1 files changed, 46 insertions, 3 deletions
diff --git a/src/devices/timer.c b/src/devices/timer.c
index a4521de..aea6aa2 100644
--- a/src/devices/timer.c
+++ b/src/devices/timer.c
@@ -1,6 +1,7 @@
 #include "devices/timer.h"
 #include <debug.h>
 #include <inttypes.h>
+#include <list.h>
 #include <round.h>
 #include <stdio.h>
 #include "threads/interrupt.h"
@@ -17,6 +18,13 @@
 #error TIMER_FREQ <= 1000 recommended
 #endif
 
+struct waiting_thread
+  {
+    struct list_elem elem;
+    struct semaphore sema;
+    int64_t tick_target;
+  };
+
 /* Number of timer ticks since OS booted. */
 static int64_t ticks;
 
@@ -24,6 +32,8 @@ static int64_t ticks;
    Initialized by timer_calibrate(). */
 static unsigned loops_per_tick;
 
+static struct list waiting_threads;
+
 static intr_handler_func timer_interrupt;
 static bool too_many_loops (unsigned loops);
 static void busy_wait (int64_t loops);
@@ -43,6 +53,8 @@ timer_init (void)
   outb (0x40, count & 0xff);
   outb (0x40, count >> 8);
 
+  list_init (&waiting_threads);
+
   intr_register_ext (0x20, timer_interrupt, "8254 Timer");
 }
 
@@ -98,9 +110,27 @@ timer_sleep (int64_t ticks)
 {
   int64_t start = timer_ticks ();
 
-  ASSERT (intr_get_level () == INTR_ON);
-  while (timer_elapsed (start) < ticks) 
-    thread_yield ();
+  struct waiting_thread new_waiting_thread;
+  new_waiting_thread.tick_target = start + ticks;
+  sema_init (&new_waiting_thread.sema, 0);
+
+  // disable interrupts so the timer interrupt handler can't modify the list
+  enum intr_level old_level = intr_disable ();
+
+  struct list_elem *before = list_begin (&waiting_threads);
+  while (before != list_end (&waiting_threads)) {
+    struct list_elem *next = list_next (before);
+    struct waiting_thread *waiting_thread = list_entry (before, struct waiting_thread, elem);
+    if (new_waiting_thread.tick_target < waiting_thread->tick_target) {
+      break;
+    }
+    before = next;
+  }
+  list_insert (before, &new_waiting_thread.elem);
+
+  intr_set_level (old_level);
+
+  sema_down (&new_waiting_thread.sema);
 }
 
 /* Suspends execution for approximately MS milliseconds. */
@@ -137,6 +167,19 @@ timer_interrupt (struct intr_frame *args UNUSED)
 {
   ticks++;
   thread_tick ();
+
+  struct list_elem *e = list_begin (&waiting_threads);
+  while (e != list_end (&waiting_threads)) {
+    struct list_elem *next = list_next (e);
+    struct waiting_thread *waiting_thread = list_entry (e, struct waiting_thread, elem);
+    if (ticks >= waiting_thread->tick_target) {
+      sema_up (&waiting_thread->sema);
+      list_remove (e);
+      e = next;
+    } else {
+      break;
+    }
+  }
 }
 
 /* Returns true if LOOPS iterations waits for more than one timer