src/threads/synch.c

@@ -119,14 +119,14 @@ sema_up (struct semaphore *sema)
 
   old_level = intr_disable ();
   if (!list_empty (&sema->waiters))
-  {
-    /* Enforces wake-up of the highest priority thread waiting for the
-       semaphore. */
-    struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
-    list_remove (e);
-    thread_unblock (list_entry (e, struct thread, elem));
-    thread_unblocked = true;
-  }
+    {
+      /* Enforces wake-up of the highest priority thread waiting for the
+         semaphore. */
+      struct list_elem *e = list_max (&sema->waiters, priority_less, NULL);
+      list_remove (e);
+      thread_unblock (list_entry (e, struct thread, elem));
+      thread_unblocked = true;
+    }
   sema->value++;
   intr_set_level (old_level);
 
@@ -134,12 +134,12 @@ sema_up (struct semaphore *sema)
      priority that the current running thread, including the case when called
      within an interrupt handler. */
   if (thread_unblocked)
-  {
-    if (intr_context ())
-      intr_yield_on_return ();
-    else
-      thread_yield ();
-  }
+    {
+      if (intr_context ())
+        intr_yield_on_return ();
+      else
+        thread_yield ();
+    }
 }
 
 static void sema_test_helper (void *sema_);

src/threads/thread.c

@@ -688,6 +688,7 @@ 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);
 

src/threads/thread.h

@@ -32,6 +32,9 @@ typedef int tid_t;
 #define NICE_DEFAULT 0 /* Default niceness. */
 #define NICE_MAX 20    /* Highest niceness. */
 
+/* File Descriptors. */
+#define MINIMUM_USER_FD 2 /* Minimum file descriptor for user programs. */
+
 /* A process result, synchronised between parent and child. */
 struct process_result
 {
@@ -137,7 +140,9 @@ struct thread
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
-    struct hash open_files;             /* Hash Table of FD -> Struct File */
+    unsigned int fd_counter;            /* File descriptor counter for thread's
+                                           open files. */
+    struct hash open_files;             /* Hash Table of FD -> Struct File. */
 #endif
 
     /* Owned by thread.c. */

src/userprog/process.c

@@ -84,9 +84,9 @@ process_execute (const char *cmd)
 
   /* Validates that the current file to be executed can be opened/exists. */
   lock_acquire (&filesys_lock);
-  bool valid_file = filesys_open (file_name) != NULL;
+  struct file *file = filesys_open (file_name);
   lock_release (&filesys_lock);
-  if (!valid_file)
+  if (file == NULL)
     return TID_ERROR;
 
   /* Create a new thread to execute the command, by initializing
@@ -128,7 +128,6 @@ static void
 start_process (void *proc_start_data)
 {
   struct intr_frame if_;
-
   struct process_start_data *data = proc_start_data;
 
   /* Initialize interrupt frame and load executable. */
@@ -136,41 +135,46 @@ start_process (void *proc_start_data)
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
-  
+
+  /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
-  
-  /* Prevent writing to the file being executed. */
+
   struct file *exec_file = filesys_open (data->file_name);
   if (exec_file == NULL)
     {
+      /* If the executable file cannot be opened, free resources and quit. */
       lock_release (&filesys_lock);
       sema_up (&data->loaded);
       thread_exit ();
     }
+
+  /* Deny write to the executable file to prevent writing to it and release the
+     file system lock. */
   file_deny_write (exec_file);
   lock_release (&filesys_lock);
+
   thread_current ()->exec_file = exec_file;
 
+  /* Load the ELF executable file, and store the success of the operation in
+     the 'success' variable in data. */
   data->success = load (data->file_name, &if_.eip, &if_.esp);
 
-  /* If load failed, quit. */
-  if (!data->success) 
+  /* If load was sucessful, initialize user process stack and free page used
+ to store the command that executed the process. */
+  if (data->success)
     {
-      sema_up (&data->loaded);
-      thread_exit ();
+      data->success =
+        process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
     }
 
-  /* Initialize user process stack and free page used to store the
-     command that executed the process. */
-  data->success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
+  /* Signal that the process has finished attempting to load. */
   bool success = data->success;
   sema_up (&data->loaded);
   
-  /* If stack initialization failed, free resources and quit. */
+  /* If the load was unsuccessful or if it was but the stack initialization
+     failed, exit the thread. */
   if (!success)
-    {
       thread_exit ();
-    }
 
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
@@ -187,6 +191,10 @@ start_process (void *proc_start_data)
 static bool
 process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
   {
+    ASSERT (cmd_saveptr != NULL);
+    ASSERT (esp != NULL);
+    ASSERT (file_name != NULL);
+
     /* Load command line argument *data* to user process stack.
        This can't cause overflow due to enforcing that the size of
        command line input must fit in a page. Also keep track
@@ -198,8 +206,12 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
     int arg_count = 0;
     while (arg != NULL)
       {
+        /* filename has already been validated to be a safe-to-access string,
+           so we can safely use strlen here. Filename has already been
+           split from the command line arguments. */
         push_to_stack (esp, arg, (strlen (arg) + 1) * sizeof (char));
-        
+
+        /* Try to allocate memory for the argument pointer. */
         struct arg_elem *arg_elem = malloc (sizeof (struct arg_elem));
         if (arg_elem == NULL)
           {
@@ -208,9 +220,11 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
             return false;
           }
 
+        /* Store the argument pointer in the linked list. */
         arg_elem->arg = *esp;
         list_push_front (&arg_list, &arg_elem->elem);
 
+        /* Increment the argument count and get the next argument. */
         arg_count++;
         arg = strtok_r (NULL, " ", &cmd_saveptr);
       }
@@ -231,7 +245,10 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
     int overflow_bytes = (PHYS_BASE - *esp) + remaining_size - PGSIZE;
     if (overflow_bytes > 0)
       {
+        /* Calculate the number of pages needed to allocate. */
         int pages_needed = DIV_CEIL (overflow_bytes, PGSIZE);
+
+        /* Allocate the pages and map them to the user process. */
         for (int i = 1; i < pages_needed + 1; i++)
           {
             uint8_t *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
@@ -296,11 +313,12 @@ push_to_stack (void **esp, void *data, size_t data_size)
  * This function will be implemented in task 2.
  * For now, it does nothing. */
 int
-process_wait (tid_t child_tid UNUSED) 
+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))
     {
@@ -308,27 +326,40 @@ process_wait (tid_t child_tid UNUSED)
           = 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. */
+
+      /* 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;
+
   /* Wait for child to die. */
   sema_down (&child_result->sema);
+
   /* We need lock release in process_exit, so we need to acquire (and possibly
      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.
      No need to use lock since this is the only thread with access to
      the struct process_result now. */
   list_remove (&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);
+
   free (child_result);
   return exit_status;
 }
@@ -345,9 +376,10 @@ process_exit (void)
   /* Clean up all open files */
   hash_destroy (&cur->open_files, fd_cleanup);
 
-  /* Close the executable file. */
+  /* Close the executable file, implicitly allowing it to be written to. */
   if (cur->exec_file != NULL)
     {
+      /* Acquire the file system lock to prevent race conditions. */
       lock_acquire (&filesys_lock);
       file_close (cur->exec_file);
       lock_release (&filesys_lock);

src/userprog/syscall.c

@@ -16,8 +16,6 @@
 #define MAX_SYSCALL_ARGS 3
 #define EXIT_FAILURE -1
 
-static unsigned fd_counter = MIN_USER_FD;
-
 struct open_file
   {
     int fd;                     /* File Descriptor / Identifier */
@@ -142,6 +140,7 @@ syscall_exit (int status)
 static pid_t
 syscall_exec (const char *cmd_line)
 {
+  /* Validate the user string before executing the process. */
   validate_user_string (cmd_line);
 
   return process_execute (cmd_line); /* Returns the PID of the new process */
@@ -161,12 +160,15 @@ syscall_wait (pid_t pid)
 static bool
 syscall_create (const char *file, unsigned initial_size)
 {
+  /* Validate the user string before creating the file. */
   validate_user_string (file);
 
+  /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   bool status = filesys_create (file, initial_size);
   lock_release (&filesys_lock);
 
+  /* Return the status of the file creation. */
   return status;
 }
 
@@ -176,12 +178,15 @@ syscall_create (const char *file, unsigned initial_size)
 static bool
 syscall_remove (const char *file)
 {
+  /* Validate the user string before removing the file. */
   validate_user_string (file);
 
+  /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   bool status = filesys_remove (file);
   lock_release (&filesys_lock);
 
+  /* Return the status of the file removal. */
   return status;
 }
 
@@ -192,11 +197,15 @@ syscall_remove (const char *file)
 static int
 syscall_open (const char *file)
 {
+  /* Validate the user string before opening the file. */
   validate_user_string (file);
 
+  /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   struct file *ptr = filesys_open (file);
   lock_release (&filesys_lock);
+
+  /* If the file could not be opened, return failure. */
   if (ptr == NULL)
     return EXIT_FAILURE;
 
@@ -206,12 +215,14 @@ syscall_open (const char *file)
     = (struct open_file*) malloc (sizeof (struct open_file));
   if (file_info == NULL)
     {
+      /* If we could not allocate memory for the file_info struct, close the
+         file and return failure. */
       file_close (ptr);
       return EXIT_FAILURE;
     }
 
   /* Populate the above struct, with a unique FD and the current open file */
-  file_info->fd = fd_counter++;
+  file_info->fd = thread_current ()->fd_counter++;
   file_info->file = ptr;
 
   /* Add the new FD->file mapping to the hashtable for the current thread */
@@ -227,14 +238,17 @@ syscall_open (const char *file)
 static int
 syscall_filesize (int fd)
 {
+  /* Try to get the file from the FD. If it does not exist, return failure. */
   struct open_file *file_info = fd_get_file (fd);
   if (file_info == NULL)
       return EXIT_FAILURE;
 
+  /* Acquire the file system lock to prevent any race conditions. */
   lock_acquire (&filesys_lock);
   int bytes = file_length (file_info->file);
   lock_release (&filesys_lock);
 
+  /* Return the number of bytes in the file. */
   return bytes;
 }
 
@@ -247,9 +261,10 @@ syscall_read (int fd, void *buffer, unsigned size)
 {
   /* Only console (fd = 0) or other files, not including STDOUT, (fd > 1) are
      allowed. */
-  if (fd < 0 || fd == STDOUT_FILENO)
+  if (fd < STDIN_FILENO || fd == STDOUT_FILENO)
     return EXIT_FAILURE;
 
+  /* Validate the user buffer for the provided size before reading. */
   validate_user_pointer (buffer, size);
 
   if (fd == STDIN_FILENO)
@@ -259,18 +274,24 @@ syscall_read (int fd, void *buffer, unsigned size)
       for (int i = 0; i < size; i++)
         write_buffer[i] = input_getc ();
 
+      /* In case of console, read is always (eventually) successful. So return
+         the size for the number of bytes read. */
       return size;
     }
   else
     {
       /* Reading from a file. */
+      /* Find the file from the FD. If it does not exist, return failure. */
       struct open_file *file_info = fd_get_file (fd);
       if (file_info == NULL)
         return EXIT_FAILURE;
 
+      /* Acquire the file system lock to prevent race-conditions. */
       lock_acquire (&filesys_lock);
       int bytes_written = file_read (file_info->file, buffer, size);
       lock_release (&filesys_lock);
+
+      /* Return the number of bytes read. */
       return bytes_written;
     }
 }
@@ -287,25 +308,32 @@ syscall_write (int fd, const void *buffer, unsigned size)
   if (fd <= 0)
     return 0;
 
+  /* Validate the user buffer for the provided size before writing. */
   validate_user_pointer (buffer, size);
 
   if (fd == STDOUT_FILENO)
     {
       /* Writing to the console. */
       putbuf (buffer, size);
+
+      /* In case of console, write is always successful. So return the size for
+         the number of bytes written. */
       return size;
     }
   else
     {
       /* Writing to a file. */
+      /* Find the file from the FD. If it does not exist, return failure. */
       struct open_file *file_info = fd_get_file (fd);
       if (file_info == NULL)
         return 0;
 
+      /* Acquire the file system lock to prevent race conditions. */
       lock_acquire (&filesys_lock);
       int bytes = file_write (file_info->file, buffer, size);
       lock_release (&filesys_lock);
 
+      /* Return the number of bytes written. */
       return bytes;
     }
 }
@@ -317,9 +345,11 @@ syscall_write (int fd, const void *buffer, unsigned size)
 static void
 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);
@@ -332,14 +362,17 @@ syscall_seek (int fd, unsigned position)
 static unsigned
 syscall_tell (int fd)
 {
+  /* Find the file from the FD. If it does not exist, return 0. */
   struct open_file *file_info = fd_get_file (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;
 }
 
@@ -349,14 +382,21 @@ syscall_tell (int fd)
 static void
 syscall_close (int fd)
 {
+  /* 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 */
+      /* First, remove the file from the hash table of open files. */
       hash_delete (&thread_current ()->open_files, &file_info->elem);
 
+      /* Then, close the file, acquiring the file system lock to prevent race
+         conditions. */
       lock_acquire (&filesys_lock);
       file_close (file_info->file);
       lock_release (&filesys_lock);
+
+      /* Free the memory allocated for the file_info struct. */
       free (file_info);
     }
 }
@@ -366,7 +406,12 @@ syscall_close (int fd)
 unsigned
 fd_hash (const struct hash_elem *element, void *aux UNUSED)
 {
-  return hash_int (hash_entry (element, struct open_file, elem)->fd);
+  /* We use the FD as the hash value. This is because the FD is incremented
+     sequentially and is therefore unique for each file. It positively affects
+     the performance of the hash table: 1. It is unique so no need to call
+     expensive hash functions. 2. It being sequential means that the hash table
+     is more likely to be weight balanced. */
+  return hash_entry (element, struct open_file, elem)->fd;
 }
 
 /* Comparator function for the open_file table. Compares two entries based on
@@ -421,17 +466,19 @@ fd_get_file (int fd)
 static void
 validate_user_pointer (const void *start, size_t size)
 {
+  /* If the size is 0, we do not need to check anything. */
   if (size == 0)
     return;
 
   const void *end = start + size - 1;
 
+  /* Check if the start and end pointers are valid user virtual addresses. */
   if (start == NULL || !is_user_vaddr (start) || !is_user_vaddr (end))
     syscall_exit (EXIT_FAILURE);
 
   /* We now need to check if the entire memory block is mapped to physical
      memory by the page table. */
-  for (const void *ptr = start; ptr <= end; ptr += PGSIZE)
+  for (const void *ptr = pg_round_down (start); ptr <= end; ptr += PGSIZE)
     if (pagedir_get_page (thread_current ()->pagedir, ptr) == NULL)
       syscall_exit (EXIT_FAILURE);
 }
@@ -442,9 +489,11 @@ validate_user_pointer (const void *start, size_t size)
 static void
 validate_user_string (const char *str)
 {
+  /* Check if the string pointer is a valid user virtual address. */
   if (str == NULL || !is_user_vaddr (str))
     syscall_exit (EXIT_FAILURE);
 
+  /* Calculate the offset of the string within the (first) page. */
   size_t offset = (uintptr_t) str % PGSIZE;
 
   /* We move page by page, checking if the page is mapped to physical memory. */
@@ -452,6 +501,8 @@ validate_user_string (const char *str)
   {
     void *page = pg_round_down (str);
 
+    /* If we reach addresses that are not mapped to physical memory before the
+       end of the string, the thread is terminated. */
     if (!is_user_vaddr(page) ||
         pagedir_get_page (thread_current ()->pagedir, page) == NULL)
       syscall_exit (EXIT_FAILURE);

src/userprog/syscall.h

@@ -4,8 +4,6 @@
 #include <hash.h>
 #include "threads/synch.h"
 
-#define MIN_USER_FD 2
-
 typedef int pid_t;
 
 struct lock filesys_lock;