X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?p=pintos-anon;a=blobdiff_plain;f=src%2Fthreads%2Fthread.c;h=d68c123fb2476420d09557f4f52d4a6fb12949f4;hp=bae893d5f872d70f1cce8803fe03bf9e2fc62846;hb=09fa5c11aff62bd728d4fe8de28a7abaf252fed3;hpb=102ba3ee754e0cdc61ad66a3322e92e3d15fe171

diff --git a/src/threads/thread.c b/src/threads/thread.c
index bae893d..d68c123 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -1,141 +1,193 @@
-#include "thread.h"
+#include "threads/thread.h"
+#include <debug.h>
 #include <stddef.h>
-#include "debug.h"
-#include "interrupt.h"
-#include "intr-stubs.h"
-#include "lib.h"
-#include "mmu.h"
-#include "palloc.h"
-#include "random.h"
-#include "switch.h"
+#include <random.h>
+#include <stdio.h>
+#include <string.h>
+#include "threads/flags.h"
+#include "threads/interrupt.h"
+#include "threads/intr-stubs.h"
+#include "threads/palloc.h"
+#include "threads/switch.h"
+#include "threads/synch.h"
+#include "threads/vaddr.h"
+#ifdef USERPROG
+#include "userprog/process.h"
+#endif
 
-/* Offset of `stack' member within `struct thread'.
-   Used by switch.S, which can't figure it out on its own. */
-uint32_t thread_stack_ofs = offsetof (struct thread, stack);
+/* Random value for struct thread's `magic' member.
+   Used to detect stack overflow.  See the big comment at the top
+   of thread.h for details. */
+#define THREAD_MAGIC 0xcd6abf4b
 
 /* List of processes in THREAD_READY state, that is, processes
    that are ready to run but not actually running. */
-static struct list run_queue;
+static struct list ready_list;
+
+/* List of all processes.  Processes are added to this list
+   when they are first scheduled and removed when they exit. */
+static struct list all_list;
 
-/* Thread to run when nothing else is ready. */
+/* Idle thread. */
 static struct thread *idle_thread;
 
-static struct thread *find_next_to_run (void);
+/* Initial thread, the thread running init.c:main(). */
+static struct thread *initial_thread;
 
-/* Idle thread.  Executes when no other thread is ready to run. */
-static void
-idle (void *aux UNUSED) 
-{
-  for (;;) 
-    {
-      /* Wait for an interrupt. */
-      DEBUG (idle, "idle");
-      asm ("hlt");
+/* Lock used by allocate_tid(). */
+static struct lock tid_lock;
 
-      /* Let someone else run. */
-      intr_disable ();
-      thread_sleep ();
-      intr_enable ();
-    }
-}
+/* Stack frame for kernel_thread(). */
+struct kernel_thread_frame 
+  {
+    void *eip;                  /* Return address. */
+    thread_func *function;      /* Function to call. */
+    void *aux;                  /* Auxiliary data for function. */
+  };
 
-/* Initializes the threading system and starts an initial thread
-   which is immediately scheduled.  Never returns to the caller.
-   The initial thread is named NAME and executes FUNCTION passing
-   AUX as the argument. */
+/* Statistics. */
+static long long idle_ticks;    /* # of timer ticks spent idle. */
+static long long kernel_ticks;  /* # of timer ticks in kernel threads. */
+static long long user_ticks;    /* # of timer ticks in user programs. */
+
+/* Scheduling. */
+#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 struct thread *running_thread (void);
+static struct thread *next_thread_to_run (void);
+static void init_thread (struct thread *, const char *name, int priority);
+static bool is_thread (struct thread *) UNUSED;
+static void *alloc_frame (struct thread *, size_t size);
+static void schedule (void);
+void thread_schedule_tail (struct thread *prev);
+static tid_t allocate_tid (void);
+
+/* Initializes the threading system by transforming the code
+   that's currently running into a thread.  This can't work in
+   general and it is possible in this case only because loader.S
+   was careful to put the bottom of the stack at a page boundary.
+
+   Also initializes the run queue and the tid lock.
+
+   After calling this function, be sure to initialize the page
+   allocator before trying to create any threads with
+   thread_create().
+
+   It is not safe to call thread_current() until this function
+   finishes. */
 void
 thread_init (void) 
 {
-  ASSERT (intr_get_level () == IF_OFF);
+  ASSERT (intr_get_level () == INTR_OFF);
 
-  /* Initialize run queue. */
-  list_init (&run_queue);
+  lock_init (&tid_lock);
+  list_init (&ready_list);
+  list_init (&all_list);
 
-  /* Create idle thread. */
-  idle_thread = thread_create ("idle", idle, NULL);
-  idle_thread->status = THREAD_BLOCKED;
+  /* Set up a thread structure for the running thread. */
+  initial_thread = running_thread ();
+  init_thread (initial_thread, "main", PRI_DEFAULT);
+  initial_thread->status = THREAD_RUNNING;
+  initial_thread->tid = allocate_tid ();
 }
 
+/* Starts preemptive thread scheduling by enabling interrupts.
+   Also creates the idle thread. */
 void
 thread_start (void) 
 {
-  struct thread *t = find_next_to_run ();
-  if (t->status == THREAD_READY)
-    list_remove (&t->rq_elem);
-  t->status = THREAD_RUNNING;
-  switch_threads (NULL, t);
+  /* Create the idle thread. */
+  struct semaphore idle_started;
+  sema_init (&idle_started, 0);
+  thread_create ("idle", PRI_MIN, idle, &idle_started);
 
-  NOT_REACHED ();
-}
-
-/* Stack frame for kernel_thread(). */
-struct kernel_thread_frame 
-  {
-    void *eip;                  /* Return address. */
-    void (*function) (void *);  /* Function to call. */
-    void *aux;                  /* Auxiliary data for function. */
-  };
+  /* Start preemptive thread scheduling. */
+  intr_enable ();
 
-/* Function used as the basis for a kernel thread. */
-static void
-kernel_thread (void (*function) (void *aux), void *aux) 
-{
-  ASSERT (function != NULL);
-
-  intr_enable ();       /* The scheduler runs with interrupts off. */
-  function (aux);       /* Execute the thread function. */
-  thread_exit ();       /* If function() returns, kill the thread. */
+  /* Wait for the idle thread to initialize idle_thread. */
+  sema_down (&idle_started);
 }
 
-/* Creates a new thread named NAME and initializes its fields.
-   Returns the new thread if successful or a null pointer on
-   failure. */
-static struct thread *
-new_thread (const char *name) 
+/* Called by the timer interrupt handler at each timer tick.
+   Thus, this function runs in an external interrupt context. */
+void
+thread_tick (void) 
 {
-  struct thread *t;
+  struct thread *t = thread_current ();
 
-  ASSERT (name != NULL);
-  
-  t = palloc_get (PAL_ZERO);
-  if (t != NULL)
-    {
-      strlcpy (t->name, name, sizeof t->name);
-      t->stack = (uint8_t *) t + PGSIZE;
-      t->status = THREAD_INITIALIZING;
-    }
-  
-  return t;
+  /* Update statistics. */
+  if (t == idle_thread)
+    idle_ticks++;
+#ifdef USERPROG
+  else if (t->pagedir != NULL)
+    user_ticks++;
+#endif
+  else
+    kernel_ticks++;
+
+  /* Enforce preemption. */
+  if (++thread_ticks >= TIME_SLICE)
+    intr_yield_on_return ();
 }
 
-/* Allocates a SIZE-byte frame within thread T's stack and
-   returns a pointer to the frame's base. */
-static void *
-alloc_frame (struct thread *t, size_t size) 
+/* Prints thread statistics. */
+void
+thread_print_stats (void) 
 {
-  /* Stack data is always allocated in word-size units. */
-  ASSERT (size % sizeof (uint32_t) == 0);
-
-  t->stack -= size;
-  return t->stack;
+  printf ("Thread: %lld idle ticks, %lld kernel ticks, %lld user ticks\n",
+          idle_ticks, kernel_ticks, user_ticks);
 }
 
-/* Creates a new kernel thread named NAME, which executes
-   FUNCTION passing AUX as the argument.  The thread is added to
-   the ready queue.  Thus, it may be scheduled even before
-   thread_create() returns.  If you need to ensure ordering, then
-   use synchronization, such as a semaphore. */
-struct thread *
-thread_create (const char *name, void (*function) (void *aux), void *aux) 
+/* Creates a new kernel thread named NAME with the given initial
+   PRIORITY, which executes FUNCTION passing AUX as the argument,
+   and adds it to the ready queue.  Returns the thread identifier
+   for the new thread, or TID_ERROR if creation fails.
+
+   If thread_start() has been called, then the new thread may be
+   scheduled before thread_create() returns.  It could even exit
+   before thread_create() returns.  Contrariwise, the original
+   thread may run for any amount of time before the new thread is
+   scheduled.  Use a semaphore or some other form of
+   synchronization if you need to ensure ordering.
+
+   The code provided sets the new thread's `priority' member to
+   PRIORITY, but no actual priority scheduling is implemented.
+   Priority scheduling is the goal of Problem 1-3. */
+tid_t
+thread_create (const char *name, int priority,
+               thread_func *function, void *aux) 
 {
   struct thread *t;
   struct kernel_thread_frame *kf;
   struct switch_entry_frame *ef;
   struct switch_threads_frame *sf;
+  tid_t tid;
+  enum intr_level old_level;
 
   ASSERT (function != NULL);
 
-  t = new_thread (name);
+  /* Allocate thread. */
+  t = palloc_get_page (PAL_ZERO);
+  if (t == NULL)
+    return TID_ERROR;
+
+  /* Initialize thread. */
+  init_thread (t, name, priority);
+  tid = t->tid = allocate_tid ();
+
+  /* Prepare thread for first run by initializing its stack.
+     Do this atomically so intermediate values for the 'stack' 
+     member cannot be observed. */
+  old_level = intr_disable ();
 
   /* Stack frame for kernel_thread(). */
   kf = alloc_frame (t, sizeof *kf);
@@ -150,163 +202,391 @@ thread_create (const char *name, void (*function) (void *aux), void *aux)
   /* Stack frame for switch_threads(). */
   sf = alloc_frame (t, sizeof *sf);
   sf->eip = switch_entry;
+  sf->ebp = 0;
+
+  intr_set_level (old_level);
 
   /* Add to run queue. */
-  thread_ready (t);
+  thread_unblock (t);
 
-  return t;
+  return tid;
 }
 
-struct thread *
-thread_current (void) 
+/* Puts the current thread to sleep.  It will not be scheduled
+   again until awoken by thread_unblock().
+
+   This function must be called with interrupts turned off.  It
+   is usually a better idea to use one of the synchronization
+   primitives in synch.h. */
+void
+thread_block (void) 
 {
-  uint32_t *esp;
-  asm ("movl %%esp, %0\n" : "=g" (esp));
-  return pg_round_down (esp);
+  ASSERT (!intr_context ());
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  thread_current ()->status = THREAD_BLOCKED;
+  schedule ();
 }
 
-#ifdef USERPROG
-bool
-thread_execute (const char *filename) 
+/* 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.)
+
+   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) 
 {
-  struct thread *t;
-  struct intr_frame *if_;
-  struct switch_entry_frame *ef;
-  struct switch_threads_frame *sf;
-  void (*start) (void);
+  enum intr_level old_level;
 
-  ASSERT (filename != NULL);
+  ASSERT (is_thread (t));
 
-  t = new_thread (filename);
-  if (t == NULL)
-    return false;
-  
-  if (!addrspace_load (&t->addrspace, filename, &start)) 
-    PANIC ("%s: program load failed", filename);
-
-  /* Interrupt frame. */
-  if_ = alloc_frame (t, sizeof *if_);
-  if_->es = SEL_UDSEG;
-  if_->ds = SEL_UDSEG;
-  if_->eip = start;
-  if_->cs = SEL_UCSEG;
-  if_->eflags = FLAG_IF | 2;
-  if_->esp = PHYS_BASE;
-  if_->ss = SEL_UDSEG;
+  old_level = intr_disable ();
+  ASSERT (t->status == THREAD_BLOCKED);
+  list_push_back (&ready_list, &t->elem);
+  t->status = THREAD_READY;
+  intr_set_level (old_level);
+}
 
-  /* Stack frame for switch_entry(). */
-  ef = alloc_frame (t, sizeof *ef);
-  ef->eip = intr_exit;
+/* Returns the name of the running thread. */
+const char *
+thread_name (void) 
+{
+  return thread_current ()->name;
+}
 
-  /* Stack frame for switch_threads(). */
-  sf = alloc_frame (t, sizeof *sf);
-  sf->eip = switch_entry;
+/* Returns the running thread.
+   This is running_thread() plus a couple of sanity checks.
+   See the big comment at the top of thread.h for details. */
+struct thread *
+thread_current (void) 
+{
+  struct thread *t = running_thread ();
+  
+  /* Make sure T is really a thread.
+     If either of these assertions fire, then your thread may
+     have overflowed its stack.  Each thread has less than 4 kB
+     of stack, so a few big automatic arrays or moderate
+     recursion can cause stack overflow. */
+  ASSERT (is_thread (t));
+  ASSERT (t->status == THREAD_RUNNING);
 
-  /* Add to run queue. */
-  thread_ready (t);
+  return t;
+}
 
-  return true;
+/* Returns the running thread's tid. */
+tid_t
+thread_tid (void) 
+{
+  return thread_current ()->tid;
 }
+
+/* Deschedules the current thread and destroys it.  Never
+   returns to the caller. */
+void
+thread_exit (void) 
+{
+  ASSERT (!intr_context ());
+
+#ifdef USERPROG
+  process_exit ();
 #endif
 
+  /* Remove thread from all threads list, set our status to dying,
+     and schedule another process.  That process will destroy us
+     when it calls thread_schedule_tail(). */
+  intr_disable ();
+  list_remove (&thread_current()->allelem);
+  thread_current ()->status = THREAD_DYING;
+  schedule ();
+  NOT_REACHED ();
+}
+
+/* Yields the CPU.  The current thread is not put to sleep and
+   may be scheduled again immediately at the scheduler's whim. */
+void
+thread_yield (void) 
+{
+  struct thread *cur = thread_current ();
+  enum intr_level old_level;
+  
+  ASSERT (!intr_context ());
+
+  old_level = intr_disable ();
+  if (cur != idle_thread) 
+    list_push_back (&ready_list, &cur->elem);
+  cur->status = THREAD_READY;
+  schedule ();
+  intr_set_level (old_level);
+}
+
+/* Invoke function 'func' on all threads, passing along 'aux'.
+   This function must be called with interrupts off. */
 void
-thread_ready (struct thread *t) 
+thread_foreach (thread_action_func *func, void *aux)
 {
-  if (t->status != THREAD_READY) 
+  struct list_elem *e;
+
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  for (e = list_begin (&all_list); e != list_end (&all_list);
+       e = list_next (e))
     {
-      list_push_back (&run_queue, &t->rq_elem);
-      t->status = THREAD_READY;
+      struct thread *t = list_entry (e, struct thread, allelem);
+      func (t, aux);
     }
 }
 
-static struct thread *
-find_next_to_run (void) 
+/* Sets the current thread's priority to NEW_PRIORITY. */
+void
+thread_set_priority (int new_priority) 
 {
-  if (list_empty (&run_queue))
-    return idle_thread;
-  else
-    return list_entry (list_pop_front (&run_queue), struct thread, rq_elem);
+  thread_current ()->priority = new_priority;
 }
 
+/* Returns the current thread's priority. */
+int
+thread_get_priority (void) 
+{
+  return thread_current ()->priority;
+}
+
+/* Sets the current thread's nice value to NICE. */
 void
-thread_destroy (struct thread *t) 
+thread_set_nice (int nice UNUSED) 
 {
-  ASSERT (t->status == THREAD_DYING);
-  ASSERT (t != thread_current ());
+  /* Not yet implemented. */
+}
 
-  palloc_free (t);
+/* Returns the current thread's nice value. */
+int
+thread_get_nice (void) 
+{
+  /* Not yet implemented. */
+  return 0;
 }
 
-void schedule_tail (struct thread *prev);
+/* Returns 100 times the system load average. */
+int
+thread_get_load_avg (void) 
+{
+  /* Not yet implemented. */
+  return 0;
+}
 
-void
-schedule_tail (struct thread *prev) 
+/* Returns 100 times the current thread's recent_cpu value. */
+int
+thread_get_recent_cpu (void) 
+{
+  /* Not yet implemented. */
+  return 0;
+}
+
+/* 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
+idle (void *idle_started_ UNUSED) 
 {
-  ASSERT (intr_get_level () == IF_OFF);
+  struct semaphore *idle_started = idle_started_;
+  idle_thread = thread_current ();
+  sema_up (idle_started);
 
-#ifdef USERPROG
-  addrspace_activate (&thread_current ()->addrspace);
-#endif
+  for (;;) 
+    {
+      /* Let someone else run. */
+      intr_disable ();
+      thread_block ();
 
-  if (prev != NULL && prev->status == THREAD_DYING) 
-    thread_destroy (prev);
+      /* Re-enable interrupts and wait for the next one.
+
+         The `sti' instruction disables interrupts until the
+         completion of the next instruction, so these two
+         instructions are executed atomically.  This atomicity is
+         important; otherwise, an interrupt could be handled
+         between re-enabling interrupts and waiting for the next
+         one to occur, wasting as much as one clock tick worth of
+         time.
+
+         See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3a]
+         7.11.1 "HLT Instruction". */
+      asm volatile ("sti; hlt" : : : "memory");
+    }
 }
 
+/* Function used as the basis for a kernel thread. */
 static void
-thread_schedule (void) 
+kernel_thread (thread_func *function, void *aux) 
 {
-  struct thread *cur, *next, *prev;
+  ASSERT (function != NULL);
 
-  ASSERT (intr_get_level () == IF_OFF);
+  intr_enable ();       /* The scheduler runs with interrupts off. */
+  function (aux);       /* Execute the thread function. */
+  thread_exit ();       /* If function() returns, kill the thread. */
+}
+
+/* Returns the running thread. */
+struct thread *
+running_thread (void) 
+{
+  uint32_t *esp;
 
-  cur = thread_current ();
-  ASSERT (cur->status != THREAD_RUNNING);
+  /* Copy the CPU's stack pointer into `esp', and then round that
+     down to the start of a page.  Because `struct thread' is
+     always at the beginning of a page and the stack pointer is
+     somewhere in the middle, this locates the curent thread. */
+  asm ("mov %%esp, %0" : "=g" (esp));
+  return pg_round_down (esp);
+}
 
-  next = find_next_to_run ();
+/* Returns true if T appears to point to a valid thread. */
+static bool
+is_thread (struct thread *t)
+{
+  return t != NULL && t->magic == THREAD_MAGIC;
+}
 
-  next->status = THREAD_RUNNING;
-  if (cur != next)
-    {
-      prev = switch_threads (cur, next);
+/* Does basic initialization of T as a blocked thread named
+   NAME. */
+static void
+init_thread (struct thread *t, const char *name, int priority)
+{
+  enum intr_level old_level;
+
+  ASSERT (t != NULL);
+  ASSERT (PRI_MIN <= priority && priority <= PRI_MAX);
+  ASSERT (name != NULL);
 
-      /* Prevent GCC from reordering anything around the thread
-         switch. */
-      asm volatile ("" : : : "memory");
+  memset (t, 0, sizeof *t);
+  t->status = THREAD_BLOCKED;
+  strlcpy (t->name, name, sizeof t->name);
+  t->stack = (uint8_t *) t + PGSIZE;
+  t->priority = priority;
+  t->magic = THREAD_MAGIC;
 
-      schedule_tail (prev); 
-    }
+  old_level = intr_disable ();
+  list_push_back (&all_list, &t->allelem);
+  intr_set_level (old_level);
+}
+
+/* Allocates a SIZE-byte frame at the top of thread T's stack and
+   returns a pointer to the frame's base. */
+static void *
+alloc_frame (struct thread *t, size_t size) 
+{
+  /* Stack data is always allocated in word-size units. */
+  ASSERT (is_thread (t));
+  ASSERT (size % sizeof (uint32_t) == 0);
+
+  t->stack -= size;
+  return t->stack;
+}
+
+/* Chooses and returns the next thread to be scheduled.  Should
+   return a thread from the run queue, unless the run queue is
+   empty.  (If the running thread can continue running, then it
+   will be in the run queue.)  If the run queue is empty, return
+   idle_thread. */
+static struct thread *
+next_thread_to_run (void) 
+{
+  if (list_empty (&ready_list))
+    return idle_thread;
+  else
+    return list_entry (list_pop_front (&ready_list), struct thread, elem);
 }
 
+/* Completes a thread switch by activating the new thread's page
+   tables, and, if the previous thread is dying, destroying it.
+
+   At this function's invocation, we just switched from thread
+   PREV, the new thread is already running, and interrupts are
+   still disabled.  This function is normally invoked by
+   thread_schedule() as its final action before returning, but
+   the first time a thread is scheduled it is called by
+   switch_entry() (see switch.S).
+
+   It's not safe to call printf() until the thread switch is
+   complete.  In practice that means that printf()s should be
+   added at the end of the function.
+
+   After this function and its caller returns, the thread switch
+   is complete. */
 void
-thread_yield (void) 
+thread_schedule_tail (struct thread *prev)
 {
-  enum if_level old_level;
+  struct thread *cur = running_thread ();
   
-  ASSERT (!intr_context ());
+  ASSERT (intr_get_level () == INTR_OFF);
 
-  old_level = intr_disable ();
-  thread_ready (thread_current ());
-  thread_schedule ();
-  intr_set_level (old_level);
+  /* Mark us as running. */
+  cur->status = THREAD_RUNNING;
+
+  /* Start new time slice. */
+  thread_ticks = 0;
+
+#ifdef USERPROG
+  /* Activate the new address space. */
+  process_activate ();
+#endif
+
+  /* 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.  (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);
+      palloc_free_page (prev);
+    }
 }
 
-void
-thread_exit (void) 
+/* Schedules a new process.  At entry, interrupts must be off and
+   the running process's state must have been changed from
+   running to some other state.  This function finds another
+   thread to run and switches to it.
+
+   It's not safe to call printf() until thread_schedule_tail()
+   has completed. */
+static void
+schedule (void) 
 {
-  ASSERT (!intr_context ());
+  struct thread *cur = running_thread ();
+  struct thread *next = next_thread_to_run ();
+  struct thread *prev = NULL;
 
-  intr_disable ();
-  thread_current ()->status = THREAD_DYING;
-  thread_schedule ();
-  NOT_REACHED ();
+  ASSERT (intr_get_level () == INTR_OFF);
+  ASSERT (cur->status != THREAD_RUNNING);
+  ASSERT (is_thread (next));
+
+  if (cur != next)
+    prev = switch_threads (cur, next);
+  thread_schedule_tail (prev);
 }
 
-void
-thread_sleep (void) 
+/* Returns a tid to use for a new thread. */
+static tid_t
+allocate_tid (void) 
 {
-  ASSERT (!intr_context ());
-  ASSERT (intr_get_level () == IF_OFF);
+  static tid_t next_tid = 1;
+  tid_t tid;
 
-  thread_current ()->status = THREAD_BLOCKED;
-  thread_schedule ();
+  lock_acquire (&tid_lock);
+  tid = next_tid++;
+  lock_release (&tid_lock);
+
+  return tid;
 }
+
+/* Offset of `stack' member within `struct thread'.
+   Used by switch.S, which can't figure it out on its own. */
+uint32_t thread_stack_ofs = offsetof (struct thread, stack);