diff --git a/progress.md b/progress.md
index 8012cc6..62e5078 100644
--- a/progress.md
+++ b/progress.md
@@ -81,7 +81,7 @@
  - [ ] 9.1 Race Conditions
  - [ ] 9.2 Mutual Exclusion: sync.Mutex
  - [x] 9.3 Read/Write Mutexes: sync.RWMutex
- - [ ] 9.4 Memory Synchronization
+ - [x] 9.4 Memory Synchronization
  - [ ] 9.5 Lazy Initialization: sync.Once
  - [x] 9.6 The Race Detector
  - [ ] 9.7 Example: Concurrent Non-Blocking Cache