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authorCarlos O'Donell <carlos@systemhalted.org>2017-09-28 11:05:18 -0600
committerCarlos O'Donell <carlos@systemhalted.org>2017-10-06 09:31:52 -0700
commit1e26d35193efbb29239c710a4c46a64708643320 (patch)
tree711bdaefe5af9f9566c3a9e101b7328f565faa61 /malloc
parentd13867625894fda6c6a5034dadfa8ff86983ea12 (diff)
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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.
Diffstat (limited to 'malloc')
-rw-r--r--malloc/Makefile3
-rw-r--r--malloc/malloc.c8
-rw-r--r--malloc/tst-malloc-tcache-leak.c112
3 files changed, 120 insertions, 3 deletions
diff --git a/malloc/Makefile b/malloc/Makefile
index 50b487eeb57..6cf78e1177f 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -34,6 +34,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
tst-interpose-nothread \
tst-interpose-thread \
tst-alloc_buffer \
+ tst-malloc-tcache-leak \
tests-static := \
tst-interpose-static-nothread \
@@ -242,3 +243,5 @@ tst-dynarray-fail-ENV = MALLOC_TRACE=$(objpfx)tst-dynarray-fail.mtrace
$(objpfx)tst-dynarray-fail-mem.out: $(objpfx)tst-dynarray-fail.out
$(common-objpfx)malloc/mtrace $(objpfx)tst-dynarray-fail.mtrace > $@; \
$(evaluate-test)
+
+$(objpfx)tst-malloc-tcache-leak: $(shared-thread-library)
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 1c2a0b05b78..d3fcadd20e4 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2916,7 +2916,7 @@ typedef struct tcache_perthread_struct
tcache_entry *entries[TCACHE_MAX_BINS];
} tcache_perthread_struct;
-static __thread char tcache_shutting_down = 0;
+static __thread bool tcache_shutting_down = false;
static __thread tcache_perthread_struct *tcache = NULL;
/* Caller must ensure that we know tc_idx is valid and there's room
@@ -2953,8 +2953,12 @@ tcache_thread_freeres (void)
if (!tcache)
return;
+ /* Disable the tcache and prevent it from being reinitialized. */
tcache = NULL;
+ tcache_shutting_down = true;
+ /* Free all of the entries and the tcache itself back to the arena
+ heap for coalescing. */
for (i = 0; i < TCACHE_MAX_BINS; ++i)
{
while (tcache_tmp->entries[i])
@@ -2966,8 +2970,6 @@ tcache_thread_freeres (void)
}
__libc_free (tcache_tmp);
-
- tcache_shutting_down = 1;
}
text_set_element (__libc_thread_subfreeres, tcache_thread_freeres);
diff --git a/malloc/tst-malloc-tcache-leak.c b/malloc/tst-malloc-tcache-leak.c
new file mode 100644
index 00000000000..22c679b65b1
--- /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>