Implement hash table for child process results

src/userprog/process.c

@@ -1,5 +1,6 @@
 #include "userprog/process.h"
 #include <debug.h>
+#include <hash.h>
 #include <inttypes.h>
 #include <list.h>
 #include <round.h>
@@ -55,6 +56,7 @@ struct process_start_data
   };
 
 static thread_func start_process NO_RETURN;
+static void destruct_process_result (struct hash_elem *e, void *aux);
 static bool load (const char *cmdline, void (**eip) (void), void **esp);
 
 /* Starts a new thread running a user program executed via
@@ -315,32 +317,15 @@ push_to_stack (void **esp, void *data, size_t data_size)
 int
 process_wait (tid_t child_tid)
 {
-  struct process_result *child_result = NULL;
-  struct list_elem *e;
-  struct thread *cur = thread_current ();
-
-  for (e = list_begin (&cur->child_results);
-       e != list_end (&cur->child_results); e = list_next (e))
-    {
-      struct process_result *result
-          = list_entry (e, struct process_result, elem);
-      if (result->tid == child_tid)
-        {
-          /* Found the child process. */
-          child_result = result;
-          break;
-        }
-
-      /* List is ordered, allowing us to break early if the child_tid is
-         greater than the current result's tid. */
-      else if (result->tid > child_tid)
-        break;
-    }
-
-  /* If the child process was not found, return -1. */
-  if (child_result == NULL)
+  struct thread *t = thread_current ();
+  struct process_result fake_result;
+  fake_result.tid = child_tid;
+  struct hash_elem *e = hash_find (&t->child_results, &fake_result.elem);
+  if (e == NULL)
     return -1;
 
+  struct process_result *child_result
+      = hash_entry (e, struct process_result, elem);
   /* Wait for child to die. */
   sema_down (&child_result->sema);
 
@@ -348,18 +333,17 @@ process_wait (tid_t child_tid)
      wait) for it here to ensure we don't free the lock memory before it is
      released in process_exit. */
   lock_acquire (&child_result->lock);
-
-  /* To prevent waiting for child twice, remove it from the list.
+  /* To prevent waiting for child twice, remove it from the table.
      No need to use lock since this is the only thread with access to
      the struct process_result now. */
-  list_remove (&child_result->elem);
+  hash_delete (&t->child_results, &child_result->elem);
 
   /* Get the exit status of the child */
   int exit_status = child_result->exit_status;
 
   /* Release the lock */
   lock_release (&child_result->lock);
-
+  /* Result no-longer used by parent, nor child. Deallocate it. */
   free (child_result);
   return exit_status;
 }
@@ -385,49 +369,12 @@ process_exit (void)
       lock_release (&filesys_lock);
     }
 
-  /* Update process result. */
+  /* Update own process result. */
   if (cur->result != NULL)
-    {
-      lock_acquire (&cur->result->lock);
-      cur->result->exit_status = cur->exit_status;
-      /* Parent has died, child has to free the struct process_result * */
-      if (sema_try_down (&cur->result->sema))
-        {
-          lock_release (&cur->result->lock);
-          free (cur->result);
-        }
-      /* Parent is still alive and will be the one to free the
-         struct process_result *, and may be waiting so call sema_up */
-      else
-        {
-          sema_up (&cur->result->sema);
-          lock_release (&cur->result->lock);
-        }
-    }
+    destruct_process_result (&cur->result->elem, cur);
 
   /* Free child process results or signal parent's death. */
-  struct list_elem *e;
-  for (e = list_begin (&cur->child_results);
-       e != list_end (&cur->child_results);)
-    {
-      struct process_result *result
-          = list_entry (e, struct process_result, elem);
-      struct list_elem *next = list_next (e);
-      lock_acquire (&result->lock);
-      /* Child has died (and was not waited for). Free the result. */
-      if (sema_try_down (&result->sema))
-        {
-          lock_release (&result->lock);
-          free (result);
-        }
-      /* Child is still alive, signal via sema that parent has died. */
-      else
-        {
-          sema_up (&result->sema);
-          lock_release (&result->lock);
-        }
-      e = next;
-    }
+  hash_destroy (&cur->child_results, destruct_process_result);
 
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
@@ -447,6 +394,34 @@ process_exit (void)
     }
 }
 
+/* Destruct a process_result, with multi-thread awareness. Takes the
+   process_result->elem and current thread.
+   If the other thread is running, simply signals death. Otherwise
+   frees the result.
+   Also set's process_result->exit_status if the process result is FOR
+   the current thread. */
+static void
+destruct_process_result (struct hash_elem *e, void *cur_)
+{
+  struct thread *cur = cur_;
+  struct process_result *result = hash_entry (e, struct process_result, elem);
+  lock_acquire (&result->lock);
+  /* Other thread has died (and was not waited for). Free the result. */
+  if (sema_try_down (&result->sema))
+    {
+      lock_release (&result->lock);
+      free (result);
+    }
+  /* Other thread is still alive, signal via sema that parent has died. */
+  else
+    {
+      if (cur->tid == result->tid)
+        result->exit_status = cur->exit_status;
+      sema_up (&result->sema);
+      lock_release (&result->lock);
+    }
+}
+
 /* Sets up the CPU for running user code in the current
    thread.
    This function is called on every context switch. */