diff --git a/usertests.c b/usertests.c
index 740b81d..11464f3 100644
--- a/usertests.c
+++ b/usertests.c
@@ -138,6 +138,76 @@ writetest1(void)
   printf(stdout, "big files ok\n");
 }
 
+void
+writetest2(void)
+{
+  int fd, fd1, n;
+
+  printf(stdout, "files with holes\n");
+
+  fd = open("hole", O_CREATE|O_RDWR);
+  if(fd < 0){
+    printf(stdout, "error: creat hole failed!\n");
+    exit();
+  }
+
+  buf[0] = 1;
+  if(write(fd, buf, 512) != 512) {
+    printf(stdout, "error: write hole file failed\n");
+    exit();
+  }
+
+  // now truncate, while fd is still open
+  fd1 = open("hole", O_CREATE | O_RDWR);
+  if(fd < 0){
+    printf(stdout, "error: 2nd creat hole failed!\n");
+    exit();
+  }
+  close(fd1);
+
+  // now write another block, should be 2nd in file
+  buf[0] = 2;
+  if(write(fd, buf, 512) != 512) {
+    printf(stdout, "error: 2nd write hole file failed\n");
+    exit();
+  }
+
+  close(fd);
+
+  fd = open("hole", O_RDONLY);
+  if(fd < 0){
+    printf(stdout, "error: open hole failed!\n");
+    exit();
+  }
+
+  n = read(fd, buf, 512);
+  if(n != 512){
+    printf(stdout, "error: 1st hole read failed\n");
+    exit();
+  }
+  if(buf[0] != 0){
+    printf(stdout, "error: 1st hole block didn't have zeros\n");
+    exit();
+  }
+
+  n = read(fd, buf, 512);
+  if(n != 512){
+    printf(stdout, "error: 2nd hole read failed\n");
+    exit();
+  }
+  if(buf[0] != 1){
+    printf(stdout, "error: 2nd hole block wrong content\n");
+    exit();
+  }
+
+  close(fd);
+  if(unlink("hole") < 0) {
+    printf(stdout, "unlink hole failed\n");
+    exit();
+  }
+  printf(stdout, "hole files ok\n");
+}
+
 void
 createtest(void)
 {
@@ -461,7 +531,7 @@ twofiles(void)
   printf(1, "twofiles ok\n");
 }
 
-// two processes create and delete files in same directory
+// two processes create and delete different files in same directory
 void
 createdelete(void)
 {
@@ -647,7 +717,7 @@ linktest(void)
   printf(1, "linktest ok\n");
 }
 
-// test concurrent create of the same file
+// test concurrent create and unlink of the same file
 void
 concreate(void)
 {
@@ -713,7 +783,22 @@ concreate(void)
 
   for(i = 0; i < 40; i++){
     file[1] = '0' + i;
-    unlink(file);
+    pid = fork();
+    if(pid < 0){
+      printf(1, "fork failed\n");
+      exit();
+    }
+    if(((i % 3) == 0 && pid == 0) ||
+       ((i % 3) == 1 && pid != 0)){
+      fd = open(file, 0);
+      close(fd);
+    } else {
+      unlink(file);
+    }
+    if(pid == 0)
+      exit();
+    else
+      wait();
   }
 
   printf(1, "concreate ok\n");
@@ -1095,6 +1180,7 @@ main(int argc, char *argv[])
   opentest();
   writetest();
   writetest1();
+  writetest2();
   createtest();
 
   mem();