Refactor donate_priority to include comments explaining its purpose and logic w/ S

src/threads/synch.c

@@ -194,16 +194,25 @@ lock_init (struct lock *lock)
   sema_init (&lock->semaphore, 1);
 }
 
+/* Allows for the donor to donate 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. */
 static void
 donate_priority (struct thread *donor, struct thread *donee) {
   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;
 
       donee->priority = donor->priority;
+
+      /* 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)
         donee = NULL;
       else