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malloc: Fix list_lock/arena lock deadlock [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.
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ChangeLog
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ChangeLog
@ -1,3 +1,22 @@
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2015-12-21 Florian Weimer <fweimer@redhat.com>
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[BZ #19182]
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* malloc/arena.c (list_lock): Document lock ordering requirements.
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(free_list_lock): New lock.
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(ptmalloc_lock_all): Comment on free_list_lock.
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(ptmalloc_unlock_all2): Reinitialize free_list_lock.
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(detach_arena): Update comment. free_list_lock is now needed.
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(_int_new_arena): Use free_list_lock around detach_arena call.
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Acquire arena lock after list_lock. Add comment, including FIXME
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about incorrect synchronization.
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(get_free_list): Switch to free_list_lock.
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(reused_arena): Acquire free_list_lock around detach_arena call
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and attached threads counter update. Add two FIXMEs about
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incorrect synchronization.
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(arena_thread_freeres): Switch to free_list_lock.
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* malloc/malloc.c (struct malloc_state): Update comments to
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mention free_list_lock.
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2015-12-21 Siddhesh Poyarekar <siddhesh.poyarekar@linaro.org>
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* sysdeps/ieee754/dbl-64/s_sin.c (csloww, csloww1, csloww2):
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@ -68,15 +68,28 @@ extern int sanity_check_heap_info_alignment[(sizeof (heap_info)
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static __thread mstate thread_arena attribute_tls_model_ie;
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/* Arena free list. list_lock protects the free_list variable below,
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and the next_free and attached_threads members of the mstate
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objects. No other (malloc) locks must be taken while list_lock is
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active, otherwise deadlocks may occur. */
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/* Arena free list. free_list_lock synchronizes access to the
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free_list variable below, and the next_free and attached_threads
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members of struct malloc_state objects. No other locks must be
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acquired after free_list_lock has been acquired. */
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static mutex_t list_lock = _LIBC_LOCK_INITIALIZER;
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static mutex_t free_list_lock = _LIBC_LOCK_INITIALIZER;
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static size_t narenas = 1;
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static mstate free_list;
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/* list_lock prevents concurrent writes to the next member of struct
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malloc_state objects.
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Read access to the next member is supposed to synchronize with the
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atomic_write_barrier and the write to the next member in
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_int_new_arena. This suffers from data races; see the FIXME
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comments in _int_new_arena and reused_arena.
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list_lock also prevents concurrent forks. When list_lock is
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acquired, no arena lock must be acquired, but it is permitted to
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acquire arena locks after list_lock. */
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static mutex_t list_lock = _LIBC_LOCK_INITIALIZER;
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/* Mapped memory in non-main arenas (reliable only for NO_THREADS). */
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static unsigned long arena_mem;
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@ -207,6 +220,9 @@ ptmalloc_lock_all (void)
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if (__malloc_initialized < 1)
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return;
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/* We do not acquire free_list_lock here because we completely
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reconstruct free_list in ptmalloc_unlock_all2. */
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if (mutex_trylock (&list_lock))
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{
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if (thread_arena == ATFORK_ARENA_PTR)
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@ -286,6 +302,7 @@ ptmalloc_unlock_all2 (void)
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/* Push all arenas to the free list, except save_arena, which is
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attached to the current thread. */
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mutex_init (&free_list_lock);
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if (save_arena != NULL)
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((mstate) save_arena)->attached_threads = 1;
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free_list = NULL;
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@ -303,6 +320,7 @@ ptmalloc_unlock_all2 (void)
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if (ar_ptr == &main_arena)
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break;
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}
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mutex_init (&list_lock);
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atfork_recursive_cntr = 0;
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}
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@ -735,7 +753,7 @@ heap_trim (heap_info *heap, size_t pad)
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/* Create a new arena with initial size "size". */
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/* If REPLACED_ARENA is not NULL, detach it from this thread. Must be
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called while list_lock is held. */
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called while free_list_lock is held. */
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static void
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detach_arena (mstate replaced_arena)
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{
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@ -788,19 +806,34 @@ _int_new_arena (size_t size)
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mstate replaced_arena = thread_arena;
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thread_arena = a;
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mutex_init (&a->mutex);
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(void) mutex_lock (&a->mutex);
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(void) mutex_lock (&list_lock);
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detach_arena (replaced_arena);
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/* Add the new arena to the global list. */
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a->next = main_arena.next;
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/* FIXME: The barrier is an attempt to synchronize with read access
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in reused_arena, which does not acquire list_lock while
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traversing the list. */
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atomic_write_barrier ();
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main_arena.next = a;
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(void) mutex_unlock (&list_lock);
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(void) mutex_lock (&free_list_lock);
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detach_arena (replaced_arena);
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(void) mutex_unlock (&free_list_lock);
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/* Lock this arena. NB: Another thread may have been attached to
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this arena because the arena is now accessible from the
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main_arena.next list and could have been picked by reused_arena.
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This can only happen for the last arena created (before the arena
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limit is reached). At this point, some arena has to be attached
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to two threads. We could acquire the arena lock before list_lock
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to make it less likely that reused_arena picks this new arena,
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but this could result in a deadlock with ptmalloc_lock_all. */
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(void) mutex_lock (&a->mutex);
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return a;
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}
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@ -814,7 +847,7 @@ get_free_list (void)
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mstate result = free_list;
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if (result != NULL)
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{
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(void) mutex_lock (&list_lock);
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(void) mutex_lock (&free_list_lock);
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result = free_list;
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if (result != NULL)
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{
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@ -825,7 +858,7 @@ get_free_list (void)
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detach_arena (replaced_arena);
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}
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(void) mutex_unlock (&list_lock);
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(void) mutex_unlock (&free_list_lock);
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if (result != NULL)
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{
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@ -845,6 +878,7 @@ static mstate
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reused_arena (mstate avoid_arena)
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{
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mstate result;
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/* FIXME: Access to next_to_use suffers from data races. */
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static mstate next_to_use;
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if (next_to_use == NULL)
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next_to_use = &main_arena;
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@ -857,6 +891,7 @@ reused_arena (mstate avoid_arena)
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if (!arena_is_corrupt (result) && !mutex_trylock (&result->mutex))
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goto out;
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/* FIXME: This is a data race, see _int_new_arena. */
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result = result->next;
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}
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while (result != next_to_use);
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@ -888,11 +923,12 @@ out:
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/* Attach the arena to the current thread. Note that we may have
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selected an arena which was on free_list. */
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{
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/* Update the arena thread attachment counters. */
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mstate replaced_arena = thread_arena;
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(void) mutex_lock (&list_lock);
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(void) mutex_lock (&free_list_lock);
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detach_arena (replaced_arena);
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++result->attached_threads;
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(void) mutex_unlock (&list_lock);
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(void) mutex_unlock (&free_list_lock);
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}
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LIBC_PROBE (memory_arena_reuse, 2, result, avoid_arena);
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@ -988,7 +1024,7 @@ arena_thread_freeres (void)
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if (a != NULL)
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{
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(void) mutex_lock (&list_lock);
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(void) mutex_lock (&free_list_lock);
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/* If this was the last attached thread for this arena, put the
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arena on the free list. */
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assert (a->attached_threads > 0);
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@ -997,7 +1033,7 @@ arena_thread_freeres (void)
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a->next_free = free_list;
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free_list = a;
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}
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(void) mutex_unlock (&list_lock);
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(void) mutex_unlock (&free_list_lock);
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}
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}
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text_set_element (__libc_thread_subfreeres, arena_thread_freeres);
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@ -1710,12 +1710,12 @@ struct malloc_state
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struct malloc_state *next;
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/* Linked list for free arenas. Access to this field is serialized
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by list_lock in arena.c. */
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by free_list_lock in arena.c. */
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struct malloc_state *next_free;
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/* Number of threads attached to this arena. 0 if the arena is on
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the free list. Access to this field is serialized by list_lock
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in arena.c. */
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the free list. Access to this field is serialized by
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free_list_lock in arena.c. */
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INTERNAL_SIZE_T attached_threads;
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/* Memory allocated from the system in this arena. */
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