static tid_t allocate_tid (void);
/* Initializes the threading system by transforming the code
- that's currently running into a thread. Note that this is
- possible only because the loader was careful to put the bottom
- of the stack at a page boundary; it won't work in general.
- Also initializes the run queue.
+ 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
/* 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. 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. Use a semaphore or some other form
- of synchronization if you need to ensure ordering. Returns
- the thread identifier for the new thread, or TID_ERROR if
- creation fails.
+ 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.
return tid;
}
-/* Transitions a blocked thread T from its current state to the
- ready-to-run state. This is an error if T is not blocked.
- (Use thread_yield() to make the running thread ready.) */
+/* 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)
+{
+ ASSERT (!intr_context ());
+ ASSERT (intr_get_level () == INTR_OFF);
+
+ thread_current ()->status = THREAD_BLOCKED;
+ schedule ();
+}
+
+/* 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.) */
void
thread_unblock (struct thread *t)
{
intr_set_level (old_level);
}
-/* 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. */
+/* Sets the current thread's priority to NEW_PRIORITY. */
void
-thread_block (void)
+thread_set_priority (int new_priority)
{
- ASSERT (!intr_context ());
- ASSERT (intr_get_level () == INTR_OFF);
+ thread_current ()->priority = new_priority;
+}
- thread_current ()->status = THREAD_BLOCKED;
- schedule ();
+/* Returns the current thread's priority. */
+int
+thread_get_priority (void)
+{
+ return thread_current ()->priority;
}
\f
/* Idle thread. Executes when no other thread is ready to run. */
/* Let someone else run. */
intr_disable ();
thread_block ();
- intr_enable ();
- /* Use CPU `hlt' instruction to wait for interrupt. */
- asm ("hlt");
+ /* 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-v3] 7.7. */
+ asm ("sti; hlt");
}
}
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 ("movl %%esp, %0\n" : "=g" (esp));
+ asm ("mov %0, %%esp" : "=g" (esp));
return pg_round_down (esp);
}
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
/* 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. */
+ thread to run and switches to it.
+
+ It's not safe to call printf() until schedule_tail() has
+ completed. */
static void
schedule (void)
{