fix: do not acquire filesys_lock for tell and seek

Use a single `exit_status` instead of two

See merge request lab2425_autumn/pintos_22!50

src/threads/thread.c

@@ -71,7 +71,7 @@ static void kernel_thread (thread_func *, void *aux);
 static void idle (void *aux UNUSED);
 static struct thread *running_thread (void);
 static struct thread *next_thread_to_run (void);
-static void init_process_result (struct thread *t);
+static bool init_process_result (struct thread *t);
 static void init_thread (struct thread *, const char *name, int nice,
                          int priority, fp32_t recent_cpu);
 static bool is_thread (struct thread *) UNUSED;
@@ -84,6 +84,10 @@ 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);
+static unsigned process_result_hash (const struct hash_elem *e,
+                                     void *aux UNUSED);
+static bool process_result_less (const struct hash_elem *a,
+                                 const struct hash_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
@@ -122,6 +126,13 @@ thread_init (void)
 void
 thread_start (void) 
 {
+  /* Malloc has been initalised, we can allocate the child results table
+     for the main thread. */
+  struct thread *t = thread_current ();
+  if (!hash_init (&t->child_results, process_result_hash, process_result_less,
+                  t))
+    PANIC ("Failed to initialise child results table for main thread.");
+
   /* Create the idle thread. */
   struct semaphore idle_started;
   sema_init (&idle_started, 0);
@@ -241,11 +252,25 @@ thread_create (const char *name, int priority,
   struct thread *parent_thread = thread_current ();
   init_thread (t, name, parent_thread->nice, priority, parent_thread->recent_cpu);
   tid = t->tid = allocate_tid ();
-  init_process_result (t);
+  if (!init_process_result (t))
+    {
+      palloc_free_page (t);
+      return TID_ERROR;
+    }
 
-  #ifdef USERPROG
+#ifdef USERPROG
-    hash_init (&t->open_files, fd_hash, fd_less, NULL);
+  /* Initialize the thread's file descriptor table. */
-  #endif
+  t->fd_counter = MINIMUM_USER_FD;
+
+  if (!hash_init (&t->open_files, fd_hash, fd_less, NULL)
+      || !hash_init (&t->child_results, process_result_hash,
+                     process_result_less, t))
+    {
+      palloc_free_page (t);
+      free (t->result);
+      return TID_ERROR;
+    }
+#endif
 
   /* Prepare thread for first run by initializing its stack.
      Do this atomically so intermediate values for the 'stack' 
@@ -269,9 +294,7 @@ thread_create (const char *name, int priority,
 
   intr_set_level (old_level);
 
-  /* No need to synchronise child_results since it is only ever accessed by one
+  hash_insert (&parent_thread->child_results, &t->result->elem);
-     thread. By the nature of increasing TIDs, this list is ordered. */
-  list_push_back (&parent_thread->child_results, &t->result->elem);
 
   /* Add to run queue. */
   thread_unblock (t);
@@ -649,15 +672,18 @@ is_thread (struct thread *t)
 }
 
 /* Allocate and initialise a process result for given thread. */
-static void
+static bool
 init_process_result (struct thread *t)
 {
   struct process_result *result = malloc (sizeof (struct process_result));
+  if (result == NULL)
+    return false;
   result->tid = t->tid;
-  result->exit_status = t->exit_status;
+  result->exit_status = -1;
   lock_init (&result->lock);
   sema_init (&result->sema, 0);
   t->result = result;
+  return true;
 }
 
 /* Does basic initialization of T as a blocked thread named
@@ -688,10 +714,6 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
-  t->fd_counter = MINIMUM_USER_FD;
-  t->exit_status = -1;
-  list_init (&t->child_results);
-
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);
@@ -822,6 +844,29 @@ allocate_tid (void)
   return tid;
 }
 
+/* Hashing function needed for child_results table.
+   Returns hash of process_result's TID. */
+static unsigned
+process_result_hash (const struct hash_elem *e, void *aux UNUSED)
+{
+  const struct process_result *result
+      = hash_entry (e, struct process_result, elem);
+  return hash_int (result->tid);
+}
+
+/* Comparator function needed for child_results table.
+   Returns less than comparison on process_results' TIDs. */
+static bool
+process_result_less (const struct hash_elem *a_, const struct hash_elem *b_,
+                     void *aux UNUSED)
+{
+  const struct process_result *a
+      = hash_entry (a_, struct process_result, elem);
+  const struct process_result *b
+      = hash_entry (b_, struct process_result, elem);
+  return a->tid < b->tid;
+}
+
 /* Offset of `stack' member within `struct thread'.
    Used by switch.S, which can't figure it out on its own. */
 uint32_t thread_stack_ofs = offsetof (struct thread, stack);

src/threads/thread.h

@@ -44,7 +44,7 @@ struct process_result
   struct lock lock;      /* Lock the exit_status and sema. */
   struct semaphore sema; /* Semaphore to signal the parent that the exit_status
                             has been set. */
-  struct list_elem elem; /* List element for the parent's children list. */
+  struct hash_elem elem; /* Hash element for the parent's children map. */
 };
 
 /* A kernel thread or user process.
@@ -128,15 +128,13 @@ struct thread
 
     /* Process wait properties. */
     struct process_result *result;      /* Result of the process. */
-    struct list child_results;          /* List of children's of this thread
+    struct hash child_results;          /* Map of children's of this thread
-                                           process results. */
+                                           TID to process result. */
     struct file *exec_file;             /* Thread's currently running file */
 
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
-    int exit_status;                    /* Exit Status: 0 = successful exit. */
-
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */

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 UNUSED);
 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 thread *t = thread_current ();
-  struct list_elem *e;
+  struct process_result fake_result;
-  struct thread *cur = thread_current ();
+  fake_result.tid = child_tid;
-
+  struct hash_elem *e = hash_find (&t->child_results, &fake_result.elem);
-  for (e = list_begin (&cur->child_results);
+  if (e == NULL)
-       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)
     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 table.
-  /* To prevent waiting for child twice, remove it from the list.
      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;
 }
@@ -371,8 +355,6 @@ process_exit (void)
   struct thread *cur = thread_current ();
   uint32_t *pd;
 
-  printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
-
   /* Clean up all open files */
   hash_destroy (&cur->open_files, fd_cleanup);
 
@@ -385,49 +367,15 @@ process_exit (void)
       lock_release (&filesys_lock);
     }
 
-  /* Update process result. */
   if (cur->result != NULL)
     {
-      lock_acquire (&cur->result->lock);
+      printf ("%s: exit(%d)\n", cur->name, cur->result->exit_status);
-      cur->result->exit_status = cur->exit_status;
+      /* Update own process result. */
-      /* Parent has died, child has to free the struct process_result * */
+      destruct_process_result (&cur->result->elem, cur);
-      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);
-        }
     }
 
   /* Free child process results or signal parent's death. */
-  struct list_elem *e;
+  hash_destroy (&cur->child_results, destruct_process_result);
-  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;
-    }
 
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
@@ -447,6 +395,28 @@ process_exit (void)
     }
 }
 
+/* Destruct a process_result, with multi-thread awareness.
+   If the other thread is running, simply signals death. Otherwise
+   frees the result. */
+static void
+destruct_process_result (struct hash_elem *e, void *aux UNUSED)
+{
+  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
+    {
+      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. */

src/userprog/syscall.c

@@ -131,7 +131,7 @@ syscall_exit (int status)
 {
   /* Sets exit_status of the thread to status. thread_exit () will call
      process_exit () if user programs are allowed. */
-  thread_current ()->exit_status = status;
+  thread_current ()->result->exit_status = status;
   thread_exit ();
 }
 
@@ -271,7 +271,7 @@ syscall_read (int fd, void *buffer, unsigned size)
     {
       /* Reading from the console. */
       char *write_buffer = buffer;
-      for (int i = 0; i < size; i++)
+      for (unsigned i = 0; i < size; i++)
         write_buffer[i] = input_getc ();
 
       /* In case of console, read is always (eventually) successful. So return
@@ -348,12 +348,7 @@ syscall_seek (int fd, unsigned position)
   /* Find the file from the FD. If it does not exist, do nothing. */
   struct open_file *file_info = fd_get_file (fd);
   if (file_info != NULL)
-  {
-    /* File exists: Acquire the file system lock to prevent race conditions. */
-    lock_acquire (&filesys_lock);
     file_seek (file_info->file, position);
-    lock_release (&filesys_lock);
-  }
 }
 
 /* Handles the syscall for returning the next byte in a file referenced by
@@ -367,10 +362,7 @@ syscall_tell (int fd)
   if (file_info == NULL)
     return 0;
 
-  /* Acquire the file system lock to prevent race conditions. */
-  lock_acquire (&filesys_lock);
   unsigned pos = file_tell (file_info->file);
-  lock_release (&filesys_lock);
 
   /* Return the current position in the file. */
   return pos;