diff --git a/activesupport/lib/active_support/core_ext/date/calculations.rb b/activesupport/lib/active_support/core_ext/date/calculations.rb
index 2075d79bd1..34961f22e1 100644
--- a/activesupport/lib/active_support/core_ext/date/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/date/calculations.rb
@@ -17,7 +17,7 @@ class Date
     # If <tt>Date.beginning_of_week</tt> has not been set for the current request, returns the week start specified in <tt>config.beginning_of_week</tt>.
     # If no config.beginning_of_week was specified, returns :monday.
     def beginning_of_week
-      Thread.current[:beginning_of_week] || beginning_of_week_default || :monday
+      ::ActiveSupport::IsolatedExecutionState[:beginning_of_week] || beginning_of_week_default || :monday
     end
 
     # Sets <tt>Date.beginning_of_week</tt> to a week start (e.g. :monday) for current request/thread.
@@ -25,7 +25,7 @@ class Date
     # This method accepts any of the following day symbols:
     # :monday, :tuesday, :wednesday, :thursday, :friday, :saturday, :sunday
     def beginning_of_week=(week_start)
-      Thread.current[:beginning_of_week] = find_beginning_of_week!(week_start)
+      ::ActiveSupport::IsolatedExecutionState[:beginning_of_week] = find_beginning_of_week!(week_start)
     end
 
     # Returns week start day symbol (e.g. :monday), or raises an +ArgumentError+ for invalid day symbol.
diff --git a/activesupport/lib/active_support/core_ext/time/zones.rb b/activesupport/lib/active_support/core_ext/time/zones.rb
index 4443405a85..76d8639063 100644
--- a/activesupport/lib/active_support/core_ext/time/zones.rb
+++ b/activesupport/lib/active_support/core_ext/time/zones.rb
@@ -12,7 +12,7 @@ class Time
     # Returns the TimeZone for the current request, if this has been set (via Time.zone=).
     # If <tt>Time.zone</tt> has not been set for the current request, returns the TimeZone specified in <tt>config.time_zone</tt>.
     def zone
-      Thread.current[:time_zone] || zone_default
+      ::ActiveSupport::IsolatedExecutionState[:time_zone] || zone_default
     end
 
     # Sets <tt>Time.zone</tt> to a TimeZone object for the current request/thread.
@@ -39,7 +39,7 @@ class Time
     #     end
     #   end
     def zone=(time_zone)
-      Thread.current[:time_zone] = find_zone!(time_zone)
+      ::ActiveSupport::IsolatedExecutionState[:time_zone] = find_zone!(time_zone)
     end
 
     # Allows override of <tt>Time.zone</tt> locally inside supplied block;
diff --git a/activesupport/lib/active_support/execution_context.rb b/activesupport/lib/active_support/execution_context.rb
index 01d7742e9a..1c95188bae 100644
--- a/activesupport/lib/active_support/execution_context.rb
+++ b/activesupport/lib/active_support/execution_context.rb
@@ -46,7 +46,7 @@ module ActiveSupport
 
       private
         def store
-          Thread.current[:active_support_execution_context] ||= {}
+          IsolatedExecutionState[:active_support_execution_context] ||= {}
         end
     end
   end
diff --git a/activesupport/lib/active_support/execution_wrapper.rb b/activesupport/lib/active_support/execution_wrapper.rb
index 36e80b23c7..0c11087d26 100644
--- a/activesupport/lib/active_support/execution_wrapper.rb
+++ b/activesupport/lib/active_support/execution_wrapper.rb
@@ -113,11 +113,11 @@ module ActiveSupport
     self.active = Concurrent::Hash.new
 
     def self.active? # :nodoc:
-      @active[Thread.current]
+      @active[IsolatedExecutionState.unique_id]
     end
 
     def run! # :nodoc:
-      self.class.active[Thread.current] = true
+      self.class.active[IsolatedExecutionState.unique_id] = true
       run
     end
 
@@ -132,7 +132,7 @@ module ActiveSupport
     def complete!
       complete
     ensure
-      self.class.active.delete Thread.current
+      self.class.active.delete(IsolatedExecutionState.unique_id)
     end
 
     def complete # :nodoc:
diff --git a/activesupport/lib/active_support/isolated_execution_state.rb b/activesupport/lib/active_support/isolated_execution_state.rb
index 5d972a4acd..7e14b9b4f6 100644
--- a/activesupport/lib/active_support/isolated_execution_state.rb
+++ b/activesupport/lib/active_support/isolated_execution_state.rb
@@ -25,6 +25,10 @@ module ActiveSupport
         end
       end
 
+      def unique_id
+        self[:__id__] ||= Object.new
+      end
+
       def [](key)
         current[key]
       end
diff --git a/activesupport/lib/active_support/logger_thread_safe_level.rb b/activesupport/lib/active_support/logger_thread_safe_level.rb
index 4f860939e7..042f484f82 100644
--- a/activesupport/lib/active_support/logger_thread_safe_level.rb
+++ b/activesupport/lib/active_support/logger_thread_safe_level.rb
@@ -18,8 +18,7 @@ module ActiveSupport
     end
 
     def local_level
-      # Note: Thread#[] is fiber-local
-      Thread.current[:logger_thread_safe_level]
+      IsolatedExecutionState[:logger_thread_safe_level]
     end
 
     def local_level=(level)
@@ -31,7 +30,7 @@ module ActiveSupport
       else
         raise ArgumentError, "Invalid log level: #{level.inspect}"
       end
-      Thread.current[:logger_thread_safe_level] = level
+      IsolatedExecutionState[:logger_thread_safe_level] = level
     end
 
     def level
diff --git a/activesupport/lib/active_support/notifications/fanout.rb b/activesupport/lib/active_support/notifications/fanout.rb
index 4935a8ffcf..0759d3a086 100644
--- a/activesupport/lib/active_support/notifications/fanout.rb
+++ b/activesupport/lib/active_support/notifications/fanout.rb
@@ -226,12 +226,12 @@ module ActiveSupport
           end
 
           def start(name, id, payload)
-            timestack = Thread.current[:_timestack] ||= []
+            timestack = IsolatedExecutionState[:_timestack] ||= []
             timestack.push Time.now
           end
 
           def finish(name, id, payload)
-            timestack = Thread.current[:_timestack]
+            timestack = IsolatedExecutionState[:_timestack]
             started = timestack.pop
             @delegate.call(name, started, Time.now, id, payload)
           end
@@ -243,12 +243,12 @@ module ActiveSupport
           end
 
           def start(name, id, payload)
-            timestack = Thread.current[:_timestack_monotonic] ||= []
+            timestack = IsolatedExecutionState[:_timestack_monotonic] ||= []
             timestack.push Process.clock_gettime(Process::CLOCK_MONOTONIC)
           end
 
           def finish(name, id, payload)
-            timestack = Thread.current[:_timestack_monotonic]
+            timestack = IsolatedExecutionState[:_timestack_monotonic]
             started = timestack.pop
             @delegate.call(name, started, Process.clock_gettime(Process::CLOCK_MONOTONIC), id, payload)
           end
@@ -256,14 +256,14 @@ module ActiveSupport
 
         class EventObject < Evented
           def start(name, id, payload)
-            stack = Thread.current[:_event_stack] ||= []
+            stack = IsolatedExecutionState[:_event_stack] ||= []
             event = build_event name, id, payload
             event.start!
             stack.push event
           end
 
           def finish(name, id, payload)
-            stack = Thread.current[:_event_stack]
+            stack = IsolatedExecutionState[:_event_stack]
             event = stack.pop
             event.payload = payload
             event.finish!
diff --git a/activesupport/lib/active_support/tagged_logging.rb b/activesupport/lib/active_support/tagged_logging.rb
index 0bc4a21401..26852e590a 100644
--- a/activesupport/lib/active_support/tagged_logging.rb
+++ b/activesupport/lib/active_support/tagged_logging.rb
@@ -57,7 +57,7 @@ module ActiveSupport
       def current_tags
         # We use our object ID here to avoid conflicting with other instances
         thread_key = @thread_key ||= "activesupport_tagged_logging_tags:#{object_id}"
-        Thread.current[thread_key] ||= []
+        IsolatedExecutionState[thread_key] ||= []
       end
 
       def tags_text
diff --git a/activesupport/lib/active_support/xml_mini.rb b/activesupport/lib/active_support/xml_mini.rb
index 1930c417fd..e2b52daa65 100644
--- a/activesupport/lib/active_support/xml_mini.rb
+++ b/activesupport/lib/active_support/xml_mini.rb
@@ -181,11 +181,11 @@ module ActiveSupport
       end
 
       def current_thread_backend
-        Thread.current[:xml_mini_backend]
+        IsolatedExecutionState[:xml_mini_backend]
       end
 
       def current_thread_backend=(name)
-        Thread.current[:xml_mini_backend] = name && cast_backend_name_to_module(name)
+        IsolatedExecutionState[:xml_mini_backend] = name && cast_backend_name_to_module(name)
       end
 
       def cast_backend_name_to_module(name)
diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index cb28444bba..c328234a39 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -1205,13 +1205,11 @@ class TimeWithZoneMethodsForTimeAndDateTimeTest < ActiveSupport::TestCase
 
   def test_time_zone_setter_is_thread_safe
     Time.use_zone "Paris" do
-      t1 = Thread.new { Time.zone = "Alaska" }.join
-      t2 = Thread.new { Time.zone = "Hawaii" }.join
-      assert t1.stop?, "Thread 1 did not finish running"
-      assert t2.stop?, "Thread 2 did not finish running"
+      t1 = Thread.new { Time.zone = "Alaska"; Time.zone }
+      t2 = Thread.new { Time.zone = "Hawaii"; Time.zone }
       assert_equal ActiveSupport::TimeZone["Paris"], Time.zone
-      assert_equal ActiveSupport::TimeZone["Alaska"], t1[:time_zone]
-      assert_equal ActiveSupport::TimeZone["Hawaii"], t2[:time_zone]
+      assert_equal ActiveSupport::TimeZone["Alaska"], t1.value
+      assert_equal ActiveSupport::TimeZone["Hawaii"], t2.value
     end
   end
 
diff --git a/activesupport/test/logger_test.rb b/activesupport/test/logger_test.rb
index 23e77ad270..2461b48bc6 100644
--- a/activesupport/test/logger_test.rb
+++ b/activesupport/test/logger_test.rb
@@ -272,6 +272,9 @@ class LoggerTest < ActiveSupport::TestCase
   end
 
   def test_logger_level_main_fiber_safety
+    previous_isolation_level = ActiveSupport::IsolatedExecutionState.isolation_level
+    ActiveSupport::IsolatedExecutionState.isolation_level = :fiber
+
     @logger.level = Logger::INFO
     assert_level(Logger::INFO)
 
@@ -283,9 +286,14 @@ class LoggerTest < ActiveSupport::TestCase
       assert_level(Logger::ERROR)
       fiber.resume
     end
+  ensure
+    ActiveSupport::IsolatedExecutionState.isolation_level = previous_isolation_level
   end
 
   def test_logger_level_local_fiber_safety
+    previous_isolation_level = ActiveSupport::IsolatedExecutionState.isolation_level
+    ActiveSupport::IsolatedExecutionState.isolation_level = :fiber
+
     @logger.level = Logger::INFO
     assert_level(Logger::INFO)
 
@@ -313,6 +321,25 @@ class LoggerTest < ActiveSupport::TestCase
     end.resume
 
     assert_level(Logger::INFO)
+  ensure
+    ActiveSupport::IsolatedExecutionState.isolation_level = previous_isolation_level
+  end
+
+  def test_logger_level_thread_safety
+    previous_isolation_level = ActiveSupport::IsolatedExecutionState.isolation_level
+    ActiveSupport::IsolatedExecutionState.isolation_level = :thread
+
+    @logger.level = Logger::INFO
+    assert_level(Logger::INFO)
+
+    enumerator = Enumerator.new do |yielder|
+      @logger.level = Logger::DEBUG
+      yielder.yield @logger.level
+    end
+    assert_equal Logger::DEBUG, enumerator.next
+    assert_level(Logger::DEBUG)
+  ensure
+    ActiveSupport::IsolatedExecutionState.isolation_level = previous_isolation_level
   end
 
   def test_temporarily_logging_at_a_noisier_level