X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Fthread.h;h=d6a3990f1047d795b4255efa278af5f1a1119e32;hb=f2f8875638593bd5365cfd6a5ba7c9578e52322f;hp=e8927def25ebc99594a541425096ddc6eff8c4e4;hpb=349cc721b17effa62c8b18a7dccc5defc44472d3;p=pintos-anon

diff --git a/src/threads/thread.h b/src/threads/thread.h
index e8927de..d6a3990 100644
--- a/src/threads/thread.h
+++ b/src/threads/thread.h
@@ -1,35 +1,92 @@
 #ifndef HEADER_THREAD_H
 #define HEADER_THREAD_H 1
 
+#include <debug.h>
+#include <list.h>
 #include <stdint.h>
-#include "debug.h"
-#include "list.h"
 
 #ifdef USERPROG
-#include "addrspace.h"
+#include "userprog/addrspace.h"
 #endif
 
-enum thread_status 
+/* States in a thread's life cycle. */
+enum thread_status
   {
-    THREAD_RUNNING,
-    THREAD_READY,
-    THREAD_BLOCKED,
-    THREAD_DYING
+    THREAD_RUNNING,     /* Running thread. */
+    THREAD_READY,       /* Not running but ready to run. */
+    THREAD_BLOCKED,     /* Waiting for an event to trigger. */
+    THREAD_DYING        /* About to be destroyed. */
   };
 
-struct thread 
+/* A kernel thread or user process.
+
+   Each thread structure is stored in its own 4 kB page.  The
+   thread structure itself sits at the very bottom of the page
+   (at offset 0).  The rest of the page is reserved for the
+   thread's kernel stack, which grows downward from the top of
+   the page (at offset 4 kB).  Here's an illustration:
+
+        4 kB +---------------------------------+
+             |          kernel stack           |
+             |                |                |
+             |                |                |
+             |                V                |
+             |         grows downward          |
+             |                                 |
+             |                                 |
+             |                                 |
+             |                                 |
+             |                                 |
+             |                                 |
+             |                                 |
+             |                                 |
+             +---------------------------------+
+             |             magic               |
+             |               :                 |
+             |               :                 |
+             |              name               |
+             |             status              |
+        0 kB +---------------------------------+
+
+   The upshot of this is twofold:
+
+      1. First, `struct thread' must not be allowed to grow too
+         big.  If it does, then there will not be enough room for
+         the kernel stack.  Our base `struct thread' is only a
+         few bytes in size.  It probably should stay well under 1
+         kB.
+
+      2. Second, kernel stacks must not be allowed to grow too
+         large.  If a stack overflows, it will corrupt the thread
+         state.  Thus, kernel functions should not allocate large
+         structures or arrays as non-static local variables.  Use
+         dynamic allocation with malloc() or palloc_get()
+         instead.
+
+   The first symptom of either of these problems will probably be
+   an assertion failure in thread_current(), which checks that
+   the `magic' member of the running thread's `struct thread' is
+   set to THREAD_MAGIC.  Stack overflow will normally change this
+   value, triggering the assertion. */
+/* The `elem' member has a dual purpose.  It can be an element in
+   the run queue (thread.c), or it can be an element in a
+   semaphore wait list (synch.c).  It can be used these two ways
+   only because they are mutually exclusive: only a thread in the
+   ready state is on the run queue, whereas only a thread in the
+   blocked state is on a semaphore wait list. */
+struct thread
   {
     /* These members are owned by the thread_*() functions. */
     enum thread_status status;          /* Thread state. */
     char name[16];                      /* Name (for debugging purposes). */
     uint8_t *stack;                     /* Saved stack pointer. */
-    list_elem rq_elem;                  /* Run queue list element. */
+    list_elem elem;                     /* List element. */
 
 #ifdef USERPROG
     /* These members are owned by the addrspace_*() functions. */
     uint32_t *pagedir;                  /* Page directory. */
 #endif
-    
+
     /* Marker to detect stack overflow. */
     unsigned magic;                     /* Always set to THREAD_MAGIC. */
   };