X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Fthread.c;h=16b8c8b692e8664b81bab789eb3eeb946e38f3b7;hb=13753f29344700c01d9dc80834e51c7303ed18f7;hp=dc23efb4b2dfb3bb608084940991a3c916d50ae6;hpb=bb15e05316dc37273c24dcda094806a0250a328b;p=pintos-anon

diff --git a/src/threads/thread.c b/src/threads/thread.c
index dc23efb..16b8c8b 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -96,7 +96,8 @@ thread_start (void)
   intr_enable ();
 }
 
-/* Called by the timer interrupt handler at each timer tick. */
+/* Called by the timer interrupt handler at each timer tick.
+   Thus, this function runs in an external interrupt context. */
 void
 thread_tick (void) 
 {
@@ -328,10 +329,24 @@ thread_get_recent_cpu (void)
   return 0;
 }
 
-/* Idle thread.  Executes when no other thread is ready to run. */
+/* Idle thread.  Executes when no other thread is ready to run.
+
+   The idle thread is initially put on the ready list by
+   thread_start().  It will be scheduled once initially, at which
+   point it initializes idle_thread and immediately blocks.
+   After that, the idle thread never appears in the ready list.
+   It is returned by next_thread_to_run() as a special case when
+   the ready list is empty. */
 static void
 idle (void *aux UNUSED) 
 {
+  /* Initialize idle_thread.
+
+     Until we run for the first time, idle_thread remains a null
+     pointer.  That's okay because we know that, at that point,
+     the ready list has at least one element (the idle thread),
+     so next_thread_to_run() will not attempt to return the idle
+     thread. */
   idle_thread = thread_current ();
 
   for (;;) 
@@ -350,7 +365,8 @@ idle (void *aux UNUSED)
          one to occur, wasting as much as one clock tick worth of
          time.
 
-         See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3] 7.7. */
+         See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3a]
+         7.11.1 "HLT Instruction". */
       asm ("sti; hlt");
     }
 }
@@ -467,12 +483,13 @@ schedule_tail (struct thread *prev)
 
   /* If the thread we switched from is dying, destroy its struct
      thread.  This must happen late so that thread_exit() doesn't
-     pull out the rug under itself. */
-  if (prev != NULL && prev->status == THREAD_DYING) 
+     pull out the rug under itself.  (We don't free
+     initial_thread because its memory was not obtained via
+     palloc().) */
+  if (prev != NULL && prev->status == THREAD_DYING && prev != initial_thread) 
     {
       ASSERT (prev != cur);
-      if (prev != initial_thread)
-        palloc_free_page (prev);
+      palloc_free_page (prev);
     }
 }