Make Monitor a core class

[Feature #21788]

It allows monitor to access internal routines and remove some overhead.

Before:

```
ruby 4.0.0dev (2025-12-13T04:52:13Z master 71dd272506) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     2.111M i/100ms
             Monitor     1.736M i/100ms
Calculating -------------------------------------
               Mutex     25.050M (± 0.4%) i/s   (39.92 ns/i) -    126.631M in   5.055208s
             Monitor     19.809M (± 0.1%) i/s   (50.48 ns/i) -    100.672M in   5.082015s
```

After:

```
ruby 4.0.0dev (2025-12-13T06:49:18Z core-monitor 6fabf389fd) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     2.144M i/100ms
             Monitor     1.859M i/100ms
Calculating -------------------------------------
               Mutex     24.771M (± 0.4%) i/s   (40.37 ns/i) -    124.342M in   5.019716s
             Monitor     23.722M (± 0.4%) i/s   (42.15 ns/i) -    118.998M in   5.016361s
```

Bench:

```ruby
require 'bundler/inline'

gemfile do
  gem "benchmark-ips"
end

mutex = Mutex.new
require "monitor"
monitor = Monitor.new

Benchmark.ips do |x|
  x.report("Mutex") { mutex.synchronize { } }
  x.report("Monitor") { monitor.synchronize { } }
end
```

7 files changed, 455 insertions, 546 deletions

diff --git a/ext/monitor/depend b/ext/monitor/depend
deleted file mode 100644
index 0c7d54afc8..0000000000
--- a/ext/monitor/depend
+++ /dev/null
@@ -1,162 +0,0 @@
-# AUTOGENERATED DEPENDENCIES START
-monitor.o: $(RUBY_EXTCONF_H)
-monitor.o: $(arch_hdrdir)/ruby/config.h
-monitor.o: $(hdrdir)/ruby/assert.h
-monitor.o: $(hdrdir)/ruby/backward.h
-monitor.o: $(hdrdir)/ruby/backward/2/assume.h
-monitor.o: $(hdrdir)/ruby/backward/2/attributes.h
-monitor.o: $(hdrdir)/ruby/backward/2/bool.h
-monitor.o: $(hdrdir)/ruby/backward/2/inttypes.h
-monitor.o: $(hdrdir)/ruby/backward/2/limits.h
-monitor.o: $(hdrdir)/ruby/backward/2/long_long.h
-monitor.o: $(hdrdir)/ruby/backward/2/stdalign.h
-monitor.o: $(hdrdir)/ruby/backward/2/stdarg.h
-monitor.o: $(hdrdir)/ruby/defines.h
-monitor.o: $(hdrdir)/ruby/intern.h
-monitor.o: $(hdrdir)/ruby/internal/abi.h
-monitor.o: $(hdrdir)/ruby/internal/anyargs.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/char.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/double.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/fixnum.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/gid_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/int.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/intptr_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/long.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/long_long.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/mode_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/off_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/pid_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/short.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/size_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/st_data_t.h
-monitor.o: $(hdrdir)/ruby/internal/arithmetic/uid_t.h
-monitor.o: $(hdrdir)/ruby/internal/assume.h
-monitor.o: $(hdrdir)/ruby/internal/attr/alloc_size.h
-monitor.o: $(hdrdir)/ruby/internal/attr/artificial.h
-monitor.o: $(hdrdir)/ruby/internal/attr/cold.h
-monitor.o: $(hdrdir)/ruby/internal/attr/const.h
-monitor.o: $(hdrdir)/ruby/internal/attr/constexpr.h
-monitor.o: $(hdrdir)/ruby/internal/attr/deprecated.h
-monitor.o: $(hdrdir)/ruby/internal/attr/diagnose_if.h
-monitor.o: $(hdrdir)/ruby/internal/attr/enum_extensibility.h
-monitor.o: $(hdrdir)/ruby/internal/attr/error.h
-monitor.o: $(hdrdir)/ruby/internal/attr/flag_enum.h
-monitor.o: $(hdrdir)/ruby/internal/attr/forceinline.h
-monitor.o: $(hdrdir)/ruby/internal/attr/format.h
-monitor.o: $(hdrdir)/ruby/internal/attr/maybe_unused.h
-monitor.o: $(hdrdir)/ruby/internal/attr/noalias.h
-monitor.o: $(hdrdir)/ruby/internal/attr/nodiscard.h
-monitor.o: $(hdrdir)/ruby/internal/attr/noexcept.h
-monitor.o: $(hdrdir)/ruby/internal/attr/noinline.h
-monitor.o: $(hdrdir)/ruby/internal/attr/nonnull.h
-monitor.o: $(hdrdir)/ruby/internal/attr/noreturn.h
-monitor.o: $(hdrdir)/ruby/internal/attr/packed_struct.h
-monitor.o: $(hdrdir)/ruby/internal/attr/pure.h
-monitor.o: $(hdrdir)/ruby/internal/attr/restrict.h
-monitor.o: $(hdrdir)/ruby/internal/attr/returns_nonnull.h
-monitor.o: $(hdrdir)/ruby/internal/attr/warning.h
-monitor.o: $(hdrdir)/ruby/internal/attr/weakref.h
-monitor.o: $(hdrdir)/ruby/internal/cast.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/apple.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/clang.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/gcc.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/intel.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/msvc.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_is/sunpro.h
-monitor.o: $(hdrdir)/ruby/internal/compiler_since.h
-monitor.o: $(hdrdir)/ruby/internal/config.h
-monitor.o: $(hdrdir)/ruby/internal/constant_p.h
-monitor.o: $(hdrdir)/ruby/internal/core.h
-monitor.o: $(hdrdir)/ruby/internal/core/rarray.h
-monitor.o: $(hdrdir)/ruby/internal/core/rbasic.h
-monitor.o: $(hdrdir)/ruby/internal/core/rbignum.h
-monitor.o: $(hdrdir)/ruby/internal/core/rclass.h
-monitor.o: $(hdrdir)/ruby/internal/core/rdata.h
-monitor.o: $(hdrdir)/ruby/internal/core/rfile.h
-monitor.o: $(hdrdir)/ruby/internal/core/rhash.h
-monitor.o: $(hdrdir)/ruby/internal/core/robject.h
-monitor.o: $(hdrdir)/ruby/internal/core/rregexp.h
-monitor.o: $(hdrdir)/ruby/internal/core/rstring.h
-monitor.o: $(hdrdir)/ruby/internal/core/rstruct.h
-monitor.o: $(hdrdir)/ruby/internal/core/rtypeddata.h
-monitor.o: $(hdrdir)/ruby/internal/ctype.h
-monitor.o: $(hdrdir)/ruby/internal/dllexport.h
-monitor.o: $(hdrdir)/ruby/internal/dosish.h
-monitor.o: $(hdrdir)/ruby/internal/error.h
-monitor.o: $(hdrdir)/ruby/internal/eval.h
-monitor.o: $(hdrdir)/ruby/internal/event.h
-monitor.o: $(hdrdir)/ruby/internal/fl_type.h
-monitor.o: $(hdrdir)/ruby/internal/gc.h
-monitor.o: $(hdrdir)/ruby/internal/glob.h
-monitor.o: $(hdrdir)/ruby/internal/globals.h
-monitor.o: $(hdrdir)/ruby/internal/has/attribute.h
-monitor.o: $(hdrdir)/ruby/internal/has/builtin.h
-monitor.o: $(hdrdir)/ruby/internal/has/c_attribute.h
-monitor.o: $(hdrdir)/ruby/internal/has/cpp_attribute.h
-monitor.o: $(hdrdir)/ruby/internal/has/declspec_attribute.h
-monitor.o: $(hdrdir)/ruby/internal/has/extension.h
-monitor.o: $(hdrdir)/ruby/internal/has/feature.h
-monitor.o: $(hdrdir)/ruby/internal/has/warning.h
-monitor.o: $(hdrdir)/ruby/internal/intern/array.h
-monitor.o: $(hdrdir)/ruby/internal/intern/bignum.h
-monitor.o: $(hdrdir)/ruby/internal/intern/class.h
-monitor.o: $(hdrdir)/ruby/internal/intern/compar.h
-monitor.o: $(hdrdir)/ruby/internal/intern/complex.h
-monitor.o: $(hdrdir)/ruby/internal/intern/cont.h
-monitor.o: $(hdrdir)/ruby/internal/intern/dir.h
-monitor.o: $(hdrdir)/ruby/internal/intern/enum.h
-monitor.o: $(hdrdir)/ruby/internal/intern/enumerator.h
-monitor.o: $(hdrdir)/ruby/internal/intern/error.h
-monitor.o: $(hdrdir)/ruby/internal/intern/eval.h
-monitor.o: $(hdrdir)/ruby/internal/intern/file.h
-monitor.o: $(hdrdir)/ruby/internal/intern/hash.h
-monitor.o: $(hdrdir)/ruby/internal/intern/io.h
-monitor.o: $(hdrdir)/ruby/internal/intern/load.h
-monitor.o: $(hdrdir)/ruby/internal/intern/marshal.h
-monitor.o: $(hdrdir)/ruby/internal/intern/numeric.h
-monitor.o: $(hdrdir)/ruby/internal/intern/object.h
-monitor.o: $(hdrdir)/ruby/internal/intern/parse.h
-monitor.o: $(hdrdir)/ruby/internal/intern/proc.h
-monitor.o: $(hdrdir)/ruby/internal/intern/process.h
-monitor.o: $(hdrdir)/ruby/internal/intern/random.h
-monitor.o: $(hdrdir)/ruby/internal/intern/range.h
-monitor.o: $(hdrdir)/ruby/internal/intern/rational.h
-monitor.o: $(hdrdir)/ruby/internal/intern/re.h
-monitor.o: $(hdrdir)/ruby/internal/intern/ruby.h
-monitor.o: $(hdrdir)/ruby/internal/intern/select.h
-monitor.o: $(hdrdir)/ruby/internal/intern/select/largesize.h
-monitor.o: $(hdrdir)/ruby/internal/intern/set.h
-monitor.o: $(hdrdir)/ruby/internal/intern/signal.h
-monitor.o: $(hdrdir)/ruby/internal/intern/sprintf.h
-monitor.o: $(hdrdir)/ruby/internal/intern/string.h
-monitor.o: $(hdrdir)/ruby/internal/intern/struct.h
-monitor.o: $(hdrdir)/ruby/internal/intern/thread.h
-monitor.o: $(hdrdir)/ruby/internal/intern/time.h
-monitor.o: $(hdrdir)/ruby/internal/intern/variable.h
-monitor.o: $(hdrdir)/ruby/internal/intern/vm.h
-monitor.o: $(hdrdir)/ruby/internal/interpreter.h
-monitor.o: $(hdrdir)/ruby/internal/iterator.h
-monitor.o: $(hdrdir)/ruby/internal/memory.h
-monitor.o: $(hdrdir)/ruby/internal/method.h
-monitor.o: $(hdrdir)/ruby/internal/module.h
-monitor.o: $(hdrdir)/ruby/internal/newobj.h
-monitor.o: $(hdrdir)/ruby/internal/scan_args.h
-monitor.o: $(hdrdir)/ruby/internal/special_consts.h
-monitor.o: $(hdrdir)/ruby/internal/static_assert.h
-monitor.o: $(hdrdir)/ruby/internal/stdalign.h
-monitor.o: $(hdrdir)/ruby/internal/stdbool.h
-monitor.o: $(hdrdir)/ruby/internal/stdckdint.h
-monitor.o: $(hdrdir)/ruby/internal/symbol.h
-monitor.o: $(hdrdir)/ruby/internal/value.h
-monitor.o: $(hdrdir)/ruby/internal/value_type.h
-monitor.o: $(hdrdir)/ruby/internal/variable.h
-monitor.o: $(hdrdir)/ruby/internal/warning_push.h
-monitor.o: $(hdrdir)/ruby/internal/xmalloc.h
-monitor.o: $(hdrdir)/ruby/missing.h
-monitor.o: $(hdrdir)/ruby/ruby.h
-monitor.o: $(hdrdir)/ruby/st.h
-monitor.o: $(hdrdir)/ruby/subst.h
-monitor.o: monitor.c
-# AUTOGENERATED DEPENDENCIES END
diff --git a/ext/monitor/extconf.rb b/ext/monitor/extconf.rb
deleted file mode 100644
index 78c53fa0c5..0000000000
--- a/ext/monitor/extconf.rb
+++ /dev/null
@@ -1,2 +0,0 @@
-require 'mkmf'
-create_makefile('monitor')
diff --git a/ext/monitor/monitor.c b/ext/monitor/monitor.c
deleted file mode 100644
index c43751c4e2..0000000000
--- a/ext/monitor/monitor.c
+++ /dev/null
@@ -1,301 +0,0 @@
-#include "ruby/ruby.h"
-
-/* Thread::Monitor */
-
-struct rb_monitor {
-    long count;
-    VALUE owner;
-    VALUE mutex;
-};
-
-static void
-monitor_mark(void *ptr)
-{
-    struct rb_monitor *mc = ptr;
-    rb_gc_mark_movable(mc->owner);
-    rb_gc_mark_movable(mc->mutex);
-}
-
-static void
-monitor_compact(void *ptr)
-{
-    struct rb_monitor *mc = ptr;
-    mc->owner = rb_gc_location(mc->owner);
-    mc->mutex = rb_gc_location(mc->mutex);
-}
-
-static const rb_data_type_t monitor_data_type = {
-    .wrap_struct_name = "monitor",
-    .function = {
-        .dmark = monitor_mark,
-        .dfree = RUBY_TYPED_DEFAULT_FREE,
-        .dsize = NULL, // Fully embeded
-        .dcompact = monitor_compact,
-    },
-    .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_EMBEDDABLE,
-};
-
-static VALUE
-monitor_alloc(VALUE klass)
-{
-    struct rb_monitor *mc;
-    VALUE obj;
-
-    obj = TypedData_Make_Struct(klass, struct rb_monitor, &monitor_data_type, mc);
-    RB_OBJ_WRITE(obj, &mc->mutex, rb_mutex_new());
-    RB_OBJ_WRITE(obj, &mc->owner, Qnil);
-    mc->count = 0;
-
-    return obj;
-}
-
-static struct rb_monitor *
-monitor_ptr(VALUE monitor)
-{
-    struct rb_monitor *mc;
-    TypedData_Get_Struct(monitor, struct rb_monitor, &monitor_data_type, mc);
-    return mc;
-}
-
-static bool
-mc_owner_p(struct rb_monitor *mc, VALUE current_fiber)
-{
-    return mc->owner == current_fiber;
-}
-
-/*
- * call-seq:
- *   try_enter -> true or false
- *
- * Attempts to enter exclusive section.  Returns +false+ if lock fails.
- */
-static VALUE
-monitor_try_enter(VALUE monitor)
-{
-    struct rb_monitor *mc = monitor_ptr(monitor);
-
-    VALUE current_fiber = rb_fiber_current();
-    if (!mc_owner_p(mc, current_fiber)) {
-        if (!rb_mutex_trylock(mc->mutex)) {
-            return Qfalse;
-        }
-        RB_OBJ_WRITE(monitor, &mc->owner, current_fiber);
-        mc->count = 0;
-    }
-    mc->count += 1;
-    return Qtrue;
-}
-
-
-struct monitor_args {
-    VALUE monitor;
-    struct rb_monitor *mc;
-    VALUE current_fiber;
-};
-
-static inline void
-monitor_args_init(struct monitor_args *args, VALUE monitor)
-{
-    args->monitor = monitor;
-    args->mc = monitor_ptr(monitor);
-    args->current_fiber = rb_fiber_current();
-}
-
-static void
-monitor_enter0(struct monitor_args *args)
-{
-    if (!mc_owner_p(args->mc, args->current_fiber)) {
-        rb_mutex_lock(args->mc->mutex);
-        RB_OBJ_WRITE(args->monitor, &args->mc->owner, args->current_fiber);
-        args->mc->count = 0;
-    }
-    args->mc->count++;
-}
-
-/*
- * call-seq:
- *   enter -> nil
- *
- * Enters exclusive section.
- */
-static VALUE
-monitor_enter(VALUE monitor)
-{
-    struct monitor_args args;
-    monitor_args_init(&args, monitor);
-    monitor_enter0(&args);
-    return Qnil;
-}
-
-static inline void
-monitor_check_owner0(struct monitor_args *args)
-{
-    if (!mc_owner_p(args->mc, args->current_fiber)) {
-        rb_raise(rb_eThreadError, "current fiber not owner");
-    }
-}
-
-/* :nodoc: */
-static VALUE
-monitor_check_owner(VALUE monitor)
-{
-    struct monitor_args args;
-    monitor_args_init(&args, monitor);
-    monitor_check_owner0(&args);
-    return Qnil;
-}
-
-static void
-monitor_exit0(struct monitor_args *args)
-{
-    monitor_check_owner0(args);
-
-    if (args->mc->count <= 0) rb_bug("monitor_exit: count:%d", (int)args->mc->count);
-    args->mc->count--;
-
-    if (args->mc->count == 0) {
-        RB_OBJ_WRITE(args->monitor, &args->mc->owner, Qnil);
-        rb_mutex_unlock(args->mc->mutex);
-    }
-}
-
-/*
- * call-seq:
- *   exit -> nil
- *
- * Leaves exclusive section.
- */
-static VALUE
-monitor_exit(VALUE monitor)
-{
-    struct monitor_args args;
-    monitor_args_init(&args, monitor);
-    monitor_exit0(&args);
-    return Qnil;
-}
-
-/* :nodoc: */
-static VALUE
-monitor_locked_p(VALUE monitor)
-{
-    struct rb_monitor *mc = monitor_ptr(monitor);
-    return rb_mutex_locked_p(mc->mutex);
-}
-
-/* :nodoc: */
-static VALUE
-monitor_owned_p(VALUE monitor)
-{
-    struct rb_monitor *mc = monitor_ptr(monitor);
-    return rb_mutex_locked_p(mc->mutex) && mc_owner_p(mc, rb_fiber_current()) ? Qtrue : Qfalse;
-}
-
-static VALUE
-monitor_exit_for_cond(VALUE monitor)
-{
-    struct rb_monitor *mc = monitor_ptr(monitor);
-    long cnt = mc->count;
-    RB_OBJ_WRITE(monitor, &mc->owner, Qnil);
-    mc->count = 0;
-    return LONG2NUM(cnt);
-}
-
-struct wait_for_cond_data {
-    VALUE monitor;
-    VALUE cond;
-    VALUE timeout;
-    VALUE count;
-};
-
-static VALUE
-monitor_wait_for_cond_body(VALUE v)
-{
-    struct wait_for_cond_data *data = (struct wait_for_cond_data *)v;
-    struct rb_monitor *mc = monitor_ptr(data->monitor);
-    // cond.wait(monitor.mutex, timeout)
-    VALUE signaled = rb_funcall(data->cond, rb_intern("wait"), 2, mc->mutex, data->timeout);
-    return RTEST(signaled) ? Qtrue : Qfalse;
-}
-
-static VALUE
-monitor_enter_for_cond(VALUE v)
-{
-    // assert(rb_mutex_owned_p(mc->mutex) == Qtrue)
-    // but rb_mutex_owned_p is not exported...
-
-    struct wait_for_cond_data *data = (struct wait_for_cond_data *)v;
-    struct rb_monitor *mc = monitor_ptr(data->monitor);
-    RB_OBJ_WRITE(data->monitor, &mc->owner, rb_fiber_current());
-    mc->count = NUM2LONG(data->count);
-    return Qnil;
-}
-
-/* :nodoc: */
-static VALUE
-monitor_wait_for_cond(VALUE monitor, VALUE cond, VALUE timeout)
-{
-    VALUE count = monitor_exit_for_cond(monitor);
-    struct wait_for_cond_data data = {
-        monitor,
-        cond,
-        timeout,
-        count,
-    };
-
-    return rb_ensure(monitor_wait_for_cond_body, (VALUE)&data,
-                     monitor_enter_for_cond, (VALUE)&data);
-}
-
-static VALUE
-monitor_sync_body(VALUE monitor)
-{
-    return rb_yield_values(0);
-}
-
-static VALUE
-monitor_sync_ensure(VALUE v_args)
-{
-    monitor_exit0((struct monitor_args *)v_args);
-    return Qnil;
-}
-
-/*
- * call-seq:
- *   synchronize { } -> result of the block
- *
- * Enters exclusive section and executes the block.  Leaves the exclusive
- * section automatically when the block exits.  See example under
- * +MonitorMixin+.
- */
-static VALUE
-monitor_synchronize(VALUE monitor)
-{
-    struct monitor_args args;
-    monitor_args_init(&args, monitor);
-    monitor_enter0(&args);
-    return rb_ensure(monitor_sync_body, (VALUE)&args, monitor_sync_ensure, (VALUE)&args);
-}
-
-void
-Init_monitor(void)
-{
-#ifdef HAVE_RB_EXT_RACTOR_SAFE
-    rb_ext_ractor_safe(true);
-#endif
-
-    VALUE rb_cMonitor = rb_define_class("Monitor", rb_cObject);
-    rb_define_alloc_func(rb_cMonitor, monitor_alloc);
-
-    rb_define_method(rb_cMonitor, "try_enter", monitor_try_enter, 0);
-    rb_define_method(rb_cMonitor, "enter", monitor_enter, 0);
-    rb_define_method(rb_cMonitor, "exit", monitor_exit, 0);
-    rb_define_method(rb_cMonitor, "synchronize", monitor_synchronize, 0);
-
-    /* internal methods for MonitorMixin */
-    rb_define_method(rb_cMonitor, "mon_locked?", monitor_locked_p, 0);
-    rb_define_method(rb_cMonitor, "mon_check_owner", monitor_check_owner, 0);
-    rb_define_method(rb_cMonitor, "mon_owned?", monitor_owned_p, 0);
-
-    /* internal methods for MonitorMixin::ConditionVariable */
-    rb_define_method(rb_cMonitor, "wait_for_cond", monitor_wait_for_cond, 2);
-}
diff --git a/ext/monitor/lib/monitor.rb b/lib/monitor.rb
index 82d0a75c56..21329a5de7 100644
--- a/ext/monitor/lib/monitor.rb
+++ b/lib/monitor.rb
@@ -6,9 +6,6 @@
 # This library is distributed under the terms of the Ruby license.
 # You can freely distribute/modify this library.
 #
-
-require 'monitor.so'
-
 #
 # In concurrent programming, a monitor is an object or module intended to be
 # used safely by more than one thread. The defining characteristic of a
@@ -89,65 +86,14 @@ require 'monitor.so'
 # MonitorMixin module.
 #
 module MonitorMixin
+  ConditionVariable = Monitor::ConditionVariable # :nodoc:
+
   #
   # FIXME: This isn't documented in Nutshell.
   #
   # Since MonitorMixin.new_cond returns a ConditionVariable, and the example
   # above calls while_wait and signal, this class should be documented.
   #
-  class ConditionVariable
-    #
-    # Releases the lock held in the associated monitor and waits; reacquires the lock on wakeup.
-    #
-    # If +timeout+ is given, this method returns after +timeout+ seconds passed,
-    # even if no other thread doesn't signal.
-    #
-    def wait(timeout = nil)
-      @monitor.mon_check_owner
-      @monitor.wait_for_cond(@cond, timeout)
-    end
-
-    #
-    # Calls wait repeatedly while the given block yields a truthy value.
-    #
-    def wait_while
-      while yield
-        wait
-      end
-    end
-
-    #
-    # Calls wait repeatedly until the given block yields a truthy value.
-    #
-    def wait_until
-      until yield
-        wait
-      end
-    end
-
-    #
-    # Wakes up the first thread in line waiting for this lock.
-    #
-    def signal
-      @monitor.mon_check_owner
-      @cond.signal
-    end
-
-    #
-    # Wakes up all threads waiting for this lock.
-    #
-    def broadcast
-      @monitor.mon_check_owner
-      @cond.broadcast
-    end
-
-    private
-
-    def initialize(monitor) # :nodoc:
-      @monitor = monitor
-      @cond = Thread::ConditionVariable.new
-    end
-  end
 
   def self.extend_object(obj) # :nodoc:
     super(obj)
@@ -245,26 +191,7 @@ module MonitorMixin
   end
 end
 
-# Use the Monitor class when you want to have a lock object for blocks with
-# mutual exclusion.
-#
-#   require 'monitor'
-#
-#   lock = Monitor.new
-#   lock.synchronize do
-#     # exclusive access
-#   end
-#
-class Monitor
-  #
-  # Creates a new MonitorMixin::ConditionVariable associated with the
-  # Monitor object.
-  #
-  def new_cond
-    ::MonitorMixin::ConditionVariable.new(self)
-  end
-
-  # for compatibility
+class Monitor # :nodoc:
   alias try_mon_enter try_enter
   alias mon_try_enter try_enter
   alias mon_enter enter
diff --git a/spec/ruby/core/kernel/require_spec.rb b/spec/ruby/core/kernel/require_spec.rb
index 60d17242fe..da2f48fb61 100644
--- a/spec/ruby/core/kernel/require_spec.rb
+++ b/spec/ruby/core/kernel/require_spec.rb
@@ -21,13 +21,16 @@ describe "Kernel#require" do
     provided << "set"
     provided << "pathname"
   end
+  ruby_version_is "4.1" do
+    provided << "monitor"
+  end
 
   it "#{provided.join(', ')} are already required" do
     out = ruby_exe("puts $LOADED_FEATURES", options: '--disable-gems --disable-did-you-mean')
     features = out.lines.map { |line| File.basename(line.chomp, '.*') }
 
     # Ignore CRuby internals
-    features -= %w[encdb transdb windows_1252 windows_31j]
+    features -= %w[encdb transdb windows_1252 windows_31]
     features.reject! { |feature| feature.end_with?('-fake') }
 
     features.sort.should == provided.sort
@@ -37,6 +40,10 @@ describe "Kernel#require" do
       requires = requires.map { |f| f == "pathname" ? "pathname.so" : f }
     end
 
+    ruby_version_is "4.1" do
+      requires = requires.map { |f| f == "monitor" ? "monitor.so" : f }
+    end
+
     code = requires.map { |f| "puts require #{f.inspect}\n" }.join
     required = ruby_exe(code, options: '--disable-gems')
     required.should == "false\n" * requires.size
diff --git a/thread_sync.c b/thread_sync.c
index fa6a60ab62..cf4e3843ff 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -1261,6 +1261,263 @@ rb_condvar_broadcast(rb_execution_context_t *ec, VALUE self)
     return self;
 }
 
+/* Thread::Monitor */
+
+struct rb_monitor {
+    long count;
+    rb_serial_t ec_serial;
+    VALUE mutex;
+};
+
+static void
+monitor_mark(void *ptr)
+{
+    struct rb_monitor *mc = ptr;
+    rb_gc_mark_movable(mc->mutex);
+}
+
+static void
+monitor_compact(void *ptr)
+{
+    struct rb_monitor *mc = ptr;
+    mc->mutex = rb_gc_location(mc->mutex);
+}
+
+static const rb_data_type_t monitor_data_type = {
+    .wrap_struct_name = "monitor",
+    .function = {
+        .dmark = monitor_mark,
+        .dfree = RUBY_TYPED_DEFAULT_FREE,
+        .dsize = NULL, // Fully embeded
+        .dcompact = monitor_compact,
+    },
+    .flags = RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_EMBEDDABLE,
+};
+
+static VALUE
+monitor_alloc(VALUE klass)
+{
+    struct rb_monitor *mc;
+    VALUE obj;
+
+    obj = TypedData_Make_Struct(klass, struct rb_monitor, &monitor_data_type, mc);
+    RB_OBJ_WRITE(obj, &mc->mutex, rb_mutex_new());
+    mc->ec_serial = 0;
+    mc->count = 0;
+
+    return obj;
+}
+
+static struct rb_monitor *
+monitor_ptr(VALUE monitor)
+{
+    struct rb_monitor *mc;
+    TypedData_Get_Struct(monitor, struct rb_monitor, &monitor_data_type, mc);
+    return mc;
+}
+
+static bool
+mc_owner_p(struct rb_monitor *mc, rb_serial_t current_fiber_serial)
+{
+    return mc->ec_serial == current_fiber_serial;
+}
+
+static VALUE
+rb_monitor_try_enter(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct rb_monitor *mc = monitor_ptr(monitor);
+
+    rb_serial_t current_fiber_serial = rb_ec_serial(ec);
+    if (!mc_owner_p(mc, current_fiber_serial)) {
+        if (!rb_mut_trylock(ec, mc->mutex)) {
+            return Qfalse;
+        }
+        mc->ec_serial = current_fiber_serial;
+        mc->count = 0;
+    }
+    mc->count += 1;
+    return Qtrue;
+}
+
+struct monitor_args {
+    VALUE monitor;
+    struct rb_monitor *mc;
+    rb_serial_t current_fiber_serial;
+    rb_execution_context_t *ec;
+};
+
+static void
+monitor_enter0(struct monitor_args *args)
+{
+    if (!mc_owner_p(args->mc, args->current_fiber_serial)) {
+        struct mutex_args mut_args = {
+            .self = args->mc->mutex,
+            .mutex = mutex_ptr(args->mc->mutex),
+            .ec= args->ec,
+        };
+        do_mutex_lock(&mut_args, 1);
+        args->mc->ec_serial = args->current_fiber_serial;
+        args->mc->count = 0;
+    }
+    args->mc->count++;
+}
+
+static VALUE
+rb_monitor_enter(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct monitor_args args = {
+        .monitor = monitor,
+        .mc = monitor_ptr(monitor),
+        .ec = ec,
+        .current_fiber_serial = rb_ec_serial(ec),
+    };
+    monitor_enter0(&args);
+    return Qnil;
+}
+
+static inline void
+monitor_check_owner0(struct monitor_args *args)
+{
+    if (!mc_owner_p(args->mc, args->current_fiber_serial)) {
+        rb_raise(rb_eThreadError, "current fiber not owner");
+    }
+}
+
+static VALUE
+rb_monitor_check_owner(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct monitor_args args = {
+        .monitor = monitor,
+        .mc = monitor_ptr(monitor),
+        .ec = ec,
+        .current_fiber_serial = rb_ec_serial(ec),
+    };
+    monitor_check_owner0(&args);
+    return Qnil;
+}
+
+static void
+monitor_exit0(struct monitor_args *args)
+{
+    monitor_check_owner0(args);
+
+    if (args->mc->count <= 0) rb_bug("monitor_exit: count:%d", (int)args->mc->count);
+    args->mc->count--;
+
+    if (args->mc->count == 0) {
+        args->mc->ec_serial = 0;
+
+        struct mutex_args mut_args = {
+            .self = args->mc->mutex,
+            .mutex = mutex_ptr(args->mc->mutex),
+            .ec= args->ec,
+        };
+        do_mutex_unlock(&mut_args);
+    }
+}
+
+static VALUE
+rb_monitor_exit(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct monitor_args args = {
+        .monitor = monitor,
+        .mc = monitor_ptr(monitor),
+        .ec = ec,
+        .current_fiber_serial = rb_ec_serial(ec),
+    };
+    monitor_exit0(&args);
+    return Qnil;
+}
+
+static VALUE
+rb_monitor_locked_p(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct rb_monitor *mc = monitor_ptr(monitor);
+    return rb_mutex_locked_p(mc->mutex);
+}
+
+static VALUE
+rb_monitor_owned_p(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct rb_monitor *mc = monitor_ptr(monitor);
+    return RBOOL(rb_mutex_locked_p(mc->mutex) && mc_owner_p(mc, rb_ec_serial(ec)));
+}
+
+static VALUE
+monitor_exit_for_cond(VALUE monitor)
+{
+    struct rb_monitor *mc = monitor_ptr(monitor);
+    long cnt = mc->count;
+    mc->ec_serial = 0;
+    mc->count = 0;
+    return LONG2NUM(cnt);
+}
+
+struct wait_for_cond_data {
+    VALUE monitor;
+    VALUE cond;
+    VALUE timeout;
+    VALUE count;
+};
+
+static VALUE
+monitor_wait_for_cond_body(VALUE v)
+{
+    struct wait_for_cond_data *data = (struct wait_for_cond_data *)v;
+    struct rb_monitor *mc = monitor_ptr(data->monitor);
+    // cond.wait(monitor.mutex, timeout)
+    VALUE signaled = rb_funcall(data->cond, rb_intern("wait"), 2, mc->mutex, data->timeout);
+    return RTEST(signaled) ? Qtrue : Qfalse;
+}
+
+static VALUE
+monitor_enter_for_cond(VALUE v)
+{
+    // assert(rb_mutex_owned_p(mc->mutex) == Qtrue)
+    // but rb_mutex_owned_p is not exported...
+
+    struct wait_for_cond_data *data = (struct wait_for_cond_data *)v;
+    struct rb_monitor *mc = monitor_ptr(data->monitor);
+    mc->ec_serial = rb_ec_serial(GET_EC());
+    mc->count = NUM2LONG(data->count);
+    return Qnil;
+}
+
+static VALUE
+rb_monitor_wait_for_cond(rb_execution_context_t *ec, VALUE monitor, VALUE cond, VALUE timeout)
+{
+    VALUE count = monitor_exit_for_cond(monitor);
+    struct wait_for_cond_data data = {
+        monitor,
+        cond,
+        timeout,
+        count,
+    };
+
+    return rb_ensure(monitor_wait_for_cond_body, (VALUE)&data,
+                     monitor_enter_for_cond, (VALUE)&data);
+}
+
+static VALUE
+monitor_sync_ensure(VALUE v_args)
+{
+    monitor_exit0((struct monitor_args *)v_args);
+    return Qnil;
+}
+
+static VALUE
+rb_monitor_synchronize(rb_execution_context_t *ec, VALUE monitor)
+{
+    struct monitor_args args = {
+        .monitor = monitor,
+        .mc = monitor_ptr(monitor),
+        .ec = ec,
+        .current_fiber_serial = rb_ec_serial(ec),
+    };
+    monitor_enter0(&args);
+    return rb_ec_ensure(ec, do_ec_yield, (VALUE)ec, monitor_sync_ensure, (VALUE)&args);
+}
+
 static void
 Init_thread_sync(void)
 {
@@ -1283,6 +1540,11 @@ Init_thread_sync(void)
 
     id_sleep = rb_intern("sleep");
 
+    /* Monitor */
+    VALUE rb_cMonitor = rb_define_class_id_under_no_pin(rb_cThread, rb_intern("Monitor"), rb_cObject);
+    rb_define_alloc_func(rb_cMonitor, monitor_alloc);
+
+    rb_provide("monitor.so");
     rb_provide("thread.rb");
 }
 
diff --git a/thread_sync.rb b/thread_sync.rb
index 398c0d02b7..c9d37772d7 100644
--- a/thread_sync.rb
+++ b/thread_sync.rb
@@ -529,9 +529,187 @@ class Thread
       Primitive.rb_condvar_wait(mutex, timeout)
     end
   end
+
+  # Use the Monitor class when you want to have a lock object for blocks with
+  # mutual exclusion.
+  #
+  #   lock = Monitor.new
+  #   lock.synchronize do
+  #     # exclusive access
+  #   end
+  #
+  # Contrary to Mutex, Monitor is reentrant:
+  #
+  #   lock = Monitor.new
+  #   lock.synchronize do
+  #     lock.synchronize do
+  #       # exclusive access
+  #     end
+  #   end
+  class Monitor
+    # call-seq:
+    #   synchronize { } -> result of the block
+    #
+    # Enters exclusive section and executes the block.  Leaves the exclusive
+    # section automatically when the block exits.  See example under
+    # +MonitorMixin+.
+    def synchronize(&)
+      Primitive.rb_monitor_synchronize
+    end
+
+    # call-seq:
+    #   try_enter -> true or false
+    #
+    # Attempts to enter exclusive section.  Returns +false+ if lock fails.
+    def try_enter
+      Primitive.rb_monitor_try_enter
+    end
+
+    # call-seq:
+    #   enter -> nil
+    #
+    # Enters exclusive section.
+    def enter
+      Primitive.rb_monitor_enter
+    end
+
+    # call-seq:
+    #   exit -> nil
+    #
+    # Leaves exclusive section.
+    def exit
+      Primitive.rb_monitor_exit
+    end
+
+    # internal methods for MonitorMixin
+    def mon_check_owner # :nodoc:
+      Primitive.rb_monitor_check_owner
+    end
+
+    def mon_locked? # :nodoc:
+      Primitive.rb_monitor_locked_p
+    end
+
+    def mon_owned? # :nodoc:
+      Primitive.rb_monitor_owned_p
+    end
+
+    # internal methods for MonitorMixin::ConditionVariable
+    def wait_for_cond(cond, timeout) # :nodoc:
+      Primitive.rb_monitor_wait_for_cond(cond, timeout)
+    end
+
+    # Creates a new Monitor::ConditionVariable associated with the
+    # Monitor object.
+    #
+    def new_cond
+      ConditionVariable.new(self)
+    end
+
+    # Condition variables, allow to suspend the current thread while in
+    # the middle of a critical section until a condition is met, such as
+    # a resource being available.
+    #
+    #  Example:
+    #
+    #    monitor = Thread::Monitor.new
+    #
+    #    resource_available = false
+    #    condvar = monitor.new_cond
+    #
+    #    a1 = Thread.new {
+    #      # Thread 'a1' waits for the resource to become available and consumes
+    #      # the resource.
+    #      monitor.synchronize {
+    #        condvar.wait_until { resource_available }
+    #        # After the loop, 'resource_available' is guaranteed to be true.
+    #
+    #        resource_available = false
+    #        puts "a1 consumed the resource"
+    #      }
+    #    }
+    #
+    #    a2 = Thread.new {
+    #      # Thread 'a2' behaves like 'a1'.
+    #      monitor.synchronize {
+    #        condvar.wait_until { resource_available }
+    #        resource_available = false
+    #        puts "a2 consumed the resource"
+    #      }
+    #    }
+    #
+    #    b = Thread.new {
+    #      # Thread 'b' periodically makes the resource available.
+    #      loop {
+    #        monitor.synchronize {
+    #          resource_available = true
+    #
+    #          # Notify one waiting thread if any.  It is possible that neither
+    #          # 'a1' nor 'a2 is waiting on 'condvar' at this moment.  That's OK.
+    #          condvar.signal
+    #        }
+    #        sleep 1
+    #      }
+    #    }
+    #
+    #    # Eventually both 'a1' and 'a2' will have their resources, albeit in an
+    #    # unspecified order.
+    #    [a1, a2].each {|th| th.join}
+    class ConditionVariable
+      def initialize(monitor) # :nodoc:
+        @monitor = monitor
+        @cond = Thread::ConditionVariable.new
+      end
+
+      # Releases the lock held in the associated monitor and waits; reacquires the lock on wakeup.
+      #
+      # If +timeout+ is given, this method returns after +timeout+ seconds passed,
+      # even if no other thread doesn't signal.
+      #
+      def wait(timeout = nil)
+        @monitor.mon_check_owner
+        @monitor.wait_for_cond(@cond, timeout)
+      end
+
+      #
+      # Calls wait repeatedly while the given block yields a truthy value.
+      #
+      def wait_while
+        while yield
+          wait
+        end
+      end
+
+      #
+      # Calls wait repeatedly until the given block yields a truthy value.
+      #
+      def wait_until
+        until yield
+          wait
+        end
+      end
+
+      #
+      # Wakes up the first thread in line waiting for this lock.
+      #
+      def signal
+        @monitor.mon_check_owner
+        @cond.signal
+      end
+
+      #
+      # Wakes up all threads waiting for this lock.
+      #
+      def broadcast
+        @monitor.mon_check_owner
+        @cond.broadcast
+      end
+    end
+  end
 end
 
-Mutex = Thread::Mutex
-ConditionVariable = Thread::ConditionVariable
-Queue = Thread::Queue
-SizedQueue = Thread::SizedQueue
+Mutex = Thread::Mutex # :nodoc:
+Monitor = Thread::Monitor # :nodoc:
+ConditionVariable = Thread::ConditionVariable # :nodoc:
+Queue = Thread::Queue # :nodoc:
+SizedQueue = Thread::SizedQueue # :nodoc: