add exercise of lab5 spoc discussion

related_info/lab5/lab5-spoc-discuss/user/spin.c

@@ -0,0 +1,29 @@
+#include <stdio.h>
+#include <ulib.h>
+
+int
+main(void) {
+    int pid, ret;
+    cprintf("I am the parent. Forking the child...\n");
+    if ((pid = fork()) == 0) {
+        cprintf("I am the child. spinning ...\n");
+        while (1);
+    }
+    cprintf("I am the parent. Running the child...\n");
+
+    yield();
+    yield();
+    yield();
+
+    cprintf("I am the parent.  Killing the child...\n");
+
+    assert((ret = kill(pid)) == 0);
+    cprintf("kill returns %d\n", ret);
+
+    assert((ret = waitpid(pid, NULL)) == 0);
+    cprintf("wait returns %d\n", ret);
+
+    cprintf("spin may pass.\n");
+    return 0;
+}
+