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

# Conflicts:
#   src/threads/thread.c

.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

@@ -114,10 +114,23 @@ sema_up (struct semaphore *sema)
 
   old_level = intr_disable ();
   if (!list_empty (&sema->waiters))
-    thread_unblock (list_entry (list_pop_front (&sema->waiters),
+  {
-                                struct thread, elem));
+    /* 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. */
@@ -318,8 +459,13 @@ cond_signal (struct condition *cond, struct lock *lock UNUSED)
   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);
+      /* 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

src/threads/thread.c

@@ -78,6 +78,8 @@ static bool thread_priority_less (const struct list_elem *a,
 static void schedule (void);
 void thread_schedule_tail (struct thread *prev);
 static tid_t allocate_tid (void);
+static bool donor_priority_less (const struct list_elem *a_,
+                                 const struct list_elem *b_, void *aux UNUSED);
 
 /* Initializes the threading system by transforming the code
    that's currently running into a thread.  This can't work in
@@ -252,6 +254,7 @@ thread_create (const char *name, int priority,
 
   /* Add to run queue. */
   thread_unblock (t);
+  thread_yield ();
 
   return tid;
 }
@@ -289,10 +292,13 @@ thread_unblock (struct thread *t)
 
   old_level = intr_disable ();
   ASSERT (t->status == THREAD_BLOCKED);
+
   if (thread_mlfqs)
     list_insert_ordered (&ready_list, &t->elem, thread_priority_less, NULL);
   else
-    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);
 }
@@ -362,14 +368,17 @@ thread_yield (void)
   ASSERT (!intr_context ());
 
   old_level = intr_disable ();
+
   if (cur != idle_thread)
     {
       if (thread_mlfqs)
         list_insert_ordered (&ready_list, &cur->elem, thread_priority_less,
                              NULL);
       else
-        list_push_back (&ready_list, &cur->elem);
+        /* Insert the thread back into the ready list in priority order. */
+        list_insert_ordered(&ready_list, &cur->elem, priority_more, NULL);
     }
+
   cur->status = THREAD_READY;
   schedule ();
   intr_set_level (old_level);
@@ -392,22 +401,66 @@ thread_foreach (thread_action_func *func, void *aux)
     }
 }
 
-/* Sets the current thread's priority to NEW_PRIORITY. */
+/* Function that compares the two threads associated with the provided
-void
+   pointers to their 'elem' member. Returns true if the thread associated
-thread_set_priority (int new_priority) 
+   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)
 {
-  if (thread_mlfqs)
+
-    return;
+  struct thread *a = list_entry (a_, struct thread, elem);
-  thread_current ()->priority = new_priority;
+  struct thread *b = list_entry (b_, struct thread, elem);
+
+  return a->priority > b->priority;
+
 }
 
-/* Returns the current thread's priority. */
+/* Function that compares the two threads associated with the provided
+   pointers to their 'donor_elem' member. Returns true if the thread associated
+   with a_ has a lower priority than that of b_. */
+static bool
+donor_priority_less (const struct list_elem *a_, const struct list_elem *b_,
+               void *aux UNUSED)
+{
+  struct thread *a = list_entry (a_, struct thread, donor_elem);
+  struct thread *b = list_entry (b_, struct thread, donor_elem);
+
+  return a->priority < b->priority;
+}
+
+/* Sets the current thread's base priority to new_base_priority.
+   Updates the current thread's effective priority if necessary. */
+void
+thread_set_priority (int new_base_priority)
+{
+
+  if (thread_mlfqs)
+    return;
+
+  ASSERT (new_base_priority >= PRI_MIN);
+  ASSERT (new_base_priority <= PRI_MAX);
+
+  struct thread *t = thread_current ();
+
+  /* If the base priority is unchanged, do nothing. */
+  if (new_base_priority == t->base_priority)
+    return;
+
+  t->base_priority = new_base_priority;
+  thread_recalculate_priority ();
+
+  thread_yield ();
+}
+
+/* Returns the current thread's effective priority. */
 int
 thread_get_priority (void) 
 {
   return thread_current ()->priority;
 }
 
+
 /* Updates recent_cpu for a thread. */
 static void
 thread_update_recent_cpu (struct thread *t, void *aux UNUSED)
@@ -420,6 +473,31 @@ thread_update_recent_cpu (struct thread *t, void *aux UNUSED)
       = fp_add_int (fp_mul (recent_cpu_coeff, curr_recent_cpu), t->nice);
   // recent_cpu was updated, update priority.
   t->priority = calculate_bsd_priority (t->recent_cpu, t->nice);
+
+/* Recalculates the effective priority of the current thread. */
+void
+thread_recalculate_priority (void)
+{
+  struct thread *t = thread_current ();
+  
+  enum intr_level old_level = intr_disable ();
+  t->priority = t->base_priority;
+
+  /* If there are no donors to the current thread, then the effective
+     priority is just the base priority. */
+  if (!list_empty (&t->donors_list))
+    {
+      int max_donated_priority =
+        list_entry (list_max (&t->donors_list, donor_priority_less, NULL),
+                    struct thread, donor_elem)->priority;
+
+      /* The effective priority is the max donated priority if this is
+         higher than the base priority. */
+      if (max_donated_priority > t->priority)
+        t->priority = max_donated_priority;
+    }
+  intr_set_level (old_level);
+
 }
 
 /* Sets the current thread's nice value to NICE. */
@@ -458,6 +536,22 @@ thread_get_recent_cpu (void)
   return fp_round (fp_mul_int (thread_current ()->recent_cpu, 100));
 }
 
+/* Reinsert thread t into the ready list at its correct position
+   in descending order of priority. Used when this thread's priority
+   may have changed. Must be called with interrupts disabled. */
+void
+ready_list_reinsert (struct thread *t)
+{
+  ASSERT (intr_get_level () == INTR_OFF);
+
+  /* If the thread isn't ready to run, do nothing. */
+  if (t->status != THREAD_READY)
+    return;
+
+  list_remove (&t->elem);
+  list_insert_ordered (&ready_list, &t->elem, priority_more, NULL);
+}
+
 /* Idle thread.  Executes when no other thread is ready to run.
 
    The idle thread is initially put on the ready list by
@@ -544,12 +638,20 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->status = THREAD_BLOCKED;
   strlcpy (t->name, name, sizeof t->name);
   t->stack = (uint8_t *) t + PGSIZE;
+
   t->priority
       = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
   t->nice = nice;
   t->recent_cpu = recent_cpu;
+
+  t->base_priority = priority;
+
   t->magic = THREAD_MAGIC;
   
+  list_init (&t->donors_list);
+  t->priority = t->base_priority;
+  t->waiting_lock = NULL;
+
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);

src/threads/thread.h

@@ -95,6 +95,15 @@ struct thread
     int priority;                       /* Priority. */
     struct list_elem allelem;           /* List element for all threads list. */
 
+    /* Donation Related */
+    int base_priority;                  /* Base priority of the thread. */
+    struct list donors_list;            /* List of threads that have donated
+                                           to this thread. */
+    struct lock *waiting_lock;          /* The lock that the current thread is
+                                           waiting for. */
+    struct list_elem donor_elem;        /* List element so that thread can be
+                                           enlisted in other donors list. */
+
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
@@ -140,12 +149,17 @@ 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);
+void thread_recalculate_priority (void);
 
 int thread_get_nice (void);
 void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
 
+void ready_list_reinsert (struct thread *t);
+
 #endif /* threads/thread.h */