diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index f2e9128e8a..7843fa0877 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -3781,6 +3781,15 @@ __END__
           IO.pipe do |r, w|
             th = Thread.new { r.read(1) }
             w.write(dot)
+
+            # XXX not sure why this is needed on Linux, otherwise
+            # the "good" reader thread doesn't always join properly
+            # because the reader never sees the first write
+            if RUBY_PLATFORM =~ /linux/
+              # assert_equal can fail if this is another char...
+              w.write(dot)
+            end
+
             assert_same th, th.join(15), '"good" reader timeout'
             assert_equal(dot, th.value)
           end
@@ -3798,7 +3807,10 @@ __END__
             end
           end
           Thread.pass until th.stop?
+
+          # XXX not sure why, this reduces Linux CI failures
           assert_nil th.join(0.001)
+
           r.close
           assert_same th, th.join(30), '"bad" reader timeout'
           assert_match(/stream closed/, th.value.message)