X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Fthread.c;h=92d1aa899efe1d8bf84c6c7056ae7d45215c687b;hb=b0c619871eee10551a0d510b67ee3a2493088341;hp=8d9f33558bb5313b1e137250479b5103a88db689;hpb=f415a37905c57f61b444806bf84f5405184452aa;p=pintos-anon

diff --git a/src/threads/thread.c b/src/threads/thread.c
index 8d9f335..92d1aa8 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -50,6 +50,11 @@ static long long user_ticks;    /* # of timer ticks in user programs. */
 #define TIME_SLICE 4            /* # of timer ticks to give each thread. */
 static unsigned thread_ticks;   /* # of timer ticks since last yield. */
 
+/* If false (default), use round-robin scheduler.
+   If true, use multi-level feedback queue scheduler.
+   Controlled by kernel command-line option "-o mlfqs". */
+bool thread_mlfqs;
+
 static void kernel_thread (thread_func *, void *aux);
 
 static void idle (void *aux UNUSED);
@@ -71,7 +76,10 @@ static tid_t allocate_tid (void);
 
    After calling this function, be sure to initialize the page
    allocator before trying to create any threads with
-   thread_create(). */
+   thread_create().
+
+   It is not safe to call thread_current() until this function
+   finishes. */
 void
 thread_init (void) 
 {
@@ -92,8 +100,16 @@ thread_init (void)
 void
 thread_start (void) 
 {
-  thread_create ("idle", PRI_MIN, idle, NULL);
+  /* Create the idle thread. */
+  struct semaphore idle_started;
+  sema_init (&idle_started, 0);
+  thread_create ("idle", PRI_MIN, idle, &idle_started);
+
+  /* Start preemptive thread scheduling. */
   intr_enable ();
+
+  /* Wait for the idle thread to initialize idle_thread. */
+  sema_down (&idle_started);
 }
 
 /* Called by the timer interrupt handler at each timer tick.
@@ -200,7 +216,12 @@ thread_block (void)
 
 /* Transitions a blocked thread T to the ready-to-run state.
    This is an error if T is not blocked.  (Use thread_yield() to
-   make the running thread ready.) */
+   make the running thread ready.)
+
+   This function does not preempt the running thread.  This can
+   be important: if the caller had disabled interrupts itself,
+   it may expect that it can atomically unblock a thread and
+   update other data. */
 void
 thread_unblock (struct thread *t) 
 {
@@ -278,7 +299,8 @@ thread_yield (void)
   ASSERT (!intr_context ());
 
   old_level = intr_disable ();
-  list_push_back (&ready_list, &cur->elem);
+  if (cur != idle_thread) 
+    list_push_back (&ready_list, &cur->elem);
   cur->status = THREAD_READY;
   schedule ();
   intr_set_level (old_level);
@@ -333,21 +355,17 @@ thread_get_recent_cpu (void)
 
    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. */
+   point it initializes idle_thread, "up"s the semaphore passed
+   to it to enable thread_start() to continue, 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) 
+idle (void *idle_started_ 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. */
+  struct semaphore *idle_started = idle_started_;
   idle_thread = thread_current ();
+  sema_up (idle_started);
 
   for (;;) 
     {
@@ -367,7 +385,7 @@ idle (void *aux UNUSED)
 
          See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3a]
          7.11.1 "HLT Instruction". */
-      asm ("sti; hlt");
+      asm volatile ("sti; hlt" : : : "memory");
     }
 }