Add priority_less for comparing threads based on priority and Refactor sema up to use list_max for clarity

src/threads/synch.c

@@ -32,6 +32,10 @@
 #include "threads/interrupt.h"
 #include "threads/thread.h"
 
+static bool
+priority_less (const struct list_elem *a_, const struct list_elem *b_,
+               void *aux UNUSED);
+
 /* Initializes semaphore SEMA to VALUE.  A semaphore is a
    nonnegative integer along with two atomic operators for
    manipulating it:
@@ -117,7 +121,7 @@ sema_up (struct semaphore *sema)
     {
       /* Enforces wake-up of the highest priority thread waiting for the
          semaphore. */
-      struct list_elem *e = list_min (&sema->waiters, priority_more, NULL);
+      struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
       list_remove (e);
       thread_unblock (list_entry (e, struct thread, elem));
     }
@@ -364,6 +368,19 @@ struct semaphore_elem
     struct semaphore semaphore;         /* This semaphore. */
   };
 
+/* Function that compares the two threads associated with the provided
+   pointers to their 'elem' member. Returns true if the thread associated
+   with a_ has a lower priority than that of b_. */
+static bool
+priority_less (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;
+}
+
 /* 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_