diff --git a/ChangeLog b/ChangeLog
index 50f022c12e..bc8e22e55b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Sep 11 07:55:17 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* thread.c (rb_mutex_unlock): Mutex#unlock no longer raise
+	  an exception even if uses on trap. [Bug #8891]
+
 Tue Sep 10 14:37:01 2013  Shota Fukumori  <sorah@tubusu.net>
 
 	* vm_backtrace.c (vm_backtrace_to_ary): Ignore the second argument if
diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index dd13842bfe..bc3c025253 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -849,6 +849,23 @@ Thread.new(Thread.current) {|mth|
     end
   end
 
+  def test_mutex_unlock_on_trap
+    assert_in_out_err([], <<-INPUT, %w(locked unlocked false), [])
+      m = Mutex.new
+
+      Signal.trap("INT") { |signo|
+        m.unlock
+        puts "unlocked"
+      }
+
+      m.lock
+      puts "locked"
+      Process.kill("INT", $$)
+      sleep 0.01
+      puts m.locked?
+    INPUT
+  end
+
   def invoke_rec script, vm_stack_size, machine_stack_size, use_length = true
     env = {}
     env['RUBY_THREAD_VM_STACK_SIZE'] = vm_stack_size.to_s if vm_stack_size
diff --git a/thread.c b/thread.c
index 826a81b8b5..c2f6678b7b 100644
--- a/thread.c
+++ b/thread.c
@@ -4481,11 +4481,6 @@ rb_mutex_unlock(VALUE self)
     rb_mutex_t *mutex;
     GetMutexPtr(self, mutex);
 
-    /* When running trap handler */
-    if (!mutex->allow_trap && GET_THREAD()->interrupt_mask & TRAP_INTERRUPT_MASK) {
-	rb_raise(rb_eThreadError, "can't be called from trap context");
-    }
-
     err = rb_mutex_unlock_th(mutex, GET_THREAD());
     if (err) rb_raise(rb_eThreadError, "%s", err);