Update cond_wait to insert into waiters list in sorted order of priority

.gitignore

@@ -4,6 +4,13 @@
 #ignore pdf files (just keep source files)
 *.pdf
 
+#ignore Mac OS generated files
+.DS_Store
+
+#ignore code editor generated directories
+.idea
+.vscode
+
 #ignore junk files from latex output
 *.out
 *.log

src/threads/synch.c

@@ -68,7 +68,8 @@ sema_down (struct semaphore *sema)
   old_level = intr_disable ();
   while (sema->value == 0) 
     {
-      list_push_back (&sema->waiters, &thread_current ()->elem);
+      list_insert_ordered(&sema->waiters, &thread_current ()->elem,
+                          priority_more, NULL);
       thread_block ();
     }
   sema->value--;
@@ -113,11 +114,22 @@ sema_up (struct semaphore *sema)
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
-  if (!list_empty (&sema->waiters)) 
-    thread_unblock (list_entry (list_pop_front (&sema->waiters),
-                                struct thread, elem));
+
+  /* Wake up (unblock) the highest priority thread from the waiters list */
+  struct thread *t = NULL;
+  if (!list_empty (&sema->waiters))
+    {
+      t = list_entry (list_pop_front (&sema->waiters), struct thread, elem);
+      thread_unblock (t);
+    }
+
   sema->value++;
   intr_set_level (old_level);
+
+  /* If the unblocked thread has higher priority than the current running thread
+     then yield the CPU to the unblocked thread */
+  if (!intr_context() && t != NULL && t->priority > thread_get_priority ())
+    thread_yield ();
 }
 
 static void sema_test_helper (void *sema_);
@@ -253,6 +265,54 @@ struct semaphore_elem
     struct semaphore semaphore;         /* This semaphore. */
   };
 
+/* Function that compares the two *semaphores* associated with the provided
+   list_elem structures. [i.e., takes list_elem of semaphore_elem, and]
+   Returns true if the thread associated with the semaphore associated with a_
+   has a higher priority than that of b_.
+
+    If aux is provided, then it is a pointer to an integer representing the
+    priority of the first semaphore. This is useful when the thread has not been
+    sema'd down yet. */
+static bool
+sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux)
+{
+  int a_priority, b_priority;
+
+  /* If an aux is provided, then use it as the priority of the first semaphore.
+     Otherwise, get the priority of the first semaphore. */
+  if (aux != NULL)
+    a_priority = *(int *) aux;
+  else
+    {
+      struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
+
+      /* If waiters list is empty, return false (i.e., a has lower priority) */
+      if (list_empty(&a->semaphore.waiters))
+        return false;
+
+      /* Otherwise, get the thread with the highest priority from the waiters
+         list. By design, this is the first one in the list (See sema_down). */
+      struct thread *a_thread =
+        list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
+
+      a_priority = a_thread->priority;
+    }
+
+  struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
+
+  /* If waiters list is empty, return true (i.e., a has higher priority) */
+  if (list_empty(&b->semaphore.waiters))
+    return true;
+
+  struct thread *b_thread =
+    list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
+
+  b_priority = b_thread->priority;
+
+  return a_priority > b_priority;
+}
+
 /* Initializes condition variable COND.  A condition variable
    allows one piece of code to signal a condition and cooperating
    code to receive the signal and act upon it. */
@@ -295,7 +355,14 @@ cond_wait (struct condition *cond, struct lock *lock)
   ASSERT (lock_held_by_current_thread (lock));
   
   sema_init (&waiter.semaphore, 0);
-  list_push_back (&cond->waiters, &waiter.elem);
+
+  /* Insert the semaphore_elem into the waiters list in order of priority.
+     We pass the priority of the current thread as aux to sema_priority_more
+     because the thread has not been sema'd down yet (See sema_priority_more) */
+  int priority = thread_current ()->priority;
+  list_insert_ordered (&cond->waiters, &waiter.elem, sema_priority_more,
+                       &priority);
+
   lock_release (lock);
   sema_down (&waiter.semaphore);
   lock_acquire (lock);

src/threads/thread.c

@@ -220,6 +220,10 @@ thread_create (const char *name, int priority,
   /* Add to run queue. */
   thread_unblock (t);
 
+  /* Yield if the new thread has a higher priority than the current thread. */
+  if (priority > thread_get_priority ())
+    thread_yield ();
+
   return tid;
 }
 
@@ -256,7 +260,10 @@ thread_unblock (struct thread *t)
 
   old_level = intr_disable ();
   ASSERT (t->status == THREAD_BLOCKED);
-  list_push_back (&ready_list, &t->elem);
+
+  /* Insert the thread back into the ready list in priority order. */
+  list_insert_ordered(&ready_list, &t->elem, priority_more, NULL);
+
   t->status = THREAD_READY;
   intr_set_level (old_level);
 }
@@ -326,8 +333,11 @@ thread_yield (void)
   ASSERT (!intr_context ());
 
   old_level = intr_disable ();
+
+  /* Insert the thread back into the ready list in priority order. */
   if (cur != idle_thread) 
-    list_push_back (&ready_list, &cur->elem);
+    list_insert_ordered(&ready_list, &cur->elem, priority_more, NULL);
+
   cur->status = THREAD_READY;
   schedule ();
   intr_set_level (old_level);
@@ -350,11 +360,57 @@ thread_foreach (thread_action_func *func, void *aux)
     }
 }
 
+/* Function that compares the two threads associated with the provided
+   list_elem structures. Returns true if the thread associated with a_ has
+   a higher priority than that of b_. */
+bool
+priority_more (const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux UNUSED)
+{
+  struct thread *a = list_entry (a_, struct thread, elem);
+  struct thread *b = list_entry (b_, struct thread, elem);
+
+  return a->priority > b->priority;
+}
+
 /* Sets the current thread's priority to NEW_PRIORITY. */
 void
 thread_set_priority (int new_priority) 
 {
+  ASSERT (new_priority >= PRI_MIN);
+  ASSERT (new_priority <= PRI_MAX);
+
+  int old_priority = thread_get_priority ();
+  if (new_priority == old_priority)
+    return;
+
   thread_current ()->priority = new_priority;
+
+  enum intr_level old_level = intr_disable ();
+
+  /* If the thread is in the ready list, the list must be reordered to maintain
+    the priority order. */
+  if (thread_current ()->status == THREAD_READY) {
+    /* Remove from the ready list and reinsert it in priority order. */
+    list_remove (&thread_current ()->elem);
+    list_insert_ordered (&ready_list, &thread_current ()->elem, priority_more,
+                         NULL);
+  }
+
+  if (new_priority < old_priority && !list_empty (&ready_list)) {
+    /* If the new priority is lower than the old priority, check if the current
+       thread no longer has the highest priority. If it doesn't, yield the CPU.
+     */
+
+    struct thread *next_thread =
+      list_entry (list_front (&ready_list), struct thread, elem);
+
+    if (next_thread->priority > new_priority) {
+      thread_yield();
+    }
+  }
+
+  intr_set_level (old_level);
 }
 
 /* Returns the current thread's priority. */

src/threads/thread.h

@@ -131,6 +131,8 @@ void thread_yield (void);
 typedef void thread_action_func (struct thread *t, void *aux);
 void thread_foreach (thread_action_func *, void *);
 
+bool priority_more (const struct list_elem *a_, const struct list_elem *b_,
+                    void *aux UNUSED);
 int thread_get_priority (void);
 void thread_set_priority (int);