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course/14-mutexes/exercises/6b-rw_mutex_review/multiple_choice.json

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+{
+  "question": "Can readers and writers use RWMutexes at the same time?",
+  "answers": [
+    "No",
+    "Yes"
+  ]
+}

course/14-mutexes/exercises/6b-rw_mutex_review/readme.md

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+# Read/Write Mutex Review
+
+Maps are safe for concurrent *read* access, just not concurrent read/write or write/write access. A read/write mutex allows all the readers to access the map at the same time, but a writer will still lock out all other readers and writers.
+
+```go
+package main
+
+import (
+	"fmt"
+	"sync"
+)
+
+func main() {
+	m := map[int]int{}
+
+	mux := &sync.RWMutex{}
+
+	go writeLoop(m, mux)
+	go readLoop(m, mux)
+	go readLoop(m, mux)
+	go readLoop(m, mux)
+	go readLoop(m, mux)
+
+	// stop program from exiting, must be killed
+	block := make(chan struct{})
+	<-block
+}
+
+func writeLoop(m map[int]int, mux *sync.RWMutex) {
+	for {
+		for i := 0; i < 100; i++ {
+			mux.Lock()
+			m[i] = i
+			mux.Unlock()
+		}
+	}
+}
+
+func readLoop(m map[int]int, mux *sync.RWMutex) {
+	for {
+		mux.RLock()
+		for k, v := range m {
+			fmt.Println(k, "-", v)
+		}
+		mux.RUnlock()
+	}
+}
+```
+
+By using a `sync.RWMutex`, our program becomes *more efficient*. We can have as many `readLoop()` threads as we want, while still ensuring that the writers have exclusive access.
+