Add example code of mutex

categories/go.md

@@ -63,10 +63,56 @@ ch := make(chan Task, 3)
 ## Sync
 
 ###  atomic
+
 ### Mutex
-    - sync.Mutex()
-    - mutex.Lock()
-    - mutx.Unlock()
+
+  - sync.Mutex()
+  - mutex.Lock()
+  - mutx.Unlock()
+
+```go
+package main
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+// SafeCounter is safe to use concurrently.
+type SafeCounter struct {
+	v   map[string]int
+	mux sync.Mutex
+}
+
+// Inc increments the counter for the given key.
+func (c *SafeCounter) Inc(key string) {
+	c.mux.Lock()
+	// Lock so only one goroutine at a time can access the map c.v.
+	c.v[key]++
+	c.mux.Unlock()
+}
+
+// Value returns the current value of the counter for the given key.
+func (c *SafeCounter) Value(key string) int {
+	c.mux.Lock()
+	// Lock so only one goroutine at a time can access the map c.v.
+	defer c.mux.Unlock()
+	return c.v[key]
+}
+
+func main() {
+	c := SafeCounter{v: make(map[string]int)}
+	for i := 0; i < 1000; i++ {
+		go c.Inc("somekey")
+	}
+
+	time.Sleep(time.Second)
+	fmt.Println(c.Value("somekey"))
+}
+```
+
+
 ### RWMutex
   > A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.