#   end
# end.each(&:join)

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

# RACE CONDITION BECAUSE MRI MAKES A CONTEXT SWITCH WHEN CALLING A METHOD

# def read_from_bank_account
#   @bank_account
# end

# def write_bank_account(value)
#   @bank_account = value
# end

# 100.times.map do
#   Thread.new do
#     10_000.times do
#       value = read_from_bank_account
#       value = value + 1
#       write_bank_account(value)
#     end
#   end
# end.each(&:join)

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

# WORKS

# 100.times.map do
#   Thread.new do
#     10_000.times do
#       value = @bank_account
#       value = value + 1 if true
#       @bank_account = value
#     end
#   end
# end.each(&:join)

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

# RACE CONDITION USING RUBY 2.3.1. YOU MAY NEED TO RUN THIS SCRIPT SEVERAL TIME UNTIL YOU GET AN ERROR
# THIS WORKS ON RUBY 2.4 BECAUSE OF AN OPTIMIZATION OF UNREACHABLE CODE
# CHANGING THE UNLESS WITH: some_variable = false; unless some_variable && false THE RACE CONDITION APPEARS AGAIN

# 100.times.map do
#   Thread.new do
#     10_000.times do
#       value = @bank_account
#       value = value + 1 unless false
#       @bank_account = value
#     end
#   end
# end.each(&:join)

print @bank_account
puts @bank_account == 1_000_000 ? "\e[32m CORRECT \e[0m" : "\e[31m ERROR \e[0m"

Hey
Thanks for taking interest in my talk, I accidentally came across this gist. Just in case here is the full script&codes of the talk: https://github.com/DanielVartanov/rubyconf-2017-talk-what-does-gil-really-guarantee-you