diff --git a/contrib/getdents-debug/getdents_c/Makefile b/contrib/getdents-debug/getdents_c/Makefile
index 95e47dc..1a9d4d2 100644
--- a/contrib/getdents-debug/getdents_c/Makefile
+++ b/contrib/getdents-debug/getdents_c/Makefile
@@ -1,2 +1,2 @@
 getdents_c: *.c Makefile
-	gcc getdents.c -o getdents_c
+	gcc getdents.c -lpthread -o getdents_c
diff --git a/contrib/getdents-debug/getdents_c/getdents.c b/contrib/getdents-debug/getdents_c/getdents.c
index 936bd0b..d504976 100644
--- a/contrib/getdents-debug/getdents_c/getdents.c
+++ b/contrib/getdents-debug/getdents_c/getdents.c
@@ -1,6 +1,7 @@
 // See ../getdents/getdents.go for some info on why
 // this exists.
 
+#define _GNU_SOURCE
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -9,6 +10,38 @@
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <errno.h>
+#include <pthread.h>
+
+static pthread_mutex_t mutex;
+static int sum;
+
+struct up {
+    pthread_mutex_t *my_mutex;
+    pthread_mutex_t *other_mutex;
+    int fd;
+};
+
+void *reader(void *arg) {
+    struct up* up = (struct up*)arg;
+    int fd = up->fd;
+    char tmp[10000];
+    while(1) {
+        pthread_mutex_lock(up->my_mutex);
+        int n = syscall(SYS_getdents64, fd, tmp, sizeof(tmp));
+        if (n > 0) {
+            printf("t%ld: n=%d\n", gettid(), n);
+        } else {
+            printf("t%ld: n=0 errno=%d total %d bytes\n", gettid(), errno, sum);
+            if (n < 0) {
+                exit(1);
+            }
+            pthread_mutex_unlock(up->other_mutex);
+            break;
+        }
+        sum += n;
+        pthread_mutex_unlock(up->other_mutex);
+    }
+}
 
 int main(int argc, char *argv[])
 {
@@ -21,28 +54,28 @@ int main(int argc, char *argv[])
     const char *path = argv[1];
 
     for (int i = 1 ; ; i ++ ) {
+        sum = 0;
         int fd = open(path, O_RDONLY);
         if (fd == -1) {
             perror("open");
             exit(1);
         }
-
-        char tmp[10000];
-        int sum = 0;
-        printf("%3d: getdents64: ", i);
-        for ( ; ; ) {
-            int n = syscall(SYS_getdents64, fd, tmp, sizeof(tmp));
-            printf("n=%d; ", n);
-            if (n <= 0) {
-                printf("errno=%d total %d bytes\n", errno, sum);
-                if (n < 0) {
-                    exit(1);
-                }
-                break;
-            }
-            sum += n;
-        }
+        pthread_t reader1_thread, reader2_thread;
+        pthread_mutex_t m1 = PTHREAD_MUTEX_INITIALIZER, m2 = PTHREAD_MUTEX_INITIALIZER;
+        pthread_mutex_lock(&m2);
+        struct up up1, up2;
+        up1.fd = fd;
+        up1.my_mutex = &m1;
+        up1.other_mutex = &m2;
+        up2.fd = fd;
+        up2.my_mutex = &m2;
+        up2.other_mutex = &m1;
+        pthread_create(&reader1_thread, NULL, reader, &up1);
+        pthread_create(&reader2_thread, NULL, reader, &up2);
+        pthread_join(reader1_thread, NULL);
+        pthread_join(reader2_thread, NULL);
         close(fd);
+        printf("---\n");
         usleep(100000);
     }
 }