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glibc.6399
malloc-Fix-list_lock-arena-lock-deadlock-BZ-191...
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File malloc-Fix-list_lock-arena-lock-deadlock-BZ-19182.patch of Package glibc.6399
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-2.19/malloc/arena.c =================================================================== --- glibc-2.19.orig/malloc/arena.c +++ glibc-2.19/malloc/arena.c @@ -75,10 +75,23 @@ 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; + #if THREAD_STATS static int stat_n_heaps; # define THREAD_STAT(x) x @@ -225,6 +238,9 @@ 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; @@ -300,6 +316,7 @@ 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; @@ -317,6 +334,7 @@ ptmalloc_unlock_all2 (void) if (ar_ptr == &main_arena) break; } + mutex_init (&list_lock); atfork_recursive_cntr = 0; } @@ -751,7 +769,7 @@ 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) { @@ -805,19 +823,34 @@ _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); + THREAD_STAT (++(a->stat_lock_loop)); return a; @@ -834,7 +867,7 @@ 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) { @@ -845,7 +878,7 @@ get_free_list (void) detach_arena (replaced_arena); } - (void) mutex_unlock (&list_lock); + (void) mutex_unlock (&free_list_lock); if (result != NULL) { @@ -866,6 +899,7 @@ 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; @@ -878,6 +912,7 @@ 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); @@ -909,12 +944,13 @@ 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); @@ -1010,7 +1046,7 @@ 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); @@ -1019,7 +1055,7 @@ 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-2.19/malloc/malloc.c =================================================================== --- glibc-2.19.orig/malloc/malloc.c +++ glibc-2.19/malloc/malloc.c @@ -1707,12 +1707,12 @@ 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. */
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