Redesign reschedule points, allow one operation to reschedule many cpus at once

kernel/amd64/debug.c

@@ -26,9 +26,13 @@ static bool amd64_debug_serial_tx_empty (void) {
 
 /* Write a single character to serial */
 static void amd64_debug_serial_write (char x) {
+  spin_lock (&serial_lock);
+
   while (!amd64_debug_serial_tx_empty ())
     ;
   amd64_io_outb (PORT_COM1, (uint8_t)x);
+
+  spin_unlock (&serial_lock);
 }
 
 /*
@@ -50,14 +54,10 @@ void debugprintf (const char* fmt, ...) {
 
   const char* p = buffer;
 
-  spin_lock (&serial_lock);
-
   while (*p) {
     amd64_debug_serial_write (*p);
     p++;
   }
-
-  spin_unlock (&serial_lock);
 }
 
 /* Initialize serial */