Improve comment on thread_unblock().

[pintos-anon] / src / threads / thread.c

diff --git a/src/threads/thread.c b/src/threads/thread.c
index dc23efb4b2dfb3bb608084940991a3c916d50ae6..92d1aa899efe1d8bf84c6c7056ae7d45215c687b 100644 (file)
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -7,10 +7,10 @@
 #include "threads/flags.h"
 #include "threads/interrupt.h"
 #include "threads/intr-stubs.h"
 #include "threads/flags.h"
 #include "threads/interrupt.h"
 #include "threads/intr-stubs.h"

-#include "threads/mmu.h"

 #include "threads/palloc.h"
 #include "threads/switch.h"
 #include "threads/synch.h"
 #include "threads/palloc.h"
 #include "threads/switch.h"
 #include "threads/synch.h"

+#include "threads/vaddr.h"

 #ifdef USERPROG
 #include "userprog/process.h"
 #endif
 #ifdef USERPROG
 #include "userprog/process.h"
 #endif


@@ -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. */
 
 #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);
 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
 
    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) 
 {
 void
 thread_init (void) 
 {


@@ -92,11 +100,20 @@ thread_init (void)
 void
 thread_start (void) 
 {
 void
 thread_start (void) 
 {

-  thread_create ("idle", PRI_MAX, 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 ();
   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. */
+/* Called by the timer interrupt handler at each timer tick.
+   Thus, this function runs in an external interrupt context. */

 void
 thread_tick (void) 
 {
 void
 thread_tick (void) 
 {


@@ -199,7 +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
 
 /* 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) 
 {
 void
 thread_unblock (struct thread *t) 
 {


@@ -277,7 +299,8 @@ thread_yield (void)
   ASSERT (!intr_context ());
 
   old_level = intr_disable ();
   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);
   cur->status = THREAD_READY;
   schedule ();
   intr_set_level (old_level);


@@ -328,11 +351,21 @@ thread_get_recent_cpu (void)
   return 0;
 }
 \f
   return 0;
 }
 \f

-/* 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, "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
 static void

-idle (void *aux UNUSED) 
+idle (void *idle_started_ UNUSED) 

 {
 {

+  struct semaphore *idle_started = idle_started_;

   idle_thread = thread_current ();
   idle_thread = thread_current ();

+  sema_up (idle_started);

 
   for (;;) 
     {
 
   for (;;) 
     {


@@ -350,8 +383,9 @@ idle (void *aux UNUSED)
          one to occur, wasting as much as one clock tick worth of
          time.
 
          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. */
-      asm ("sti; hlt");
+         See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3a]
+         7.11.1 "HLT Instruction". */
+      asm volatile ("sti; hlt" : : : "memory");

     }
 }
 
     }
 }
 


@@ -467,12 +501,13 @@ 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
 
   /* 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);
     {
       ASSERT (prev != cur);

-      if (prev != initial_thread)
-        palloc_free_page (prev);
+      palloc_free_page (prev);

     }
 }
 
     }
 }