malloc: Fix tcache leak after thread destruction [BZ #22111]

The malloc tcache added in 2.26 will leak all of the elements remaining
in the cache and the cache structure itself when a thread exits. The
defect is that we do not set tcache_shutting_down early enough, and the
thread simply recreates the tcache and places the elements back onto a
new tcache which is subsequently lost as the thread exits (unfreed
memory). The fix is relatively simple, move the setting of
tcache_shutting_down earlier in tcache_thread_freeres. We add a test
case which uses mallinfo and some heuristics to look for unaccounted for
memory usage between the start and end of a thread start/join loop. It
is very reliable at detecting that there is a leak given the number of
iterations.  Without the fix the test will consume 122MiB of leaked
memory.

1 files changed, 112 insertions, 0 deletions

diff --git a/malloc/tst-malloc-tcache-leak.c b/malloc/tst-malloc-tcache-leak.c
new file mode 100644
index 0000000..22c679b
--- /dev/null
+++ b/malloc/tst-malloc-tcache-leak.c
@@ -0,0 +1,112 @@
+/* Bug 22111: Test that threads do not leak their per thread cache.
+   Copyright (C) 2015-2017 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/>.  */
+
+/* The point of this test is to start and exit a large number of
+   threads, while at the same time looking to see if the used
+   memory grows with each round of threads run.  If the memory
+   grows above some linear bound we declare the test failed and
+   that the malloc implementation is leaking memory with each
+   thread.  This is a good indicator that the thread local cache
+   is leaking chunks.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <malloc.h>
+#include <pthread.h>
+#include <assert.h>
+
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xthread.h>
+
+void *
+worker (void *data)
+{
+  void *ret;
+  /* Allocate an arbitrary amount of memory that is known to fit into
+     the thread local cache (tcache).  If we have at least 64 bins
+     (default e.g. TCACHE_MAX_BINS) we should be able to allocate 32
+     bytes and force malloc to fill the tcache.  We are assuming tcahce
+     init happens at the first small alloc, but it might in the future
+     be deferred to some other point.  Therefore to future proof this
+     test we include a full alloc/free/alloc cycle for the thread.  We
+     need a compiler barrier to avoid the removal of the useless
+     alloc/free.  We send some memory back to main to have the memory
+     freed after the thread dies, as just another check that the chunks
+     that were previously in the tcache are still OK to free after
+     thread death.  */
+  ret = xmalloc (32);
+  __asm__ volatile ("" ::: "memory");
+  free (ret);
+  return (void *) xmalloc (32);
+}
+
+static int
+do_test (void)
+{
+  pthread_t *thread;
+  struct mallinfo info_before, info_after;
+  void *retval;
+
+  /* This is an arbitrary choice. We choose a total of THREADS
+     threads created and joined.  This gives us enough iterations to
+     show a leak.  */
+  int threads = 100000;
+
+  /* Avoid there being 0 malloc'd data at this point by allocating the
+     pthread_t required to run the test.  */
+  thread = (pthread_t *) xcalloc (1, sizeof (pthread_t));
+
+  info_before = mallinfo ();
+
+  assert (info_before.uordblks != 0);
+
+  printf ("INFO: %d (bytes) are in use before starting threads.\n",
+          info_before.uordblks);
+
+  for (int loop = 0; loop < threads; loop++)
+    {
+      *thread = xpthread_create (NULL, worker, NULL);
+      retval = xpthread_join (*thread);
+      free (retval);
+    }
+
+  info_after = mallinfo ();
+  printf ("INFO: %d (bytes) are in use after all threads joined.\n",
+          info_after.uordblks);
+
+  /* We need to compare the memory in use before and the memory in use
+     after starting and joining THREADS threads.  We almost always grow
+     memory slightly, but not much. Consider that if even 1-byte leaked
+     per thread we'd have THREADS bytes of additional memory, and in
+     general the in-use at the start of main is quite low.  We will
+     always leak a full malloc chunk, and never just 1-byte, therefore
+     anything above "+ threads" from the start (constant offset) is a
+     leak.  Obviously this assumes no thread-related malloc'd internal
+     libc data structures persist beyond the thread death, and any that
+     did would limit the number of times you could call pthread_create,
+     which is a QoI we'd want to detect and fix.  */
+  if (info_after.uordblks > (info_before.uordblks + threads))
+    FAIL_EXIT1 ("Memory usage after threads is too high.\n");
+
+  /* Did not detect excessive memory usage.  */
+  free (thread);
+  exit (0);
+}
+
+#include <support/test-driver.c>