diff --git a/activesupport/test/share_lock_test.rb b/activesupport/test/share_lock_test.rb
new file mode 100644
index 0000000000000..fd50516581db4
--- /dev/null
+++ b/activesupport/test/share_lock_test.rb
@@ -0,0 +1,194 @@
+require 'abstract_unit'
+require 'concurrent/atomics'
+require 'active_support/concurrency/share_lock'
+
+class ShareLockTest < ActiveSupport::TestCase
+  def setup
+    @lock = ActiveSupport::Concurrency::ShareLock.new
+  end
+
+  def test_sharing_doesnt_block
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_latch|
+      assert_threads_not_stuck(Thread.new {@lock.sharing {} })
+    end
+  end
+
+  def test_sharing_blocks_exclusive
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      @lock.exclusive(no_wait: true) { flunk } # polling should fail
+      exclusive_thread = Thread.new { @lock.exclusive {} }
+      assert_threads_stuck_but_releasable_by_latch exclusive_thread, sharing_thread_release_latch
+    end
+  end
+
+  def test_exclusive_blocks_sharing
+    with_thread_waiting_in_lock_section(:exclusive) do |exclusive_thread_release_latch|
+      sharing_thread = Thread.new { @lock.sharing {} }
+      assert_threads_stuck_but_releasable_by_latch sharing_thread, exclusive_thread_release_latch
+    end
+  end
+
+  def test_multiple_exlusives_are_able_to_progress
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      exclusive_threads = (1..2).map do
+        Thread.new do
+          @lock.exclusive {}
+        end
+      end
+
+      assert_threads_stuck_but_releasable_by_latch exclusive_threads, sharing_thread_release_latch
+    end
+  end
+
+  def test_sharing_is_upgradeable_to_exclusive
+    upgrading_thread = Thread.new do
+      @lock.sharing do
+        @lock.exclusive {}
+      end
+    end
+    assert_threads_not_stuck upgrading_thread
+  end
+
+  def test_exclusive_upgrade_waits_for_other_sharers_to_leave
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      in_sharing = Concurrent::CountDownLatch.new
+
+      upgrading_thread = Thread.new do
+        @lock.sharing do
+          in_sharing.count_down
+          @lock.exclusive {}
+        end
+      end
+
+      in_sharing.wait
+      assert_threads_stuck_but_releasable_by_latch upgrading_thread, sharing_thread_release_latch
+    end
+  end
+
+  def test_exclusive_matching_purpose
+    [true, false].each do |use_upgrading|
+      with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+        exclusive_threads = (1..2).map do
+          Thread.new do
+            @lock.send(use_upgrading ? :sharing : :tap) do
+              @lock.exclusive(purpose: :load, compatible: [:load, :unload]) {}
+            end
+          end
+        end
+
+        assert_threads_stuck_but_releasable_by_latch exclusive_threads, sharing_thread_release_latch
+      end
+    end
+  end
+
+  def test_exclusive_conflicting_purpose
+    [true, false].each do |use_upgrading|
+      with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+        begin
+          conflicting_exclusive_threads = [
+            Thread.new do
+              @lock.send(use_upgrading ? :sharing : :tap) do
+                @lock.exclusive(purpose: :load, compatible: [:load]) {}
+              end
+            end,
+            Thread.new do
+              @lock.send(use_upgrading ? :sharing : :tap) do
+                @lock.exclusive(purpose: :unload, compatible: [:unload]) {}
+              end
+            end
+          ]
+
+          assert_threads_stuck conflicting_exclusive_threads # wait for threads to get into their respective `exclusive {}` blocks
+          sharing_thread_release_latch.count_down
+          assert_threads_stuck conflicting_exclusive_threads # assert they are stuck
+
+          no_purpose_thread = Thread.new do
+            @lock.exclusive {}
+          end
+          assert_threads_not_stuck no_purpose_thread # no purpose thread is able to squeak through
+
+          compatible_thread = Thread.new do
+            @lock.exclusive(purpose: :load, compatible: [:load, :unload])
+          end
+
+          assert_threads_not_stuck compatible_thread # compatible thread is able to squeak through
+          assert_threads_stuck conflicting_exclusive_threads # assert other threads are still stuck
+        ensure
+          conflicting_exclusive_threads.each(&:kill)
+        end
+      end
+    end
+  end
+
+  def test_exclusive_ordering
+    [true, false].each do |use_upgrading|
+      scratch_pad       = []
+      scratch_pad_mutex = Mutex.new
+
+      load_params   = [:load,   [:load]]
+      unload_params = [:unload, [:unload, :load]]
+
+      [load_params, load_params, unload_params, unload_params].permutation do |thread_params|
+        with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+          threads = thread_params.map do |purpose, compatible|
+            Thread.new do
+              @lock.send(use_upgrading ? :sharing : :tap) do
+                @lock.exclusive(purpose: purpose, compatible: compatible) do
+                  scratch_pad_mutex.synchronize { scratch_pad << purpose }
+                end
+              end
+            end
+          end
+
+          sleep(0.01)
+          scratch_pad_mutex.synchronize { assert_empty scratch_pad }
+
+          sharing_thread_release_latch.count_down
+
+          assert_threads_not_stuck threads
+          scratch_pad_mutex.synchronize do
+            assert_equal [:load, :load, :unload, :unload], scratch_pad
+            scratch_pad.clear
+          end
+        end
+      end
+    end
+  end
+
+  private
+  SUFFICIENT_TIMEOUT = 0.2
+
+  def assert_threads_stuck_but_releasable_by_latch(threads, latch)
+    assert_threads_stuck threads
+    latch.count_down
+    assert_threads_not_stuck threads
+  end
+
+  def assert_threads_stuck(threads)
+    sleep(SUFFICIENT_TIMEOUT) # give threads time to do their business
+    assert(Array(threads).all? {|t| t.join(0.001).nil?})
+  end
+
+  def assert_threads_not_stuck(threads)
+    assert_not_nil(Array(threads).all? {|t| t.join(SUFFICIENT_TIMEOUT)})
+  end
+
+  def with_thread_waiting_in_lock_section(lock_section)
+    in_section      = Concurrent::CountDownLatch.new
+    section_release = Concurrent::CountDownLatch.new
+
+    stuck_thread = Thread.new do
+      @lock.send(lock_section) do
+        in_section.count_down
+        section_release.wait
+      end
+    end
+
+    in_section.wait
+
+    yield section_release
+  ensure
+    section_release.count_down
+    stuck_thread.join # clean up
+  end
+end