Merge branch 'master' into 'BSD-merged', fixing merge conflicts

# Conflicts:
#   src/threads/thread.c

src/threads/synch.c

@@ -106,18 +106,31 @@ sema_try_down (struct semaphore *sema)
 
    This function may be called from an interrupt handler. */
 void
-sema_up (struct semaphore *sema) 
+sema_up (struct semaphore *sema)
 {
   enum intr_level old_level;
 
   ASSERT (sema != NULL);
 
   old_level = intr_disable ();
-  if (!list_empty (&sema->waiters)) 
-    thread_unblock (list_entry (list_pop_front (&sema->waiters),
-                                struct thread, elem));
+  if (!list_empty (&sema->waiters))
+  {
+    /* Enforces wake-up of the highest priority thread waiting for the
+       semaphore. */
+    struct list_elem *e = list_min (&sema->waiters, priority_more, NULL);
+    list_remove (e);
+    thread_unblock (list_entry (e, struct thread, elem));
+  }
   sema->value++;
   intr_set_level (old_level);
+
+  /* Yields the CPU in case the thread that has been woken up has a higher
+     priority that the current running thread, including the case when called
+     within an interrupt handler. */
+  if (intr_context ())
+    intr_yield_on_return ();
+  else
+    thread_yield ();
 }
 
 static void sema_test_helper (void *sema_);
@@ -181,6 +194,47 @@ lock_init (struct lock *lock)
   sema_init (&lock->semaphore, 1);
 }
 
+/* Current thread donates its priority to donee, iteratively
+   propagating the donation in the case of chains in the wait-for graph.
+   Also keeps track of the donation by updating the donors list. Expects
+   interrupts to be disabled. */
+static void
+donate_priority (struct thread *donee) {
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  struct thread *donor = thread_current ();
+  list_push_back (&donee->donors_list, &donor->donor_elem);
+  
+  while (donee != NULL)
+    {
+      /* Stop propagation of donation once a donee is reached that has
+         a higher effective priority (as its donees can't have less
+         priority than that being donated). */
+      if (donor->priority <= donee->priority)
+          break;
+
+      /* Also stop propagation of donation once a donee is reached with
+         no donees of its own (sink node in WFG). */
+      if (donee->waiting_lock == NULL)
+        {
+          /* Only the sink node of the WFG isn't waiting for a lock and
+             could be on the ready list. Thus, as its priority changed,
+             it must be reinserted into the list. */
+          enum intr_level old_level = intr_disable ();
+          donee->priority = donor->priority;
+          ready_list_reinsert (donee);
+          intr_set_level (old_level);
+          
+          donee = NULL;
+        }
+      else
+        {
+          donee->priority = donor->priority;
+          donee = donee->waiting_lock->holder;
+        }
+    }
+}
+
 /* Acquires LOCK, sleeping until it becomes available if
    necessary.  The lock must not already be held by the current
    thread.
@@ -196,8 +250,20 @@ lock_acquire (struct lock *lock)
   ASSERT (!intr_context ());
   ASSERT (!lock_held_by_current_thread (lock));
 
+  struct thread *t = thread_current ();
+  enum intr_level old_level = intr_disable ();
+
+  if (lock->holder != NULL)
+    {
+      t->waiting_lock = lock;
+      donate_priority (lock->holder);
+    }
+
+  intr_set_level (old_level);
+
   sema_down (&lock->semaphore);
   lock->holder = thread_current ();
+  t->waiting_lock = NULL;
 }
 
 /* Tries to acquires LOCK and returns true if successful or false
@@ -231,6 +297,51 @@ lock_release (struct lock *lock)
   ASSERT (lock != NULL);
   ASSERT (lock_held_by_current_thread (lock));
 
+  struct thread *current_thread = thread_current ();
+  struct thread *max_donor = NULL;
+
+  struct list orphan_list;
+  list_init (&orphan_list);
+
+  enum intr_level old_level = intr_disable ();
+  /* Loop through current thread's donors, removing the ones waiting for the
+     lock being released and keeping track of them (within orphan_list).
+     Also identifies the highest priority donor thread among them. */
+  struct list_elem *tail = list_tail (&current_thread->donors_list);
+  struct list_elem *e = list_begin (&current_thread->donors_list);
+  while (e != tail)
+    {
+      struct thread *donor = list_entry (e, struct thread, donor_elem);
+      struct list_elem *next = list_next (e);
+
+      /* Excludes donors that aren't waiting for the lock being released,
+         and tracks the rest. */
+      if (donor->waiting_lock == lock)
+        {
+          list_remove (e);
+          list_push_back (&orphan_list, e);
+
+          /* Identify highest priority donor. */
+          if (max_donor == NULL || donor->priority > max_donor->priority)
+            max_donor = donor;
+        }
+
+      e = next;
+    }
+
+  /* If there exists a maximum donor thread waiting for this lock to be
+     released, transfer the remaining orphaned donors to its donor list. */
+  if (max_donor != NULL)
+    {
+      while (!list_empty (&orphan_list))
+        list_push_back (&max_donor->donors_list, list_pop_front (&orphan_list));
+    }
+
+  intr_set_level (old_level);
+  /* Removal of donors to this thread may change its effective priority,
+     so recalculate. */
+  thread_recalculate_priority ();
+
   lock->holder = NULL;
   sema_up (&lock->semaphore);
   thread_yield ();
@@ -254,6 +365,36 @@ 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 *inserting_telem)
+{
+  struct list_elem *te_a, *te_b;
+  
+  te_b = list_front (
+    &list_entry (b, struct semaphore_elem, elem)->semaphore.waiters);
+
+  if (inserting_telem == NULL)
+    {
+      te_a = list_front (
+        &list_entry (a, struct semaphore_elem, elem)->semaphore.waiters);
+    }
+  else
+    {
+      te_a = inserting_telem;
+    }
+
+  return priority_more (te_a, te_b, NULL);
+}
+
 /* 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. */
@@ -317,9 +458,14 @@ cond_signal (struct condition *cond, struct lock *lock UNUSED)
   ASSERT (!intr_context ());
   ASSERT (lock_held_by_current_thread (lock));
 
-  if (!list_empty (&cond->waiters)) 
-    sema_up (&list_entry (list_pop_front (&cond->waiters),
-                          struct semaphore_elem, elem)->semaphore);
+  if (!list_empty (&cond->waiters))
+    {
+      /* Enforce wake-up of highest priority thread within the singleton 
+         semaphores waiting for condvar. */
+      struct list_elem *e = list_min (&cond->waiters, sema_priority_more, NULL);
+      list_remove (e);
+      sema_up (&list_entry (e, struct semaphore_elem, elem)->semaphore);
+    }
 }
 
 /* Wakes up all threads, if any, waiting on COND (protected by