[PATCH] malloc: Fix attached thread reference count handling [BZ #19243]

[Florian Weimer <fweimer at redhat dot com>]

This patch does not address the deadlock scenario around list_lock and
the fork handler.

I tried various different approaches, and there is an immediate
temptation to introduce proper synchronization into the arena selection
logic.  This is why I went with the minimal change for now.

The test, while racy, is extremely reliable in practice.  It usually
crashes within one or two seconds.  I have never seen it succeeding
without the fix.

Florian

malloc: Fix attached thread reference count handling [BZ #19243]

reused_arena can increase the attached thread count of arenas on the
free list.  This means that the assertion that the reference count is
zero is incorrect.  In this case, the reference count initialization
is incorrect as well and could cause arenas to be put on the free
list too early (while they still have attached threads).

2015-11-17  Florian Weimer  <fweimer@redhat.com>

	[BZ #19243]
	* malloc/arena.c (get_free_list): Remove assert and adjust
	reference count handling.
	* malloc/tst-malloc-thread-exit.c: New file.

diff --git a/malloc/Makefile b/malloc/Makefile
index 67ed293..aa0579c 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -28,7 +28,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
 	 tst-mallocstate tst-mcheck tst-mallocfork tst-trim1 \
 	 tst-malloc-usable tst-realloc tst-posix_memalign \
 	 tst-pvalloc tst-memalign tst-mallopt tst-scratch_buffer \
-	 tst-malloc-backtrace
+	 tst-malloc-backtrace tst-malloc-thread-exit
 test-srcs = tst-mtrace
 
 routines = malloc morecore mcheck mtrace obstack \
@@ -47,6 +47,8 @@ libmemusage-inhibit-o = $(filter-out .os,$(object-suffixes))
 
 $(objpfx)tst-malloc-backtrace: $(common-objpfx)nptl/libpthread.so \
 			       $(common-objpfx)nptl/libpthread_nonshared.a
+$(objpfx)tst-malloc-thread-exit: $(common-objpfx)nptl/libpthread.so \
+			       $(common-objpfx)nptl/libpthread_nonshared.a
 
 # These should be removed by `make clean'.
 extra-objs = mcheck-init.o libmcheck.a
diff --git a/malloc/arena.c b/malloc/arena.c
index 0f00afa..c8e043a 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -805,6 +805,8 @@ _int_new_arena (size_t size)
 }
 
 
+/* Remove an arena from free_list.  The arena may be in use because it
+   was attached concurrently to a thread by reused_arena below.  */
 static mstate
 get_free_list (void)
 {
@@ -818,10 +820,8 @@ get_free_list (void)
 	{
 	  free_list = result->next_free;
 
-	  /* Arenas on the free list are not attached to any thread.  */
-	  assert (result->attached_threads == 0);
-	  /* But the arena will now be attached to this thread.  */
-	  result->attached_threads = 1;
+	  /* The arena will be attached to this thread.  */
+	  ++result->attached_threads;
 
 	  detach_arena (replaced_arena);
 	}
diff --git a/malloc/tst-malloc-thread-exit.c b/malloc/tst-malloc-thread-exit.c
new file mode 100644
index 0000000..8977f77
--- /dev/null
+++ b/malloc/tst-malloc-thread-exit.c
@@ -0,0 +1,215 @@
+/* Test malloc with concurrent thread termination.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C 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 2.1 of the License, or (at your option) any later version.
+
+   The GNU C 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 the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* This thread spawns a number of outer threads, equal to the arena
+   limit.  The outer threads run a loop which start and join two
+   different kinds of threads: the first kind allocates (attaching an
+   arena to the thread) and waits, the second kind waits and
+   allocates.  The hope is that this will exhibit races in thread
+   termination and arena management.  */
+
+#include <errno.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define TIMEOUT 7
+
+static bool termination_requested;
+static int inner_thread_count = 4;
+static size_t malloc_size = 32;
+
+static void
+__attribute__ ((noinline, noclone))
+unoptimized_free (void *ptr)
+{
+  free (ptr);
+}
+
+static void *
+malloc_first_thread (void * closure)
+{
+  pthread_barrier_t *barrier = closure;
+  void *ptr = malloc (malloc_size);
+  if (ptr == NULL)
+    {
+      printf ("error: malloc: %m\n");
+      abort ();
+    }
+  int ret = pthread_barrier_wait (barrier);
+  if (ret != 0 && ret != PTHREAD_BARRIER_SERIAL_THREAD)
+    {
+      errno = ret;
+      printf ("error: pthread_barrier_wait: %m\n");
+      abort ();
+    }
+  unoptimized_free (ptr);
+  return NULL;
+}
+
+static void *
+wait_first_thread (void * closure)
+{
+  pthread_barrier_t *barrier = closure;
+  int ret = pthread_barrier_wait (barrier);
+  if (ret != 0 && ret != PTHREAD_BARRIER_SERIAL_THREAD)
+    {
+      errno = ret;
+      printf ("error: pthread_barrier_wait: %m\n");
+      abort ();
+    }
+  void *ptr = malloc (malloc_size);
+  if (ptr == NULL)
+    {
+      printf ("error: malloc: %m\n");
+      abort ();
+    }
+  unoptimized_free (ptr);
+  return NULL;
+}
+
+static void *
+outer_thread (void *closure)
+{
+  pthread_t *threads = calloc (sizeof (*threads), inner_thread_count);
+  if (threads == NULL)
+    {
+      printf ("error: calloc: %m\n");
+      abort ();
+    }
+
+  while (!__atomic_load_n (&termination_requested, __ATOMIC_RELAXED))
+    {
+      pthread_barrier_t barrier;
+      int ret = pthread_barrier_init (&barrier, NULL, inner_thread_count + 1);
+      if (ret != 0)
+        {
+          errno = ret;
+          printf ("pthread_barrier_init: %m\n");
+          abort ();
+        }
+      for (int i = 0; i < inner_thread_count; ++i)
+        {
+          void *(*func) (void *);
+          if ((i  % 2) == 0)
+            func = malloc_first_thread;
+          else
+            func = wait_first_thread;
+          ret = pthread_create (threads + i, NULL, func, &barrier);
+          if (ret != 0)
+            {
+              errno = ret;
+              printf ("error: pthread_create: %m\n");
+              abort ();
+            }
+        }
+      ret = pthread_barrier_wait (&barrier);
+      if (ret != 0 && ret != PTHREAD_BARRIER_SERIAL_THREAD)
+        {
+          errno = ret;
+          printf ("pthread_wait: %m\n");
+          abort ();
+        }
+      for (int i = 0; i < inner_thread_count; ++i)
+        {
+          ret = pthread_join (threads[i], NULL);
+          if (ret != 0)
+            {
+              ret = errno;
+              printf ("error: pthread_join: %m\n");
+              abort ();
+            }
+        }
+      ret = pthread_barrier_destroy (&barrier);
+      if (ret != 0)
+        {
+          ret = errno;
+          printf ("pthread_barrier_destroy: %m\n");
+          abort ();
+        }
+    }
+
+  free (threads);
+
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  /* The number of top-level threads should be equal to the number of
+     arenas.  See arena_get2.  */
+  long outer_thread_count = sysconf (_SC_NPROCESSORS_ONLN);
+  if (outer_thread_count >= 1)
+    {
+      /* See NARENAS_FROM_NCORES in malloc.c.  */
+      if (sizeof (long) == 4)
+        outer_thread_count *= 2;
+      else
+        outer_thread_count *= 8;
+    }
+
+  /* Leave some room for shutting down all threads gracefully.  */
+  int timeout = TIMEOUT - 2;
+
+  pthread_t *threads = calloc (sizeof (*threads), outer_thread_count);
+  if (threads == NULL)
+    {
+      printf ("error: calloc: %m\n");
+      abort ();
+    }
+
+  for (long i = 0; i < outer_thread_count; ++i)
+    {
+      int ret = pthread_create (threads + i, NULL, outer_thread, NULL);
+      if (ret != 0)
+        {
+          errno = ret;
+          printf ("error: pthread_create: %m\n");
+          abort ();
+        }
+    }
+
+  struct timespec ts = {timeout, 0};
+  if (nanosleep (&ts, NULL))
+    {
+      printf ("error: error: nanosleep: %m\n");
+      abort ();
+    }
+
+  __atomic_store_n (&termination_requested, true, __ATOMIC_RELAXED);
+
+  for (long i = 0; i < outer_thread_count; ++i)
+    {
+      int ret = pthread_join (threads[i], NULL);
+      if (ret != 0)
+        {
+          errno = ret;
+          printf ("error: pthread_join: %m\n");
+          abort ();
+        }
+    }
+  free (threads);
+
+  return 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
-- 
2.4.3