Drop use of volatile in favor of explicit memory barriers.

[pintos-anon] / src / devices / timer.c

diff --git a/src/devices/timer.c b/src/devices/timer.c
index 9e1b4989c833b62cfcaa5a66032c1cae1124fec0..a4521de86e32b91b7a939f68e06d6f6c7a0b83b3 100644 (file)
--- a/src/devices/timer.c
+++ b/src/devices/timer.c
@@ -5,6 +5,7 @@
 #include <stdio.h>
 #include "threads/interrupt.h"
 #include "threads/io.h"
+#include "threads/synch.h"
 #include "threads/thread.h"
   
 /* See [8254] for hardware details of the 8254 timer chip. */
@@ -16,11 +17,8 @@
 #error TIMER_FREQ <= 1000 recommended
 #endif
 
-/* Number of time ticks to elapse between process yields. */
-#define TIME_SLICE 1
-
 /* Number of timer ticks since OS booted. */
-static volatile int64_t ticks;
+static int64_t ticks;
 
 /* Number of loops per timer tick.
    Initialized by timer_calibrate(). */
@@ -45,7 +43,7 @@ timer_init (void)
   outb (0x40, count & 0xff);
   outb (0x40, count >> 8);
 
-  intr_register (0x20, 0, INTR_OFF, timer_interrupt, "8254 Timer");
+  intr_register_ext (0x20, timer_interrupt, "8254 Timer");
 }
 
 /* Calibrates loops_per_tick, used to implement brief delays. */
@@ -60,8 +58,11 @@ timer_calibrate (void)
   /* Approximate loops_per_tick as the largest power-of-two
      still less than one timer tick. */
   loops_per_tick = 1u << 10;
-  while (!too_many_loops (loops_per_tick << 1))
-    loops_per_tick <<= 1;
+  while (!too_many_loops (loops_per_tick << 1)) 
+    {
+      loops_per_tick <<= 1;
+      ASSERT (loops_per_tick != 0);
+    }
 
   /* Refine the next 8 bits of loops_per_tick. */
   high_bit = loops_per_tick;
@@ -79,6 +80,7 @@ timer_ticks (void)
   enum intr_level old_level = intr_disable ();
   int64_t t = ticks;
   intr_set_level (old_level);
+  barrier ();
   return t;
 }
 
@@ -126,7 +128,7 @@ timer_nsleep (int64_t ns)
 void
 timer_print_stats (void) 
 {
-  printf ("Timer: %"PRId64" ticks\n", ticks);
+  printf ("Timer: %"PRId64" ticks\n", timer_ticks ());
 }
 \f
 /* Timer interrupt handler. */
@@ -135,8 +137,6 @@ timer_interrupt (struct intr_frame *args UNUSED)
 {
   ticks++;
   thread_tick ();
-  if (ticks % TIME_SLICE == 0)
-    intr_yield_on_return ();
 }
 
 /* Returns true if LOOPS iterations waits for more than one timer
@@ -144,18 +144,17 @@ timer_interrupt (struct intr_frame *args UNUSED)
 static bool
 too_many_loops (unsigned loops) 
 {
-  int64_t start;
-
   /* Wait for a timer tick. */
-  start = ticks;
+  int64_t start = ticks;
   while (ticks == start)
-    continue;
+    barrier ();
 
   /* Run LOOPS loops. */
   start = ticks;
   busy_wait (loops);
 
   /* If the tick count changed, we iterated too long. */
+  barrier ();
   return start != ticks;
 }
 
@@ -170,7 +169,7 @@ static void NO_INLINE
 busy_wait (int64_t loops) 
 {
   while (loops-- > 0)
-    continue;
+    barrier ();
 }
 
 /* Sleep for approximately NUM/DENOM seconds. */