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From: Dmitry Antipov <dmantipov@yandex.ru>
To: Emacs development discussions <emacs-devel@gnu.org>
Subject: Nasty GC bug
Date: Fri, 24 Aug 2012 11:26:59 +0400	[thread overview]
Message-ID: <50372CC3.9050704@yandex.ru> (raw)

It looks like live_cons_p (M, P) may be true if P is a cons cell from
the spare_memory[X], X = 1..4. So, mark_maybe_{object, pointer} may
call to mark_object for an uninitialized cons from spare blocks, with
random results (most probably a crash). This is very hard to reproduce
because it depends from the values found on a C stack.

The same looks to be true for live_string_p and spare_memory[5, 6].

Suggested fix is to use MEM_TYPE_NON_LISP for spare memory, e.g.:

=== modified file 'src/alloc.c'
--- src/alloc.c	2012-08-21 23:39:56 +0000
+++ src/alloc.c	2012-08-24 07:23:48 +0000
@@ -3816,22 +3816,22 @@
      spare_memory[0] = malloc (SPARE_MEMORY);
    if (spare_memory[1] == 0)
      spare_memory[1] = lisp_align_malloc (sizeof (struct cons_block),
-						  MEM_TYPE_CONS);
+						  MEM_TYPE_NON_LISP);
    if (spare_memory[2] == 0)
      spare_memory[2] = lisp_align_malloc (sizeof (struct cons_block),
-					 MEM_TYPE_CONS);
+					 MEM_TYPE_NON_LISP);
    if (spare_memory[3] == 0)
      spare_memory[3] = lisp_align_malloc (sizeof (struct cons_block),
-					 MEM_TYPE_CONS);
+					 MEM_TYPE_NON_LISP);
    if (spare_memory[4] == 0)
      spare_memory[4] = lisp_align_malloc (sizeof (struct cons_block),
-					 MEM_TYPE_CONS);
+					 MEM_TYPE_NON_LISP);
    if (spare_memory[5] == 0)
      spare_memory[5] = lisp_malloc (sizeof (struct string_block),
-				   MEM_TYPE_STRING);
+				   MEM_TYPE_NON_LISP);
    if (spare_memory[6] == 0)
      spare_memory[6] = lisp_malloc (sizeof (struct string_block),
-				   MEM_TYPE_STRING);
+				   MEM_TYPE_NON_LISP);
    if (spare_memory[0] && spare_memory[1] && spare_memory[5])
      Vmemory_full = Qnil;
  #endif

Comments?

Dmitry



             reply	other threads:[~2012-08-24  7:26 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-24  7:26 Dmitry Antipov [this message]
2012-08-24  8:56 ` Nasty GC bug Paul Eggert
2012-08-24 10:58   ` Dmitry Antipov
2012-08-24 17:25     ` Paul Eggert
2012-08-27 13:25 ` Jim Meyering

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