Subject: Re: bug#19883: Correction for backtrace
From: Andy Wingo <[email-masked]>
To: Ludovic Courtès <[email-masked]>
Cc: [email-masked], David Kastrup <[email-masked]>
Date: Thu, 23 Jun 2016 12:36:34 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

On Thu 23 Jun 2016 11:50, Andy Wingo <[email-masked]> writes:

> On Thu 26 Feb 2015 16:30, David Kastrup <[email-masked]> writes:
>
>> Try ./test 2 2000 200
>
> I can reproduce the crash with your test case, thanks :) The patch below
> fixes the bug for me.  WDYT Ludovic?

Here's a patch with a test case.  I'm going to apply as it seems to be
obviously the right thing and the test case does reproduce what I think
is the bug (racing mark and finalize procedures, even if it's only
happening in one thread, finalizers and mark procedures do introduce
concurrency).  We trigger the concurrency in a simple way, via
allocation in the finalizer.  The patch does fix the original test.  GC
could happen due to another thread of course.  I'm actually not sure
where the concurrency is coming from in David's test though :/

I'm very interested in any feedback you might have!

Andy

From 8dff3af087c6eaa83ae0d72aa8b22aef5c65d65d Mon Sep 17 00:00:00 2001
From: Andy Wingo <[email-masked]>
Date: Thu, 23 Jun 2016 11:47:42 +0200
Subject: [PATCH] Fix race between SMOB marking and finalization

* libguile/smob.c (clear_smobnum): New helper.
  (finalize_smob): Re-set the smobnum to the "finalized smob" type
  before finalizing.  Fixes #19883.
  (scm_smob_prehistory): Pre-register a "finalized smob" type, which has
  no mark procedure.
* test-suite/standalone/test-smob-mark-race.c: New file.
* test-suite/standalone/Makefile.am: Arrange to build and run the new
  test.
---
 libguile/smob.c                             | 33 +++++++++++++--
 test-suite/standalone/Makefile.am           |  6 +++
 test-suite/standalone/test-smob-mark-race.c | 65 +++++++++++++++++++++++++++++
 3 files changed, 101 insertions(+), 3 deletions(-)
 create mode 100644 test-suite/standalone/test-smob-mark-race.c

diff --git a/libguile/smob.c b/libguile/smob.c
index 90849a8..ed9d91a 100644
--- a/libguile/smob.c
+++ b/libguile/smob.c
@@ -374,20 +374,43 @@ scm_gc_mark (SCM o)
 }
 
 
+static void*
+clear_smobnum (void *ptr)
+{
+  SCM smob;
+  scm_t_bits smobnum;
+
+  smob = PTR2SCM (ptr);
+
+  smobnum = SCM_SMOBNUM (smob);
+  /* Frob the object's type in place, re-setting it to be the "finalized
+     smob" type.  This will prevent other routines from accessing its
+     internals in a way that assumes that the smob data is valid.  This
+     is notably the case for SMOB's own "mark" procedure, if any; as the
+     finalizer runs without the alloc lock, it's possible for a GC to
+     occur while it's running, in which case the object is alive and yet
+     its data is invalid.  */
+  SCM_SET_SMOB_DATA_0 (smob, SCM_SMOB_DATA_0 (smob) & ~(scm_t_bits) 0xff00);
+
+  return (void *) smobnum;
+}
+
 /* Finalize SMOB by calling its SMOB type's free function, if any.  */
 static void
 finalize_smob (void *ptr, void *data)
 {
   SCM smob;
+  scm_t_bits smobnum;
   size_t (* free_smob) (SCM);
 
   smob = PTR2SCM (ptr);
+  smobnum = (scm_t_bits) GC_call_with_alloc_lock (clear_smobnum, ptr);
+
 #if 0
-  printf ("finalizing SMOB %p (smobnum: %u)\n",
-	  ptr, SCM_SMOBNUM (smob));
+  printf ("finalizing SMOB %p (smobnum: %u)\n", ptr, smobnum);
 #endif
 
-  free_smob = scm_smobs[SCM_SMOBNUM (smob)].free;
+  free_smob = scm_smobs[smobnum].free;
   if (free_smob)
     free_smob (smob);
 }
@@ -470,6 +493,7 @@ void
 scm_smob_prehistory ()
 {
   long i;
+  scm_t_bits finalized_smob_tc16;
 
   scm_i_pthread_key_create (&current_mark_stack_pointer, NULL);
   scm_i_pthread_key_create (&current_mark_stack_limit, NULL);
@@ -493,6 +517,9 @@ scm_smob_prehistory ()
       scm_smobs[i].apply      = 0;
       scm_smobs[i].apply_trampoline_objcode = SCM_BOOL_F;
     }
+
+  finalized_smob_tc16 = scm_make_smob_type ("finalized smob", 0);
+  if (SCM_TC2SMOBNUM (finalized_smob_tc16) != 0) abort ();
 }
 
 /*
diff --git a/test-suite/standalone/Makefile.am b/test-suite/standalone/Makefile.am
index 712418a..aec7418 100644
--- a/test-suite/standalone/Makefile.am
+++ b/test-suite/standalone/Makefile.am
@@ -275,4 +275,10 @@ test_smob_mark_LDADD = $(LIBGUILE_LDADD)
 check_PROGRAMS += test-smob-mark
 TESTS += test-smob-mark
 
+test_smob_mark_race_SOURCES = test-smob-mark-race.c
+test_smob_mark_race_CFLAGS = ${test_cflags}
+test_smob_mark_race_LDADD = $(LIBGUILE_LDADD)
+check_PROGRAMS += test-smob-mark-race
+TESTS += test-smob-mark-race
+
 EXTRA_DIST += ${check_SCRIPTS}
diff --git a/test-suite/standalone/test-smob-mark-race.c b/test-suite/standalone/test-smob-mark-race.c
new file mode 100644
index 0000000..eca0325
--- /dev/null
+++ b/test-suite/standalone/test-smob-mark-race.c
@@ -0,0 +1,65 @@
+/* Copyright (C) 2016 Free Software Foundation, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA
+ */
+
+#if HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#undef NDEBUG
+
+#include <libguile.h>
+#include <assert.h>
+
+static SCM
+mark_smob (SCM smob)
+{
+  assert (SCM_SMOB_DATA (smob) == 1);
+  return SCM_BOOL_F;
+}
+
+static size_t
+finalize_smob (SCM smob)
+{
+  assert (SCM_SMOB_DATA (smob) == 1);
+  SCM_SET_SMOB_DATA (smob, 0);
+  /* Allocate a bit in the hopes of triggering a new GC, making the
+     marker race with the finalizer.  */
+  scm_cons (SCM_BOOL_F, SCM_BOOL_F);
+  return 0;
+}
+
+static void
+tests (void *data, int argc, char **argv)
+{
+  scm_t_bits tc16;
+  int i;
+
+  tc16 = scm_make_smob_type ("smob with finalizer", 0);
+  scm_set_smob_mark (tc16, mark_smob);
+  scm_set_smob_free (tc16, finalize_smob);
+
+  for (i = 0; i < 1000 * 1000; i++)
+    scm_new_smob (tc16, 1);
+}
+
+int
+main (int argc, char *argv[])
+{
+  scm_boot_guile (argc, argv, tests, NULL);
+  return 0;
+}
-- 
2.8.3