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malloc: Run fork handler as late as possible [BZ #19431]

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Previously, a thread M invoking fork would acquire locks in this order:

  (M1) malloc arena locks (in the registered fork handler)
  (M2) libio list lock

A thread F invoking flush (NULL) would acquire locks in this order:

  (F1) libio list lock
  (F2) individual _IO_FILE locks

A thread G running getdelim would use this order:

  (G1) _IO_FILE lock
  (G2) malloc arena lock

After executing (M1), (F1), (G1), none of the threads can make progress.

This commit changes the fork lock order to:

  (M'1) libio list lock
  (M'2) malloc arena locks

It explicitly encodes the lock order in the implementations of fork,
and does not rely on the registration order, thus avoiding the deadlock.
This commit is contained in:
Florian Weimer 2016-04-14 09:17:02 +02:00
parent b49ab5f450
commit 29d794863c
9 changed files with 321 additions and 51 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
ChangeLog
View File

@ -1,3 +1,27 @@
2016-04-14 Florian Weimer <fweimer@redhat.com>
[BZ #19431]
Run the malloc fork handler as late as possible to avoid deadlocks.
* malloc/malloc-internal.h: New file.
* malloc/malloc.c: Include it.
* malloc/arena.c (ATFORK_MEM): Remove.
(__malloc_fork_lock_parent): Rename from ptmalloc_lock_all.
Update comment.
(__malloc_fork_unlock_parent): Rename from ptmalloc_unlock_all.
(__malloc_fork_unlock_child): Rename from ptmalloc_unlock_all2.
Remove outdated comment.
(ptmalloc_init): Do not call thread_atfork. Remove
thread_atfork_static.
* malloc/tst-malloc-fork-deadlock.c: New file.
* Makefile (tests): Add tst-malloc-fork-deadlock.
(tst-malloc-fork-deadlock): Link against libpthread.
* manual/memory.texi (Aligned Memory Blocks): Update safety
annotation comments.
* sysdeps/nptl/fork.c (__libc_fork): Call
__malloc_fork_lock_parent, __malloc_fork_unlock_parent,
__malloc_fork_unlock_child.
* sysdeps/mach/hurd/fork.c (__fork): Likewise.
2016-04-14 Florian Weimer <fweimer@redhat.com>
[BZ #19613]

3
malloc/Makefile
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@ -29,7 +29,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
tst-malloc-usable tst-realloc tst-posix_memalign \
tst-pvalloc tst-memalign tst-mallopt tst-scratch_buffer \
tst-malloc-backtrace tst-malloc-thread-exit \
tst-malloc-thread-fail
tst-malloc-thread-fail tst-malloc-fork-deadlock
test-srcs = tst-mtrace
routines = malloc morecore mcheck mtrace obstack \
@ -49,6 +49,7 @@ libmemusage-inhibit-o = $(filter-out .os,$(object-suffixes))
$(objpfx)tst-malloc-backtrace: $(shared-thread-library)
$(objpfx)tst-malloc-thread-exit: $(shared-thread-library)
$(objpfx)tst-malloc-thread-fail: $(shared-thread-library)
$(objpfx)tst-malloc-fork-deadlock: $(shared-thread-library)
# These should be removed by `make clean'.
extra-objs = mcheck-init.o libmcheck.a

58
malloc/arena.c
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@ -133,10 +133,6 @@ static void *(*save_malloc_hook)(size_t __size, const void *);
static void (*save_free_hook) (void *__ptr, const void *);
static void *save_arena;
# ifdef ATFORK_MEM
ATFORK_MEM;
# endif
/* Magic value for the thread-specific arena pointer when
malloc_atfork() is in use. */
@ -202,14 +198,14 @@ free_atfork (void *mem, const void *caller)
/* Counter for number of times the list is locked by the same thread. */
static unsigned int atfork_recursive_cntr;
/* The following two functions are registered via thread_atfork() to
make sure that the mutexes remain in a consistent state in the
fork()ed version of a thread. Also adapt the malloc and free hooks
temporarily, because the `atfork' handler mechanism may use
malloc/free internally (e.g. in LinuxThreads). */
/* The following three functions are called around fork from a
multi-threaded process. We do not use the general fork handler
mechanism to make sure that our handlers are the last ones being
called, so that other fork handlers can use the malloc
subsystem. */
static void
ptmalloc_lock_all (void)
void
__malloc_fork_lock_parent (void)
{
mstate ar_ptr;
@ -217,7 +213,7 @@ ptmalloc_lock_all (void)
return;
/* We do not acquire free_list_lock here because we completely
reconstruct free_list in ptmalloc_unlock_all2. */
reconstruct free_list in __malloc_fork_unlock_child. */
if (mutex_trylock (&list_lock))
{
@ -242,7 +238,7 @@ ptmalloc_lock_all (void)
__free_hook = free_atfork;
/* Only the current thread may perform malloc/free calls now.
save_arena will be reattached to the current thread, in
ptmalloc_lock_all, so save_arena->attached_threads is not
__malloc_fork_lock_parent, so save_arena->attached_threads is not
updated. */
save_arena = thread_arena;
thread_arena = ATFORK_ARENA_PTR;
@ -250,8 +246,8 @@ out:
++atfork_recursive_cntr;
}
static void
ptmalloc_unlock_all (void)
void
__malloc_fork_unlock_parent (void)
{
mstate ar_ptr;
@ -262,8 +258,8 @@ ptmalloc_unlock_all (void)
return;
/* Replace ATFORK_ARENA_PTR with save_arena.
save_arena->attached_threads was not changed in ptmalloc_lock_all
and is still correct. */
save_arena->attached_threads was not changed in
__malloc_fork_lock_parent and is still correct. */
thread_arena = save_arena;
__malloc_hook = save_malloc_hook;
__free_hook = save_free_hook;
@ -277,15 +273,8 @@ ptmalloc_unlock_all (void)
(void) mutex_unlock (&list_lock);
}
# ifdef __linux__
/* In NPTL, unlocking a mutex in the child process after a
fork() is currently unsafe, whereas re-initializing it is safe and
does not leak resources. Therefore, a special atfork handler is
installed for the child. */
static void
ptmalloc_unlock_all2 (void)
void
__malloc_fork_unlock_child (void)
{
mstate ar_ptr;
@ -321,11 +310,6 @@ ptmalloc_unlock_all2 (void)
atfork_recursive_cntr = 0;
}
# else
# define ptmalloc_unlock_all2 ptmalloc_unlock_all
# endif
/* Initialization routine. */
#include <string.h>
extern char **_environ;
@ -394,7 +378,6 @@ ptmalloc_init (void)
#endif
thread_arena = &main_arena;
thread_atfork (ptmalloc_lock_all, ptmalloc_unlock_all, ptmalloc_unlock_all2);
const char *s = NULL;
if (__glibc_likely (_environ != NULL))
{
@ -469,14 +452,6 @@ ptmalloc_init (void)
__malloc_initialized = 1;
}
/* There are platforms (e.g. Hurd) with a link-time hook mechanism. */
#ifdef thread_atfork_static
thread_atfork_static (ptmalloc_lock_all, ptmalloc_unlock_all, \
ptmalloc_unlock_all2)
#endif
/* Managing heaps and arenas (for concurrent threads) */
#if MALLOC_DEBUG > 1
@ -822,7 +797,8 @@ _int_new_arena (size_t size)
limit is reached). At this point, some arena has to be attached
to two threads. We could acquire the arena lock before list_lock
to make it less likely that reused_arena picks this new arena,
but this could result in a deadlock with ptmalloc_lock_all. */
but this could result in a deadlock with
__malloc_fork_lock_parent. */
(void) mutex_lock (&a->mutex);

32
malloc/malloc-internal.h Normal file
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@ -0,0 +1,32 @@
/* Internal declarations for malloc, for use within libc.
Copyright (C) 2016 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; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */
#ifndef _MALLOC_PRIVATE_H
#define _MALLOC_PRIVATE_H
/* Called in the parent process before a fork. */
void __malloc_fork_lock_parent (void) internal_function attribute_hidden;
/* Called in the parent process after a fork. */
void __malloc_fork_unlock_parent (void) internal_function attribute_hidden;
/* Called in the child process after a fork. */
void __malloc_fork_unlock_child (void) internal_function attribute_hidden;
#endif /* _MALLOC_PRIVATE_H */

1
malloc/malloc.c
View File

@ -244,6 +244,7 @@
/* For ALIGN_UP et. al. */
#include <libc-internal.h>
#include <malloc/malloc-internal.h>
/*
Debugging:

220
malloc/tst-malloc-fork-deadlock.c Normal file
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@ -0,0 +1,220 @@
/* Test concurrent fork, getline, and fflush (NULL).
Copyright (C) 2016 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; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */
#include <sys/wait.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdlib.h>
#include <malloc.h>
#include <time.h>
#include <string.h>
#include <signal.h>
static int do_test (void);
#define TEST_FUNCTION do_test ()
#include "../test-skeleton.c"
enum {
/* Number of threads which call fork. */
fork_thread_count = 4,
/* Number of threads which call getline (and, indirectly,
malloc). */
read_thread_count = 8,
};
static bool termination_requested;
static void *
fork_thread_function (void *closure)
{
while (!__atomic_load_n (&termination_requested, __ATOMIC_RELAXED))
{
pid_t pid = fork ();
if (pid < 0)
{
printf ("error: fork: %m\n");
abort ();
}
else if (pid == 0)
_exit (17);
int status;
if (waitpid (pid, &status, 0) < 0)
{
printf ("error: waitpid: %m\n");
abort ();
}
if (!WIFEXITED (status) || WEXITSTATUS (status) != 17)
{
printf ("error: waitpid returned invalid status: %d\n", status);
abort ();
}
}
return NULL;
}
static char *file_to_read;
static void *
read_thread_function (void *closure)
{
FILE *f = fopen (file_to_read, "r");
if (f == NULL)
{
printf ("error: fopen (%s): %m\n", file_to_read);
abort ();
}
while (!__atomic_load_n (&termination_requested, __ATOMIC_RELAXED))
{
rewind (f);
char *line = NULL;
size_t line_allocated = 0;
ssize_t ret = getline (&line, &line_allocated, f);
if (ret < 0)
{
printf ("error: getline: %m\n");
abort ();
}
free (line);
}
fclose (f);
return NULL;
}
static void *
flushall_thread_function (void *closure)
{
while (!__atomic_load_n (&termination_requested, __ATOMIC_RELAXED))
if (fflush (NULL) != 0)
{
printf ("error: fflush (NULL): %m\n");
abort ();
}
return NULL;
}
static void
create_threads (pthread_t *threads, size_t count, void *(*func) (void *))
{
for (size_t i = 0; i < count; ++i)
{
int ret = pthread_create (threads + i, NULL, func, NULL);
if (ret != 0)
{
errno = ret;
printf ("error: pthread_create: %m\n");
abort ();
}
}
}
static void
join_threads (pthread_t *threads, size_t count)
{
for (size_t i = 0; i < count; ++i)
{
int ret = pthread_join (threads[i], NULL);
if (ret != 0)
{
errno = ret;
printf ("error: pthread_join: %m\n");
abort ();
}
}
}
/* Create a file which consists of a single long line, and assigns
file_to_read. The hope is that this triggers an allocation in
getline which needs a lock. */
static void
create_file_with_large_line (void)
{
int fd = create_temp_file ("bug19431-large-line", &file_to_read);
if (fd < 0)
{
printf ("error: create_temp_file: %m\n");
abort ();
}
FILE *f = fdopen (fd, "w+");
if (f == NULL)
{
printf ("error: fdopen: %m\n");
abort ();
}
for (int i = 0; i < 50000; ++i)
fputc ('x', f);
fputc ('\n', f);
if (ferror (f))
{
printf ("error: fputc: %m\n");
abort ();
}
if (fclose (f) != 0)
{
printf ("error: fclose: %m\n");
abort ();
}
}
static int
do_test (void)
{
/* Make sure that we do not exceed the arena limit with the number
of threads we configured. */
if (mallopt (M_ARENA_MAX, 400) == 0)
{
printf ("error: mallopt (M_ARENA_MAX) failed\n");
return 1;
}
/* Leave some room for shutting down all threads gracefully. */
int timeout = 3;
if (timeout > TIMEOUT)
timeout = TIMEOUT - 1;
create_file_with_large_line ();
pthread_t fork_threads[fork_thread_count];
create_threads (fork_threads, fork_thread_count, fork_thread_function);
pthread_t read_threads[read_thread_count];
create_threads (read_threads, read_thread_count, read_thread_function);
pthread_t flushall_threads[1];
create_threads (flushall_threads, 1, flushall_thread_function);
struct timespec ts = {timeout, 0};
if (nanosleep (&ts, NULL))
{
printf ("error: error: nanosleep: %m\n");
abort ();
}
__atomic_store_n (&termination_requested, true, __ATOMIC_RELAXED);
join_threads (flushall_threads, 1);
join_threads (read_threads, read_thread_count);
join_threads (fork_threads, fork_thread_count);
free (file_to_read);
return 0;
}

8
manual/memory.texi
View File

@ -1051,14 +1051,6 @@ systems that do not support @w{ISO C11}.
@c _dl_addr_inside_object ok
@c determine_info ok
@c __rtld_lock_unlock_recursive (dl_load_lock) @aculock
@c thread_atfork @asulock @aculock @acsfd @acsmem
@c __register_atfork @asulock @aculock @acsfd @acsmem
@c lll_lock (__fork_lock) @asulock @aculock
@c fork_handler_alloc @asulock @aculock @acsfd @acsmem
@c calloc dup @asulock @aculock @acsfd @acsmem
@c __linkin_atfork ok
@c catomic_compare_and_exchange_bool_acq ok
@c lll_unlock (__fork_lock) @aculock
@c *_environ @mtsenv
@c next_env_entry ok
@c strcspn dup ok

13
sysdeps/mach/hurd/fork.c
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@ -26,6 +26,7 @@
#include <assert.h>
#include "hurdmalloc.h" /* XXX */
#include <tls.h>
#include <malloc/malloc-internal.h>
#undef __fork
@ -107,6 +108,12 @@ __fork (void)
/* Run things that prepare for forking before we create the task. */
RUN_HOOK (_hurd_fork_prepare_hook, ());
/* Acquire malloc locks. This needs to come last because fork
handlers may use malloc, and the libio list lock has an
indirect malloc dependency as well (via the getdelim
function). */
__malloc_fork_lock_parent ();
/* Lock things that want to be locked before we fork. */
{
void *const *p;
@ -604,6 +611,9 @@ __fork (void)
nthreads * sizeof (*threads));
}
/* Release malloc locks. */
__malloc_fork_unlock_parent ();
/* Run things that want to run in the parent to restore it to
normality. Usually prepare hooks and parent hooks are
symmetrical: the prepare hook arrests state in some way for the
@ -655,6 +665,9 @@ __fork (void)
/* Forking clears the trace flag. */
__sigemptyset (&_hurdsig_traced);
/* Release malloc locks. */
__malloc_fork_unlock_child ();
/* Run things that want to run in the child task to set up. */
RUN_HOOK (_hurd_fork_child_hook, ());

13
sysdeps/nptl/fork.c
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@ -31,7 +31,7 @@
#include <fork.h>
#include <arch-fork.h>
#include <futex-internal.h>
#include <malloc/malloc-internal.h>
static void
fresetlockfiles (void)
@ -111,6 +111,11 @@ __libc_fork (void)
_IO_list_lock ();
/* Acquire malloc locks. This needs to come last because fork
handlers may use malloc, and the libio list lock has an indirect
malloc dependency as well (via the getdelim function). */
__malloc_fork_lock_parent ();
#ifndef NDEBUG
pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
#endif
@ -168,6 +173,9 @@ __libc_fork (void)
# endif
#endif
/* Release malloc locks. */
__malloc_fork_unlock_child ();
/* Reset the file list. These are recursive mutexes. */
fresetlockfiles ();
@ -209,6 +217,9 @@ __libc_fork (void)
/* Restore the PID value. */
THREAD_SETMEM (THREAD_SELF, pid, parentpid);
/* Release malloc locks, parent process variant. */
__malloc_fork_unlock_parent ();
/* We execute this even if the 'fork' call failed. */
_IO_list_unlock ();