diff --git a/ChangeLog b/ChangeLog
index 054befacba..13304e5ad8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Sun Sep  9 20:20:31 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* lib/sync.rb (Sync_m#sync_lock): Fixed wakeup/raise unsafe code.
+	  Patched by Masaki Matsushita. [Bug #5355] [ruby-dev:44521]
+
+	* test/thread/test_sync.rb (test_sync_lock_and_wakeup,
+	  test_sync_upgrade_and_wakeup, test_sync_lock_and_raise):
+	  new test.
+
 Sun Sep  9 18:39:46 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* include/ruby/intern.h (rb_thread_blocking_region): Added
diff --git a/lib/sync.rb b/lib/sync.rb
index a57b789d55..524f3804e9 100644
--- a/lib/sync.rb
+++ b/lib/sync.rb
@@ -138,16 +138,22 @@ module Sync_m
 
     while true
       @sync_mutex.synchronize do
-        if sync_try_lock_sub(m)
-          return self
-        else
-          if sync_sh_locker[Thread.current]
-            sync_upgrade_waiting.push [Thread.current, sync_sh_locker[Thread.current]]
-            sync_sh_locker.delete(Thread.current)
+        begin
+          if sync_try_lock_sub(m)
+            return self
           else
-            sync_waiting.push Thread.current
+            if sync_sh_locker[Thread.current]
+              sync_upgrade_waiting.push [Thread.current, sync_sh_locker[Thread.current]]
+              sync_sh_locker.delete(Thread.current)
+            else
+              unless sync_waiting.include?(Thread.current) || sync_upgrade_waiting.reverse_each.any?{|w| w.first == Thread.current }
+                sync_waiting.push Thread.current
+              end
+            end
+            @sync_mutex.sleep
           end
-          @sync_mutex.sleep
+        ensure
+          sync_waiting.delete(Thread.current)
         end
       end
     end
diff --git a/test/thread/test_sync.rb b/test/thread/test_sync.rb
new file mode 100644
index 0000000000..870a8e2d39
--- /dev/null
+++ b/test/thread/test_sync.rb
@@ -0,0 +1,57 @@
+require 'test/unit'
+require 'sync'
+require 'timeout'
+
+class SyncTest < Test::Unit::TestCase
+  class Tester
+    include Sync_m
+  end
+
+  def test_sync_lock_and_wakeup
+    tester = Tester.new
+
+    tester.sync_lock(:EX)
+
+    t = Thread.new { tester.sync_lock(:EX) }
+
+    sleep 0.1 until t.stop?
+    t.wakeup
+    sleep 0.1 until t.stop?
+
+    assert_equal(tester.sync_waiting.uniq, tester.sync_waiting)
+  end
+
+  def test_sync_upgrade_and_wakeup
+    tester = Tester.new
+    tester.sync_lock(:SH)
+
+    t = Thread.new do
+      tester.sync_lock(:SH)
+      tester.sync_lock(:EX)
+    end
+
+    sleep 0.1 until t.stop?
+    t.wakeup
+    sleep 0.1 until t.stop?
+
+    tester.sync_upgrade_waiting.each { |ary|
+      assert(!tester.sync_waiting.include?(ary[0]))
+    }
+    assert_equal(tester.sync_waiting.uniq, tester.sync_waiting)
+    assert_equal(tester.sync_waiting, [])
+  end
+
+  def test_sync_lock_and_raise
+    tester= Tester.new
+    tester.sync_lock(:EX)
+
+    t = Thread.new { tester.sync_lock(:EX) }
+
+    sleep 0.1 until t.stop?
+    t.raise
+    sleep 0.1 while t.alive?
+
+    assert_equal(tester.sync_waiting.uniq, tester.sync_waiting)
+    assert_equal(tester.sync_waiting, [])
+  end
+end