Bug 22111: Fix malloc tcache leak.
Commit Message
commit message---
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 the
tcache_thread_freeres. We add a test case which uses mallinfo
and some heuristics to look for 1000x 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.
Tested on x86_64 with no regressions.
Signed-off-by: Carlos O'Donell <carlos@redhat.com>
---
OK to checkin? In particular I'd be happy to see if there is any
way to make the test better.
Once approved I'll backport this to 2.26 stable branch.
2017-09-26 Carlos O'Donell <carlos@redhat.com>
[BZ #22111]
* malloc/malloc.c (tcache_shutting_down): Use bool type.
(tcache_thread_freeres): Set tcache_shutting_down before
freeing the tcache.
* malloc/Makefile (tests): Add tst-malloc-tcache-leak.
* malloc/tst-malloc-tcache-leak.c: New file.
---
Comments
On 09/27/2017 07:44 AM, Carlos O'Donell wrote:
> commit message---
Usually, we use something like this as the subject line:
malloc: Fix tcache leak on thread destruction [BZ #22111]
That is, subsystem first, description of the fix, and bug reference last.
> 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 the
> tcache_thread_freeres. We add a test case which uses mallinfo
> and some heuristics to look for 1000x 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.
See below, commit message may need updating.
The change itself looks good to me.
> diff --git a/malloc/tst-malloc-tcache-leak.c b/malloc/tst-malloc-tcache-leak.c
> new file mode 100644
> index 0000000..ad7ea46
> --- /dev/null
> +++ b/malloc/tst-malloc-tcache-leak.c
> @@ -0,0 +1,111 @@
> +static int
> +do_test (void)
> +{
> + /* Allocate an arbitrary number of threads that can run concurrently
> + and carry out one allocation from the tcahce and then exit. We
typo: “tcahce”
> + could do it one thread at a time to minimize memory usage, but
> + it's faster to do 10 threads at a time, and we won't run out of
> + memory. */
The comment is wrong for NUMA systems at least. Running single-threaded
is faster, particularly if you set the CPU affinity first.
> +#define TNUM 10
TNUM should be parallel_threads or something like that, but I think it
is unneeded.
> + pthread_t *threads;
> + unsigned int i, loop;
> + struct mallinfo info_before, info_after;
> + void *retval;
> +
> + /* Avoid there being 0 malloc'd data at this point by allocating the
> + pthread_t's required to run the test. */
> + threads = (pthread_t *) xmalloc (sizeof (pthread_t) * TNUM);
Should use xcalloc.
> + info_before = mallinfo ();
> +
> + assert (info_before.uordblks != 0);
> +
> + printf ("INFO: %d (bytes) are in use before starting threads.\n",
> + info_before.uordblks);
> +
> + /* Again this is an arbitrary choice. We run the 10 threads 10,000
> + times for a total of 100,000 threads created and joined. This
> + gives us enough iterations to show a leak. */
> + for (loop = 0; loop < 10000; loop++)
If you keep the parallel execution (maybe it is faster on small systems,
I have not tried), the outer loop count should involve TNUM.
> + {
> +
> + for (i = 0; i < TNUM; i++)
> + {
> + threads[i] = xpthread_create (NULL, worker, NULL);
> + }
Extraneous braces.
> + for (i = 0; i < TNUM; i++)
> + {
> + retval = xpthread_join (threads[i]);
> + 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 100,000 threads. We almost always grow
> + memory slightly, but not much. If the growth factor is over 1000
> + then we know we have a leak with high confidence. Consider that if
> + even 1-byte leaked per thread we'd have 100,000 bytes of additional
> + memory, and in general the in-use at the start of main is quite
> + low. */
> + if (info_after.uordblks > (info_before.uordblks * 1000))
> + FAIL_EXIT1 ("Memory usage after threads is too high.\n");
I think you should use a fixed offset here, not something that
essentially scales with the initially allocated amount of memory.
Thanks,
Florian
@@ -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)
@@ -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);
new file mode 100644
@@ -0,0 +1,111 @@
+/* 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)
+{
+ /* 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 have the allocated
+ memory escape back to the parent to be freed to avoid any compiler
+ optimizations. */
+ return (void *) xmalloc (32);
+}
+
+static int
+do_test (void)
+{
+ /* Allocate an arbitrary number of threads that can run concurrently
+ and carry out one allocation from the tcahce and then exit. We
+ could do it one thread at a time to minimize memory usage, but
+ it's faster to do 10 threads at a time, and we won't run out of
+ memory. */
+#define TNUM 10
+ pthread_t *threads;
+ unsigned int i, loop;
+ struct mallinfo info_before, info_after;
+ void *retval;
+
+ /* Avoid there being 0 malloc'd data at this point by allocating the
+ pthread_t's required to run the test. */
+ threads = (pthread_t *) xmalloc (sizeof (pthread_t) * TNUM);
+
+ info_before = mallinfo ();
+
+ assert (info_before.uordblks != 0);
+
+ printf ("INFO: %d (bytes) are in use before starting threads.\n",
+ info_before.uordblks);
+
+ /* Again this is an arbitrary choice. We run the 10 threads 10,000
+ times for a total of 100,000 threads created and joined. This
+ gives us enough iterations to show a leak. */
+ for (loop = 0; loop < 10000; loop++)
+ {
+
+ for (i = 0; i < TNUM; i++)
+ {
+ threads[i] = xpthread_create (NULL, worker, NULL);
+ }
+
+ for (i = 0; i < TNUM; i++)
+ {
+ retval = xpthread_join (threads[i]);
+ 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 100,000 threads. We almost always grow
+ memory slightly, but not much. If the growth factor is over 1000
+ then we know we have a leak with high confidence. Consider that if
+ even 1-byte leaked per thread we'd have 100,000 bytes of additional
+ memory, and in general the in-use at the start of main is quite
+ low. */
+ if (info_after.uordblks > (info_before.uordblks * 1000))
+ FAIL_EXIT1 ("Memory usage after threads is too high.\n");
+
+ /* Did not detect excessive memory usage. */
+ free (threads);
+ exit (0);
+}
+
+#include <support/test-driver.c>