Fix BZ# 22180.

POSIX requires that dlclose() and exit() be thread safe, therefore
you can have one thread in the middle of dlclose() and another thread
executing exit() without causing any undefined behaviour on the part
of the implementation.

The existing implementation had a flaw that exit() exit handler processing
did not consider a concurrent dlclose() and would not mark already run
exit handlers using the ef_free flavour. The consequence of this is that
a concurrent exit() with dlclose() will run all the exit handlers that
dlclose() had not yet run, but then will block on the loader lock. The
concurrent dlclose() will continue to run all the exit handlers again
(twice) in violation of the Itanium C++ ABI requirements for __cxa_atexit().

This commit fixes this by having exit() mark all handlers with ef_free to
ensure that concurrent dlclose() won't re-run registered exit handlers that
have already run.

1 files changed, 79 insertions, 0 deletions

diff --git a/stdlib/test-dlclose-exit-race-helper.c b/stdlib/test-dlclose-exit-race-helper.c
new file mode 100644
index 0000000000..fa765899bc
--- /dev/null
+++ b/stdlib/test-dlclose-exit-race-helper.c
@@ -0,0 +1,79 @@
+/* Helper for exit/dlclose race test (Bug 22180).
+   Copyright (C) 2017 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/>.  */
+
+#include <stdio.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <semaphore.h>
+#include <unistd.h>
+#include <support/check.h>
+#include <support/xthread.h>
+
+/* Semaphore defined in executable to ensure we have a happens-before
+   between the first function starting and exit being called.  */
+extern sem_t order1;
+
+/* Semaphore defined in executable to ensure we have a happens-before
+   between the second function starting and the first function returning.  */
+extern sem_t order2;
+
+/* glibc function for registering DSO-specific exit functions.  */
+extern int __cxa_atexit (void (*func) (void *), void *arg, void *dso_handle);
+
+/* Hidden compiler handle to this shared object.  */
+extern void *__dso_handle __attribute__ ((__weak__));
+
+static void
+first (void *start)
+{
+  /* Let the exiting thread run.  */
+  sem_post (&order1);
+
+  /* Wait for exiting thread to finish.  */
+  sem_wait (&order2);
+
+  printf ("first\n");
+}
+
+static void
+second (void *start)
+{
+  /* We may be called from different threads.
+     This lock protects called.  */
+  static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
+  static bool called = false;
+
+  xpthread_mutex_lock (&mtx);
+  if (called)
+    FAIL_EXIT1 ("second called twice!");
+
+  called = true;
+  xpthread_mutex_unlock (&mtx);
+
+  printf ("second\n");
+}
+
+
+__attribute__ ((constructor)) static void
+constructor (void)
+{
+  sem_init (&order1, 0, 0);
+  sem_init (&order2, 0, 0);
+  __cxa_atexit (second, NULL, __dso_handle);
+  __cxa_atexit (first, NULL, __dso_handle);
+}