diff --git a/spec/compiler/codegen/class_spec.cr b/spec/compiler/codegen/class_spec.cr
index b8504543f1e5..1475d6ab21ee 100644
--- a/spec/compiler/codegen/class_spec.cr
+++ b/spec/compiler/codegen/class_spec.cr
@@ -447,6 +447,8 @@ describe "Code gen: class" do
 
   it "allows using self in class scope" do
     run(%(
+      require "prelude"
+
       class Foo
         def self.foo
           1
@@ -713,6 +715,8 @@ describe "Code gen: class" do
 
   it "codegens singleton (#718)" do
     run(%(
+      require "prelude"
+
       class Singleton
         @@instance = new
 
diff --git a/spec/compiler/codegen/class_var_spec.cr b/spec/compiler/codegen/class_var_spec.cr
index ec35957803dd..0b63d68f6342 100644
--- a/spec/compiler/codegen/class_var_spec.cr
+++ b/spec/compiler/codegen/class_var_spec.cr
@@ -106,6 +106,8 @@ describe "Codegen: class var" do
 
   it "uses var in class var initializer" do
     run(%(
+      require "prelude"
+
       class Foo
         @@var : Int32
         @@var = begin
@@ -128,6 +130,8 @@ describe "Codegen: class var" do
 
   it "reads simple class var before another complex one" do
     run(%(
+      require "prelude"
+
       class Foo
         @@var2 : Int32
         @@var2 = @@var &+ 1
@@ -145,6 +149,8 @@ describe "Codegen: class var" do
 
   it "initializes class var of union with single type" do
     run(%(
+      require "prelude"
+
       class Foo
         @@var : Int32 | String
         @@var = 42
@@ -196,6 +202,8 @@ describe "Codegen: class var" do
 
   it "initializes dependent constant before class var" do
     run(%(
+      require "prelude"
+
       def foo
         a = 1
         b = 2
@@ -233,6 +241,8 @@ describe "Codegen: class var" do
 
   it "doesn't use nilable type for initializer" do
     run(%(
+      require "prelude"
+
       class Foo
         @@foo : Int32?
         @@foo = 42
@@ -250,7 +260,9 @@ describe "Codegen: class var" do
   end
 
   it "codegens class var with begin and vars" do
-    run("
+    run(%(
+      require "prelude"
+
       class Foo
         @@foo : Int32
         @@foo = begin
@@ -265,11 +277,13 @@ describe "Codegen: class var" do
       end
 
       Foo.foo
-      ").to_i.should eq(3)
+      )).to_i.should eq(3)
   end
 
   it "codegens class var with type declaration begin and vars" do
-    run("
+    run(%(
+      require "prelude"
+
       class Foo
         @@foo : Int32 = begin
           a = 1
@@ -283,7 +297,7 @@ describe "Codegen: class var" do
       end
 
       Foo.foo
-      ").to_i.should eq(3)
+      )).to_i.should eq(3)
   end
 
   it "codegens class var with nilable reference type" do
@@ -338,6 +352,8 @@ describe "Codegen: class var" do
 
   it "gets pointerof class var complex constant" do
     run(%(
+      require "prelude"
+
       z = Foo.foo
 
       class Foo
diff --git a/spec/compiler/codegen/const_spec.cr b/spec/compiler/codegen/const_spec.cr
index 1a6ce9cd2034..8862075049b0 100644
--- a/spec/compiler/codegen/const_spec.cr
+++ b/spec/compiler/codegen/const_spec.cr
@@ -87,6 +87,8 @@ describe "Codegen: const" do
 
   it "declare constants in right order" do
     run(%(
+      require "prelude"
+
       CONST1 = 1 + 1
       CONST2 = true ? CONST1 : 0
       CONST2
@@ -94,7 +96,9 @@ describe "Codegen: const" do
   end
 
   it "uses correct types lookup" do
-    run("
+    run(%(
+      require "prelude"
+
       module Moo
         class B
           def foo
@@ -110,11 +114,13 @@ describe "Codegen: const" do
       end
 
       foo
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "codegens variable assignment in const" do
-    run("
+    run(%(
+      require "prelude"
+
       class Foo
         def initialize(@x : Int32)
         end
@@ -134,11 +140,13 @@ describe "Codegen: const" do
       end
 
       foo
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "declaring var" do
-    run("
+    run(%(
+      require "prelude"
+
       BAR = begin
         a = 1
         while 1 == 2
@@ -153,7 +161,7 @@ describe "Codegen: const" do
       end
 
       Foo.new.compile
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "initialize const that might raise an exception" do
@@ -171,7 +179,9 @@ describe "Codegen: const" do
   end
 
   it "allows implicit self in constant, called from another class (bug)" do
-    run("
+    run(%(
+      require "prelude"
+
       module Foo
         def self.foo
           1
@@ -187,11 +197,13 @@ describe "Codegen: const" do
       end
 
       Bar.new.bar
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "codegens two consts with same variable name" do
-    run("
+    run(%(
+      require "prelude"
+
       CONST1 = begin
             a = 1
           end
@@ -201,11 +213,13 @@ describe "Codegen: const" do
           end
 
       (CONST1 + CONST2).to_i
-      ").to_i.should eq(3)
+      )).to_i.should eq(3)
   end
 
   it "works with variable declared inside if" do
     run(%(
+      require "prelude"
+
       FOO = begin
         if 1 == 2
           x = 3
@@ -220,6 +234,8 @@ describe "Codegen: const" do
 
   it "codegens constant that refers to another constant that is a struct" do
     run(%(
+      require "prelude"
+
       struct Foo
         X = Foo.new(1)
         Y = X
@@ -281,6 +297,8 @@ describe "Codegen: const" do
 
   it "uses const before declaring it (hoisting)" do
     run(%(
+      require "prelude"
+
       x = CONST
 
       CONST = foo
@@ -342,6 +360,8 @@ describe "Codegen: const" do
 
   it "gets pointerof constant" do
     run(%(
+      require "prelude"
+
       z = pointerof(FOO).value
       FOO = 10
       z
@@ -350,6 +370,8 @@ describe "Codegen: const" do
 
   it "gets pointerof complex constant" do
     run(%(
+      require "prelude"
+
       z = pointerof(FOO).value
       FOO = begin
         a = 10
@@ -446,4 +468,42 @@ describe "Codegen: const" do
 
     mod.to_s.should_not contain("CONST")
   end
+
+  it "synchronize initialization of contants" do
+    run(%(
+      require "prelude"
+
+      def foo
+        v1, v2 = 1, 1
+        rand(100000..10000000).times do
+          v1, v2 = v2, v1 + v2
+        end
+        v2
+      end
+
+      ch = Channel(Int32).new
+
+      10.times do
+        spawn do
+          ch.send X
+        end
+      end
+
+      X = foo
+
+      def test(ch)
+        expected = X
+
+        10.times do
+          if ch.receive != expected
+            return false
+          end
+        end
+
+        true
+      end
+
+      test(ch)
+    )).to_b.should be_true
+  end
 end
diff --git a/spec/compiler/codegen/debug_spec.cr b/spec/compiler/codegen/debug_spec.cr
index a8b883d15409..09c20b5da858 100644
--- a/spec/compiler/codegen/debug_spec.cr
+++ b/spec/compiler/codegen/debug_spec.cr
@@ -107,6 +107,8 @@ describe "Code gen: debug" do
 
   it "has correct debug location after constant initialization in call with block (#4719)" do
     codegen(%(
+      require "prelude"
+
       fun __crystal_malloc_atomic(size : UInt32) : Void*
         x = uninitialized Void*
         x
diff --git a/spec/compiler/codegen/hooks_spec.cr b/spec/compiler/codegen/hooks_spec.cr
index eb745bb5d320..c2dfe6a4e568 100644
--- a/spec/compiler/codegen/hooks_spec.cr
+++ b/spec/compiler/codegen/hooks_spec.cr
@@ -115,7 +115,9 @@ describe "Code gen: hooks" do
   end
 
   it "does inherited macro before class body" do
-    run("
+    run(%(
+      require "prelude"
+
       class Global
         @@x = 123
 
@@ -142,7 +144,7 @@ describe "Code gen: hooks" do
       end
 
       Bar.y
-      ").to_i.should eq(123)
+      )).to_i.should eq(123)
   end
 
   it "does finished" do
diff --git a/spec/compiler/codegen/macro_spec.cr b/spec/compiler/codegen/macro_spec.cr
index c6b22b1af023..85f92ef918e1 100644
--- a/spec/compiler/codegen/macro_spec.cr
+++ b/spec/compiler/codegen/macro_spec.cr
@@ -663,6 +663,8 @@ describe "Code gen: macro" do
 
   it "transforms hooks (bug)" do
     codegen(%(
+      require "prelude"
+
       module GC
         def self.add_finalizer(object : T)
           object.responds_to?(:finalize)
@@ -800,6 +802,8 @@ describe "Code gen: macro" do
 
   it "copies base macro def to sub-subtype even after it was copied to a subtype (#448)" do
     run(%(
+      require "prelude"
+
       class Object
         def class_name : String
           {{@type.name.stringify}}
diff --git a/spec/compiler/codegen/pointer_spec.cr b/spec/compiler/codegen/pointer_spec.cr
index c95333411e36..1f6a7b75de8c 100644
--- a/spec/compiler/codegen/pointer_spec.cr
+++ b/spec/compiler/codegen/pointer_spec.cr
@@ -250,10 +250,11 @@ describe "Code gen: pointer" do
   end
 
   it "gets pointer to constant" do
-    run("
+    run(%(
+      require "prelude"
       FOO = 1
       pointerof(FOO).value
-    ").to_i.should eq(1)
+    )).to_i.should eq(1)
   end
 
   it "passes pointer of pointer to method" do
@@ -324,6 +325,8 @@ describe "Code gen: pointer" do
 
   it "does pointerof class variable with class" do
     run(%(
+      require "prelude"
+
       class Bar
         def initialize(@x : Int32)
         end
diff --git a/spec/compiler/codegen/proc_spec.cr b/spec/compiler/codegen/proc_spec.cr
index d78e5c06ea1f..98bb279a2513 100644
--- a/spec/compiler/codegen/proc_spec.cr
+++ b/spec/compiler/codegen/proc_spec.cr
@@ -595,6 +595,8 @@ describe "Code gen: proc" do
 
   it "codegens proc to implicit self in constant (#647)" do
     run(%(
+      require "prelude"
+
       module Foo
         def self.blah
           1
diff --git a/spec/compiler/codegen/struct_spec.cr b/spec/compiler/codegen/struct_spec.cr
index 02a286169700..c5da68588b74 100644
--- a/spec/compiler/codegen/struct_spec.cr
+++ b/spec/compiler/codegen/struct_spec.cr
@@ -171,7 +171,9 @@ describe "Code gen: struct" do
   end
 
   it "declares const struct" do
-    run("
+    run(%(
+      require "prelude"
+
       struct Foo
         def initialize(@x : Int32)
         end
@@ -184,11 +186,13 @@ describe "Code gen: struct" do
       FOO = Foo.new(1)
 
       FOO.x
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "uses struct in if" do
-    run("
+    run(%(
+      require "prelude"
+
       struct Foo
         def initialize(@x : Int32)
         end
@@ -206,7 +210,7 @@ describe "Code gen: struct" do
         foo = FOO
       end
       foo.x
-      ").to_i.should eq(1)
+      )).to_i.should eq(1)
   end
 
   it "uses nilable struct" do
diff --git a/src/compiler/crystal/codegen/class_var.cr b/src/compiler/crystal/codegen/class_var.cr
index e01adfeeafda..cbdffd5a8b48 100644
--- a/src/compiler/crystal/codegen/class_var.cr
+++ b/src/compiler/crystal/codegen/class_var.cr
@@ -82,6 +82,7 @@ class Crystal::CodeGenVisitor
 
   def initialize_class_var(class_var : MetaTypeVar, initializer : ClassVarInitializer)
     init_func = create_initialize_class_var_function(class_var, initializer)
+    init_func = check_main_fun(init_func.name, init_func) if init_func
 
     # For unsafe class var we just initialize them without
     # using a flag to know if they were initialized
@@ -89,7 +90,6 @@ class Crystal::CodeGenVisitor
       global = declare_class_var(class_var)
       global = ensure_class_var_in_this_module(global, class_var)
       if init_func
-        check_main_fun init_func.name, init_func
         call init_func
       end
       return global
@@ -97,20 +97,12 @@ class Crystal::CodeGenVisitor
 
     global, initialized_flag = declare_class_var_and_initialized_flag_in_this_module(class_var)
 
-    initialized_block, not_initialized_block = new_blocks "initialized", "not_initialized"
-
-    initialized = load(initialized_flag)
-    cond initialized, initialized_block, not_initialized_block
-
-    position_at_end not_initialized_block
-    store int1(1), initialized_flag
-
-    init_func = check_main_fun init_func.name, init_func
-    call init_func
-
-    br initialized_block
+    lazy_initialize_class_var(initializer.node, init_func, global, initialized_flag)
+  end
 
-    position_at_end initialized_block
+  def lazy_initialize_class_var(node, init_func, global, initialized_flag)
+    set_current_debug_location node if @debug.line_numbers?
+    run_once(initialized_flag, init_func)
 
     global
   end
@@ -317,22 +309,8 @@ class Crystal::CodeGenVisitor
 
     in_main do
       define_main_function(fun_name, ([] of LLVM::Type), llvm_type(class_var.type).pointer) do |func|
-        initialized_block, not_initialized_block = new_blocks "initialized", "not_initialized"
-
-        initialized = load(initialized_flag)
-        cond initialized, initialized_block, not_initialized_block
-
-        position_at_end not_initialized_block
-        store int1(1), initialized_flag
-
-        check_main_fun init_func.name, init_func
-        call init_func
-
-        br initialized_block
-
-        position_at_end initialized_block
-
-        ret global
+        init_func = check_main_fun init_func.name, init_func
+        ret lazy_initialize_class_var(initializer.node, init_func, global, initialized_flag)
       end
     end
   end
diff --git a/src/compiler/crystal/codegen/codegen.cr b/src/compiler/crystal/codegen/codegen.cr
index 7750c2fba9ec..d27a2bdc7004 100644
--- a/src/compiler/crystal/codegen/codegen.cr
+++ b/src/compiler/crystal/codegen/codegen.cr
@@ -13,6 +13,8 @@ module Crystal
   MALLOC_ATOMIC_NAME  = "__crystal_malloc_atomic64"
   REALLOC_NAME        = "__crystal_realloc64"
   GET_EXCEPTION_NAME  = "__crystal_get_exception"
+  ONCE_INIT           = "__crystal_once_init"
+  ONCE                = "__crystal_once"
 
   class Program
     def run(code, filename = nil, debug = Debug::Default)
@@ -243,12 +245,13 @@ module Crystal
 
       initialize_argv_and_argc
 
-      initialize_simple_constants
-
-      if @debug.line_numbers? && (filename = @program.filename)
-        set_current_debug_location Location.new(filename, 1, 1)
+      if @debug.line_numbers?
+        set_current_debug_location Location.new(@program.filename || "(no name)", 1, 1)
       end
 
+      once_init
+      initialize_simple_constants
+
       alloca_vars @program.vars, @program
 
       emit_vars_debug_info(@program.vars) if @debug.variables?
@@ -301,7 +304,9 @@ module Crystal
 
       def visit(node : FunDef)
         case node.name
-        when MALLOC_NAME, MALLOC_ATOMIC_NAME, REALLOC_NAME, RAISE_NAME, @codegen.personality_name, GET_EXCEPTION_NAME, RAISE_OVERFLOW_NAME
+        when MALLOC_NAME, MALLOC_ATOMIC_NAME, REALLOC_NAME, RAISE_NAME,
+             @codegen.personality_name, GET_EXCEPTION_NAME, RAISE_OVERFLOW_NAME,
+             ONCE_INIT, ONCE
           @codegen.accept node
         end
         false
diff --git a/src/compiler/crystal/codegen/const.cr b/src/compiler/crystal/codegen/const.cr
index 54e0b509169d..e3ba1e83420c 100644
--- a/src/compiler/crystal/codegen/const.cr
+++ b/src/compiler/crystal/codegen/const.cr
@@ -29,6 +29,8 @@ require "./codegen"
 # and can be done in any order (they have no side effects).
 
 class Crystal::CodeGenVisitor
+  @const_mutex : LLVM::Value?
+
   # The special constants ARGC_UNSAFE and ARGV_UNSAFE need to be initialized
   # as soon as the program starts, because we have access to argc and argv
   # in the main function
@@ -96,27 +98,16 @@ class Crystal::CodeGenVisitor
 
   def initialize_const(const)
     # Maybe the constant was simple and doesn't need a real initialization
-    return if const.initializer
 
     global, initialized_flag = declare_const_and_initialized_flag(const)
-
-    initialized_block, not_initialized_block = new_blocks "initialized", "not_initialized"
-
-    initialized = load(initialized_flag)
-    cond initialized, initialized_block, not_initialized_block
-
-    position_at_end not_initialized_block
-    store int1(1), initialized_flag
+    return global if const.initializer
 
     init_function_name = "~#{const.initialized_llvm_name}"
     func = @main_mod.functions[init_function_name]? || create_initialize_const_function(init_function_name, const)
     func = check_main_fun init_function_name, func
-    call func
-
-    br initialized_block
-
-    position_at_end initialized_block
 
+    set_current_debug_location const.locations.try &.first? if @debug.line_numbers?
+    run_once(initialized_flag, func)
     global
   end
 
@@ -202,26 +193,9 @@ class Crystal::CodeGenVisitor
   end
 
   def create_read_const_function(fun_name, const)
-    global, initialized_flag = declare_const_and_initialized_flag(const)
-
     in_main do
       define_main_function(fun_name, ([] of LLVM::Type), llvm_type(const.value.type).pointer) do |func|
-        initialized_block, not_initialized_block = new_blocks "initialized", "not_initialized"
-
-        initialized = load(initialized_flag)
-        cond initialized, initialized_block, not_initialized_block
-
-        position_at_end not_initialized_block
-        store int1(1), initialized_flag
-
-        init_function_name = "~#{const.initialized_llvm_name}"
-        func = @main_mod.functions[init_function_name]? || create_initialize_const_function(init_function_name, const)
-        call func
-
-        br initialized_block
-
-        position_at_end initialized_block
-
+        global = initialize_const(const)
         ret global
       end
     end
diff --git a/src/compiler/crystal/codegen/once.cr b/src/compiler/crystal/codegen/once.cr
new file mode 100644
index 000000000000..6c833b3e7fcc
--- /dev/null
+++ b/src/compiler/crystal/codegen/once.cr
@@ -0,0 +1,38 @@
+require "./codegen"
+
+class Crystal::CodeGenVisitor
+  ONCE_STATE = "~ONCE_STATE"
+  @have_once_state = false
+
+  def once_init
+    once_init_fun = @main_mod.functions[ONCE_INIT]?
+    if once_init_fun
+      @have_once_state = true
+      once_init_fun = check_main_fun ONCE_INIT, once_init_fun
+
+      once_state_global = @main_mod.globals.add(once_init_fun.return_type, ONCE_STATE)
+      once_state_global.linkage = LLVM::Linkage::Internal if @single_module
+      once_state_global.initializer = once_init_fun.return_type.null
+
+      state = call once_init_fun
+      store state, once_state_global
+    end
+  end
+
+  def run_once(flag, func)
+    once_fun = main_fun(ONCE)
+
+    once_state_global = @llvm_mod.globals[ONCE_STATE]? || begin
+      once_init_fun = main_fun(ONCE_INIT)
+      global = @llvm_mod.globals.add(once_init_fun.return_type, ONCE_STATE)
+      global.linkage = LLVM::Linkage::External
+      global
+    end
+
+    call main_fun(ONCE), [
+      load(once_state_global),
+      flag,
+      bit_cast(func.to_value, once_fun.params.last.type),
+    ]
+  end
+end
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 8d7dfc063880..9be592c62cdb 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -5690,7 +5690,8 @@ module Crystal
         when :"{{"
           members << parse_percent_macro_expression
         when :"{%"
-          members << parse_percent_macro_control
+          location = @token.location
+          members << parse_percent_macro_control.at(location)
         when :"@["
           members << parse_annotation
         when :";", :NEWLINE
diff --git a/src/crystal/once.cr b/src/crystal/once.cr
new file mode 100644
index 000000000000..db2c1aaeecde
--- /dev/null
+++ b/src/crystal/once.cr
@@ -0,0 +1,27 @@
+{% if flag?(:preview_mt) %}
+  fun __crystal_once_init : Void*
+    Mutex.new.as(Void*)
+  end
+
+  fun __crystal_once(m : Void*, f : Bool*, init : Void*)
+    unless f.value
+      m.as(Mutex).synchronize do
+        unless f.value
+          Proc(Nil).new(init, Pointer(Void).null).call
+          f.value = true
+        end
+      end
+    end
+  end
+{% else %}
+  fun __crystal_once_init : Void*
+    Pointer(Void).null
+  end
+
+  fun __crystal_once(m : Void*, f : Bool*, init : Void*)
+    unless f.value
+      Proc(Nil).new(init, Pointer(Void).null).call
+      f.value = true
+    end
+  end
+{% end %}
diff --git a/src/fiber.cr b/src/fiber.cr
index df247b72de43..3eff5957cc56 100644
--- a/src/fiber.cr
+++ b/src/fiber.cr
@@ -11,7 +11,7 @@ fun _fiber_get_stack_top : Void*
 end
 
 class Fiber
-  @@fibers = Thread::LinkedList(Fiber).new
+  @@fibers : Thread::LinkedList(Fiber)?
 
   # :nodoc:
   class_getter stack_pool = StackPool.new
@@ -30,14 +30,18 @@ class Fiber
   # :nodoc:
   property previous : Fiber?
 
+  protected def self.fibers
+    @@fibers ||= Thread::LinkedList(Fiber).new
+  end
+
   # :nodoc:
   def self.inactive(fiber : Fiber)
-    @@fibers.delete(fiber)
+    fibers.delete(fiber)
   end
 
   # :nodoc:
   def self.unsafe_each
-    @@fibers.unsafe_each { |fiber| yield fiber }
+    fibers.unsafe_each { |fiber| yield fiber }
   end
 
   def initialize(@name : String? = nil, &@proc : ->)
@@ -58,7 +62,7 @@ class Fiber
 
     makecontext(stack_ptr, fiber_main)
 
-    @@fibers.push(self)
+    Fiber.fibers.push(self)
   end
 
   # :nodoc:
@@ -71,7 +75,7 @@ class Fiber
     @stack_bottom = GC.current_thread_stack_bottom
     @name = "main"
     @current_thread.set(thread)
-    @@fibers.push(self)
+    Fiber.fibers.push(self)
   end
 
   # :nodoc:
@@ -90,7 +94,7 @@ class Fiber
     Fiber.stack_pool.release(@stack)
 
     # Remove the current fiber from the linked list
-    @@fibers.delete(self)
+    Fiber.fibers.delete(self)
 
     # Delete the resume event if it was used by `yield` or `sleep`
     @resume_event.try &.free
diff --git a/src/llvm/value_methods.cr b/src/llvm/value_methods.cr
index 1e8d2f9664b3..efba6024b4ba 100644
--- a/src/llvm/value_methods.cr
+++ b/src/llvm/value_methods.cr
@@ -96,7 +96,7 @@ module LLVM::ValueMethods
   end
 
   def to_value
-    LLVM::Value.new unwrap
+    LLVM::Value.new @unwrap
   end
 
   def dump
diff --git a/src/prelude.cr b/src/prelude.cr
index 777b85adb520..6b74c77d4eb2 100644
--- a/src/prelude.cr
+++ b/src/prelude.cr
@@ -14,6 +14,7 @@ private macro no_win(stmt)
 end
 
 # This list requires ordered statements
+require "crystal/once"
 require "lib_c"
 require "macros"
 require "object"
diff --git a/src/thread.cr b/src/thread.cr
index a88c314719ad..33c4c469b95e 100644
--- a/src/thread.cr
+++ b/src/thread.cr
@@ -8,7 +8,7 @@ class Thread
   # Use spawn and channels instead.
 
   # all thread objects, so the GC can see them (it doesn't scan thread locals)
-  @@threads = Thread::LinkedList(Thread).new
+  @@threads : Thread::LinkedList(Thread)?
 
   @th : LibC::PthreadT
   @exception : Exception?
@@ -25,8 +25,12 @@ class Thread
     property load = 0
   {% end %}
 
+  protected def self.threads
+    @@threads ||= Thread::LinkedList(Thread).new
+  end
+
   def self.unsafe_each
-    @@threads.unsafe_each { |thread| yield thread }
+    threads.unsafe_each { |thread| yield thread }
   end
 
   # Starts a new system thread.
@@ -39,7 +43,7 @@ class Thread
     }, self.as(Void*))
 
     if ret == 0
-      @@threads.push(self)
+      Thread.threads.push(self)
     else
       raise Errno.new("pthread_create", ret)
     end
@@ -52,7 +56,7 @@ class Thread
     @th = LibC.pthread_self
     @main_fiber = Fiber.new(stack_address, self)
 
-    @@threads.push(self)
+    Thread.threads.push(self)
   end
 
   private def detach
@@ -102,7 +106,9 @@ class Thread
 
     # Returns the Thread object associated to the running system thread.
     def self.current : Thread
-      @@current || raise "BUG: Thread.current returned NULL"
+      # Thread#start sets @@current as soon it starts. Thus we know
+      # that if @@current is not set then we are in the main thread
+      @@current ||= new
     end
 
     # Associates the Thread object to the running system thread.
@@ -110,12 +116,6 @@ class Thread
     end
   {% end %}
 
-  # Create the thread object for the current thread (aka the main thread of the
-  # process).
-  #
-  # TODO: consider moving to `kernel.cr` or `crystal/main.cr`
-  self.current = new
-
   def self.yield
     ret = LibC.sched_yield
     raise Errno.new("sched_yield") unless ret == 0
@@ -140,7 +140,7 @@ class Thread
     rescue ex
       @exception = ex
     ensure
-      @@threads.delete(self)
+      Thread.threads.delete(self)
       Fiber.inactive(fiber)
       detach { GC.pthread_detach(@th) }
     end
diff --git a/src/thread/linked_list.cr b/src/thread/linked_list.cr
index f64d9df95bba..2e66345cf2d5 100644
--- a/src/thread/linked_list.cr
+++ b/src/thread/linked_list.cr
@@ -6,7 +6,7 @@ class Thread
   #
   # Thread-safe doubly linked list of `T` objects that must implement
   # `#previous : T?` and `#next : T?` methods.
-  struct LinkedList(T)
+  class LinkedList(T)
     @mutex = Thread::Mutex.new
     @head : T?
     @tail : T?