ReadWriteLock donated by @alexdowad

CHANGELOG.md

@@ -15,6 +15,7 @@
 * Deprecated all clock-time based timer scheduling
   - Only support scheduling by delay
   - Effects `Concurrent.timer`, `TimerSet`, and `ScheduledTask`
+* Added new `ReadWriteLock` class
 * Consistent `at_exit` behavior for Java and Ruby thread pools.
 * Added `at_exit` handler to Ruby thread pools (already in Java thread pools)
   - Ruby handler stores the object id and retrieves from `ObjectSpace`

README.md

@@ -89,6 +89,7 @@ This library contains a variety of concurrency abstractions at high and low leve
 * [M-Structures](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/MVar.html) (MVar)
 * [Thread-local variables](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/ThreadLocalVar.html)
 * [Software transactional memory](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/TVar.html) (TVar)
+* [ReadWriteLock](http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/ReadWriteLock.html)
 
 ## Usage
 

examples/benchmark_read_write_lock.rb

@@ -0,0 +1,139 @@
+#!/usr/bin/env ruby
+
+$:.push File.join(File.dirname(__FILE__), '../lib')
+
+require 'concurrent/atomic/read_write_lock'
+require 'benchmark'
+require 'optparse'
+require 'ostruct'
+
+$options = OpenStruct.new
+$options.threads = 100
+$options.interleave = false
+$options.compare = false
+
+OptionParser.new do |opts|
+  opts.banner = "Usage: #{File.basename(__FILE__)} [options]"
+
+  opts.on('-t', '--threads=THREADS', OptionParser::DecimalInteger, "Number of threads per test (default #{$options.threads})") do |value|
+    $options.threads = value
+  end
+
+  opts.on('-i', '--[no-]interleave', 'Interleave output to check for starvation') do |value|
+    $options.interleave = value
+  end
+
+  opts.on('-c', '--[no-]compare', 'Compare with other implementations') do |value|
+    $options.compare = value
+  end
+
+  opts.on('-h', '--help', 'Prints this help') do
+    puts opts
+    exit
+  end
+end.parse!
+
+def jruby?
+  defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby"
+end
+
+# for performance comparison with ReadWriteLock
+class SimpleMutex
+  def initialize; @mutex = Mutex.new; end
+  def with_read_lock
+    @mutex.synchronize { yield }
+  end
+  alias :with_write_lock :with_read_lock
+end
+
+# for seeing whether my correctness test is doing anything...
+# and for seeing how great the overhead of the test is
+# (apart from the cost of locking)
+class FreeAndEasy
+  def with_read_lock
+    yield # thread safety is for the birds... I prefer to live dangerously
+  end
+  alias :with_write_lock :with_read_lock
+end
+
+if jruby?
+  # the Java platform comes with a read-write lock implementation
+  # performance is very close to ReadWriteLock, but just a *bit* slower
+  require 'java'
+  class JavaReadWriteLock
+    def initialize
+      @lock = java.util.concurrent.locks.ReentrantReadWriteLock.new
+    end
+    def with_read_lock
+      @lock.read_lock.lock
+      result = yield
+      @lock.read_lock.unlock
+      result
+    end
+    def with_write_lock
+      @lock.write_lock.lock
+      result = yield
+      @lock.write_lock.unlock
+      result
+    end
+  end
+end
+
+def test(lock)
+  puts "READ INTENSIVE (80% read, 20% write):"
+  single_test(lock, ($options.threads * 0.8).floor, ($options.threads * 0.2).floor)
+  puts "WRITE INTENSIVE (80% write, 20% read):"
+  single_test(lock, ($options.threads * 0.2).floor, ($options.threads * 0.8).floor)
+  puts "BALANCED (50% read, 50% write):"
+  single_test(lock, ($options.threads * 0.5).floor, ($options.threads * 0.5).floor)
+end
+
+def single_test(lock, n_readers, n_writers, reader_iterations=50, writer_iterations=50, reader_sleep=0.001, writer_sleep=0.001)
+  puts "Testing #{lock.class} with #{n_readers} readers and #{n_writers} writers. Readers iterate #{reader_iterations} times, sleeping #{reader_sleep}s each time, writers iterate #{writer_iterations} times, sleeping #{writer_sleep}s each time"
+  mutex = Mutex.new
+  bad   = false
+  data  = 0
+
+  result = Benchmark.measure do
+    readers = n_readers.times.collect do
+      Thread.new do
+        reader_iterations.times do
+          lock.with_read_lock do
+            print "r" if $options.interleave
+            mutex.synchronize { bad = true } if (data % 2) != 0
+            sleep(reader_sleep)
+            mutex.synchronize { bad = true } if (data % 2) != 0
+          end
+        end
+      end
+    end
+    writers = n_writers.times.collect do
+      Thread.new do
+        writer_iterations.times do
+          lock.with_write_lock do
+            print "w" if $options.interleave
+            # invariant: other threads should NEVER see "data" as an odd number
+            value = (data += 1)
+            # if a reader runs right now, this invariant will be violated
+            sleep(writer_sleep)
+            # this looks like a strange way to increment twice;
+            # it's designed so that if 2 writers run at the same time, at least
+            #   one increment will be lost, and we can detect that at the end
+            data  = value+1 
+          end
+        end
+      end
+    end
+
+    readers.each { |t| t.join }
+    writers.each { |t| t.join }
+    puts "BAD!!! Readers+writers overlapped!" if mutex.synchronize { bad }
+    puts "BAD!!! Writers overlapped!" if data != (n_writers * writer_iterations * 2)
+  end
+  puts result
+end
+
+test(Concurrent::ReadWriteLock.new)
+test(JavaReadWriteLock.new) if $options.compare && jruby?
+test(SimpleMutex.new) if $options.compare
+test(FreeAndEasy.new) if $options.compare