diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index 2b120b4a17..78d6d82e23 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -1239,6 +1239,27 @@ q.pop
     end
   end if Process.respond_to?(:fork)
 
+  def test_fork_while_parent_locked
+    skip 'needs fork' unless Process.respond_to?(:fork)
+    m = Thread::Mutex.new
+    failures = 0
+    run = true
+    thrs = 50.times.map do
+      Thread.new do
+        while run
+          pid = fork { m.synchronize {} }
+          m.synchronize {}
+          _, st = Process.waitpid2(pid)
+          m.synchronize { failures += 1 } unless st.success?
+        end
+      end
+    end
+    sleep 0.5
+    run = false
+    thrs.each(&:join)
+    assert_equal 0, failures, '[ruby-core:90312] [Bug #15383]'
+  end
+
   def test_subclass_no_initialize
     t = Module.new do
       break eval("class C\u{30b9 30ec 30c3 30c9} < Thread; self; end")
diff --git a/thread_sync.c b/thread_sync.c
index b79db1fee3..e478a96b88 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -45,6 +45,7 @@ typedef struct rb_mutex_struct {
     rb_thread_t *th;
     struct rb_mutex_struct *next_mutex;
     struct list_head waitq; /* protected by GVL */
+    rb_serial_t fork_gen;
 } rb_mutex_t;
 
 #if defined(HAVE_WORKING_FORK)
@@ -121,8 +122,18 @@ static rb_mutex_t *
 mutex_ptr(VALUE obj)
 {
     rb_mutex_t *mutex;
+    rb_serial_t fork_gen = GET_VM()->fork_gen;
 
     TypedData_Get_Struct(obj, rb_mutex_t, &mutex_data_type, mutex);
+
+    if (mutex->fork_gen != fork_gen) {
+        /* forked children can't reach into parent thread stacks */
+        mutex->fork_gen = fork_gen;
+        list_head_init(&mutex->waitq);
+        mutex->next_mutex = 0;
+        mutex->th = 0;
+    }
+
     return mutex;
 }