While new sharers are blocked, an existing sharer remains re-entrant

activesupport/lib/active_support/concurrency/share_lock.rb

@@ -85,7 +85,7 @@ def stop_exclusive
 
       def start_sharing(purpose: :share)
         synchronize do
-          if busy_for_sharing?(purpose)
+          if @sharing[Thread.current] == 0 && @exclusive_thread != Thread.current && busy_for_sharing?(purpose)
             @cv.wait_while { busy_for_sharing?(purpose) }
           end
           @sharing[Thread.current] += 1

activesupport/test/share_lock_test.rb

@@ -241,6 +241,35 @@ def test_new_share_attempts_block_on_waiting_exclusive
     end
   end
 
+  def test_share_remains_reentrant_ignoring_a_waiting_exclusive
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      ready = Concurrent::CyclicBarrier.new(2)
+      attempt_reentrancy = Concurrent::CountDownLatch.new
+
+      sharer = Thread.new do
+        @lock.sharing do
+          ready.wait
+          attempt_reentrancy.wait
+          @lock.sharing {}
+        end
+      end
+
+      exclusive = Thread.new do
+        @lock.sharing do
+          ready.wait
+          @lock.exclusive {}
+        end
+      end
+
+      assert_threads_stuck exclusive
+
+      attempt_reentrancy.count_down
+
+      assert_threads_not_stuck sharer
+      assert_threads_stuck exclusive
+    end
+  end
+
   def test_in_shared_section_incompatible_non_upgrading_threads_cannot_preempt_upgrading_threads
     scratch_pad       = []
     scratch_pad_mutex = Mutex.new