diff --git a/Makefile b/Makefile
index 9a024a6..8d0e4a2 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@ CFLAGS += -fno-omit-frame-pointer # More reliable backtraces in GDB
 
 all: kernel.elf
 
-kernel.elf: entry.o start.o lib/string.o
+kernel.elf: entry.o start.o lib/string.o lib/proc.o lib/spinlock.o lib/proc.o
 	@echo LD $@
 	@$(LD) $(LDFLAGS) -o $@ $^
 
diff --git a/start.c b/start.c
index 27ef439..0a9ca9b 100644
--- a/start.c
+++ b/start.c
@@ -1,4 +1,6 @@
+#include <config.h>
 #include <riscv.h>
+#include <spinlock.h>
 #include <types.h>
 
 /* QEMU memory maps a UART device here. */
@@ -20,17 +22,28 @@ void uart_puts(const char *s) {
  */
 char stack0[4096 * NCPU] __attribute__((aligned(16)));
 
+/* Keep this here and sync on it until we have synchronized printf */
+struct spinlock sl = {0};
+volatile int    greeted = 0;
+
 /* This is where entry.S drops us of. All cores land here */
 void start() {
 
     u64 a = r_mhartid();
-    if(a == 0) {
+
+    acquire(&sl);
+
+    if (!greeted) {
         uart_puts("Hello Neptune!\n");
-        uart_puts("Core number: ");
-        uart_putc(a + '0');
-        uart_putc('\n');
+        greeted = 1;
     }
 
+    uart_puts("Hart number: ");
+    uart_putc(a + '0');
+    uart_putc('\n');
+
+    release(&sl);
+
     /* Here we will do a bunch of initialization steps */
 
     // We should not arrive here, but if we do, hang in a while on wfi.