diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index 921f5a01c1..eebedb053b 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -1205,6 +1205,17 @@ q.pop
     assert_predicate(status, :success?, bug9751)
   end if Process.respond_to?(:fork)
 
+  def test_fork_while_locked
+    m = Mutex.new
+    thrs = []
+    3.times do |i|
+      thrs << Thread.new { m.synchronize { Process.waitpid2(fork{})[1] } }
+    end
+    thrs.each do |t|
+      assert_predicate t.value, :success?, '[ruby-core:85940] [Bug #14578]'
+    end
+  end if Process.respond_to?(:fork)
+
   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.c b/thread.c
index 27f5e5dabf..2c06127292 100644
--- a/thread.c
+++ b/thread.c
@@ -4236,6 +4236,7 @@ rb_thread_atfork(void)
     rb_thread_t *th = GET_THREAD();
     rb_thread_atfork_internal(th, terminate_atfork_i);
     th->join_list = NULL;
+    rb_mutex_cleanup_keeping_mutexes(th);
 
     /* We don't want reproduce CVE-2003-0900. */
     rb_reset_random_seed();
diff --git a/thread_sync.c b/thread_sync.c
index e4a28ccb5c..8f68583a78 100644
--- a/thread_sync.c
+++ b/thread_sync.c
@@ -415,6 +415,20 @@ rb_mutex_abandon_all(rb_mutex_t *mutexes)
 	list_head_init(&mutex->waitq);
     }
 }
+
+/*
+ * All other threads are dead in the a new child process, so waitqs
+ * contain references to dead threads which we need to clean up
+ */
+static void
+rb_mutex_cleanup_keeping_mutexes(const rb_thread_t *current_thread)
+{
+    rb_mutex_t *mutex = current_thread->keeping_mutexes;
+    while (mutex) {
+        list_head_init(&mutex->waitq);
+        mutex = mutex->next_mutex;
+    }
+}
 #endif
 
 static VALUE