File malloc-Fix-list_lock-arena-lock-deadlock-BZ-19182.patch of Package glibc.6721
2015-12-21 Florian Weimer <fweimer@redhat.com>
[BZ #19182]
* malloc/arena.c (list_lock): Document lock ordering requirements.
(free_list_lock): New lock.
(ptmalloc_lock_all): Comment on free_list_lock.
(ptmalloc_unlock_all2): Reinitialize free_list_lock.
(detach_arena): Update comment. free_list_lock is now needed.
(_int_new_arena): Use free_list_lock around detach_arena call.
Acquire arena lock after list_lock. Add comment, including FIXME
about incorrect synchronization.
(get_free_list): Switch to free_list_lock.
(reused_arena): Acquire free_list_lock around detach_arena call
and attached threads counter update. Add two FIXMEs about
incorrect synchronization.
(arena_thread_freeres): Switch to free_list_lock.
* malloc/malloc.c (struct malloc_state): Update comments to
mention free_list_lock.
Index: glibc-2.22/malloc/arena.c
===================================================================
--- glibc-2.22.orig/malloc/arena.c
+++ glibc-2.22/malloc/arena.c
@@ -67,10 +67,23 @@ extern int sanity_check_heap_info_alignm
/* Thread specific data */
static tsd_key_t arena_key;
-static mutex_t list_lock = MUTEX_INITIALIZER;
+static mutex_t free_list_lock = MUTEX_INITIALIZER;
static size_t narenas = 1;
static mstate free_list;
+/* list_lock prevents concurrent writes to the next member of struct
+ malloc_state objects.
+
+ Read access to the next member is supposed to synchronize with the
+ atomic_write_barrier and the write to the next member in
+ _int_new_arena. This suffers from data races; see the FIXME
+ comments in _int_new_arena and reused_arena.
+
+ list_lock also prevents concurrent forks. When list_lock is
+ acquired, no arena lock must be acquired, but it is permitted to
+ acquire arena locks after list_lock. */
+static mutex_t list_lock = MUTEX_INITIALIZER;
+
/* Mapped memory in non-main arenas (reliable only for NO_THREADS). */
static unsigned long arena_mem;
@@ -210,6 +223,9 @@ ptmalloc_lock_all (void)
if (__malloc_initialized < 1)
return;
+ /* We do not acquire free_list_lock here because we completely
+ reconstruct free_list in ptmalloc_unlock_all2. */
+
if (mutex_trylock (&list_lock))
{
void *my_arena;
@@ -285,6 +301,7 @@ ptmalloc_unlock_all2 (void)
/* Push all arenas to the free list, except save_arena, which is
attached to the current thread. */
+ mutex_init (&free_list_lock);
if (save_arena != NULL)
((mstate) save_arena)->attached_threads = 1;
free_list = NULL;
@@ -302,6 +319,7 @@ ptmalloc_unlock_all2 (void)
if (ar_ptr == &main_arena)
break;
}
+
mutex_init (&list_lock);
atfork_recursive_cntr = 0;
}
@@ -735,7 +753,7 @@ heap_trim (heap_info *heap, size_t pad)
/* Create a new arena with initial size "size". */
/* If REPLACED_ARENA is not NULL, detach it from this thread. Must be
- called while list_lock is held. */
+ called while free_list_lock is held. */
static void
detach_arena (mstate replaced_arena)
{
@@ -789,19 +807,34 @@ _int_new_arena (size_t size)
arena_lookup (replaced_arena);
tsd_setspecific (arena_key, (void *) a);
mutex_init (&a->mutex);
- (void) mutex_lock (&a->mutex);
(void) mutex_lock (&list_lock);
- detach_arena (replaced_arena);
-
/* Add the new arena to the global list. */
a->next = main_arena.next;
+ /* FIXME: The barrier is an attempt to synchronize with read access
+ in reused_arena, which does not acquire list_lock while
+ traversing the list. */
atomic_write_barrier ();
main_arena.next = a;
(void) mutex_unlock (&list_lock);
+ (void) mutex_lock (&free_list_lock);
+ detach_arena (replaced_arena);
+ (void) mutex_unlock (&free_list_lock);
+
+ /* Lock this arena. NB: Another thread may have been attached to
+ this arena because the arena is now accessible from the
+ main_arena.next list and could have been picked by reused_arena.
+ This can only happen for the last arena created (before the arena
+ 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. */
+
+ (void) mutex_lock (&a->mutex);
+
return a;
}
@@ -816,7 +849,7 @@ get_free_list (void)
mstate result = free_list;
if (result != NULL)
{
- (void) mutex_lock (&list_lock);
+ (void) mutex_lock (&free_list_lock);
result = free_list;
if (result != NULL)
{
@@ -827,7 +860,7 @@ get_free_list (void)
detach_arena (replaced_arena);
}
- (void) mutex_unlock (&list_lock);
+ (void) mutex_unlock (&free_list_lock);
if (result != NULL)
{
@@ -847,6 +880,7 @@ static mstate
reused_arena (mstate avoid_arena)
{
mstate result;
+ /* FIXME: Access to next_to_use suffers from data races. */
static mstate next_to_use;
if (next_to_use == NULL)
next_to_use = &main_arena;
@@ -859,6 +893,7 @@ reused_arena (mstate avoid_arena)
if (!arena_is_corrupt (result) && !mutex_trylock (&result->mutex))
goto out;
+ /* FIXME: This is a data race, see _int_new_arena. */
result = result->next;
}
while (result != next_to_use);
@@ -890,12 +925,13 @@ out:
/* Attach the arena to the current thread. Note that we may have
selected an arena which was on free_list. */
{
+ /* Update the arena thread attachment counters. */
mstate replaced_arena;
arena_lookup (replaced_arena);
- (void) mutex_lock (&list_lock);
+ (void) mutex_lock (&free_list_lock);
detach_arena (replaced_arena);
++result->attached_threads;
- (void) mutex_unlock (&list_lock);
+ (void) mutex_unlock (&free_list_lock);
}
LIBC_PROBE (memory_arena_reuse, 2, result, avoid_arena);
@@ -990,7 +1026,7 @@ arena_thread_freeres (void)
if (a != NULL)
{
- (void) mutex_lock (&list_lock);
+ (void) mutex_lock (&free_list_lock);
/* If this was the last attached thread for this arena, put the
arena on the free list. */
assert (a->attached_threads > 0);
@@ -999,7 +1035,7 @@ arena_thread_freeres (void)
a->next_free = free_list;
free_list = a;
}
- (void) mutex_unlock (&list_lock);
+ (void) mutex_unlock (&free_list_lock);
}
}
text_set_element (__libc_thread_subfreeres, arena_thread_freeres);
Index: glibc-2.22/malloc/malloc.c
===================================================================
--- glibc-2.22.orig/malloc/malloc.c
+++ glibc-2.22/malloc/malloc.c
@@ -1710,12 +1710,12 @@ struct malloc_state
struct malloc_state *next;
/* Linked list for free arenas. Access to this field is serialized
- by list_lock in arena.c. */
+ by free_list_lock in arena.c. */
struct malloc_state *next_free;
/* Number of threads attached to this arena. 0 if the arena is on
- the free list. Access to this field is serialized by list_lock
- in arena.c. */
+ the free list. Access to this field is serialized by
+ free_list_lock in arena.c. */
INTERNAL_SIZE_T attached_threads;
/* Memory allocated from the system in this arena. */
