glibc/malloc/arena.c

/* Malloc implementation for multiple threads without lock contention.
   Copyright (C) 2001-2015 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Wolfram Gloger <wg@malloc.de>, 2001.

   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 <stdbool.h>

/* Compile-time constants.  */

#define HEAP_MIN_SIZE (32 * 1024)
#ifndef HEAP_MAX_SIZE
# ifdef DEFAULT_MMAP_THRESHOLD_MAX
#  define HEAP_MAX_SIZE (2 * DEFAULT_MMAP_THRESHOLD_MAX)
# else
#  define HEAP_MAX_SIZE (1024 * 1024) /* must be a power of two */
# endif
#endif

/* HEAP_MIN_SIZE and HEAP_MAX_SIZE limit the size of mmap()ed heaps
   that are dynamically created for multi-threaded programs.  The
   maximum size must be a power of two, for fast determination of
   which heap belongs to a chunk.  It should be much larger than the
   mmap threshold, so that requests with a size just below that
   threshold can be fulfilled without creating too many heaps.  */

/***************************************************************************/

#define top(ar_ptr) ((ar_ptr)->top)

/* A heap is a single contiguous memory region holding (coalesceable)
   malloc_chunks.  It is allocated with mmap() and always starts at an
   address aligned to HEAP_MAX_SIZE.  */

typedef struct _heap_info
{
  mstate ar_ptr; /* Arena for this heap. */
  struct _heap_info *prev; /* Previous heap. */
  size_t size;   /* Current size in bytes. */
  size_t mprotect_size; /* Size in bytes that has been mprotected
                           PROT_READ|PROT_WRITE.  */
  /* Make sure the following data is properly aligned, particularly
     that sizeof (heap_info) + 2 * SIZE_SZ is a multiple of
     MALLOC_ALIGNMENT. */
  char pad[-6 * SIZE_SZ & MALLOC_ALIGN_MASK];
} heap_info;

/* Get a compile-time error if the heap_info padding is not correct
   to make alignment work as expected in sYSMALLOc.  */
extern int sanity_check_heap_info_alignment[(sizeof (heap_info)
                                             + 2 * SIZE_SZ) % MALLOC_ALIGNMENT
                                            ? -1 : 1];

/* Thread specific data */

static tsd_key_t arena_key;
static mutex_t list_lock = MUTEX_INITIALIZER;
static size_t narenas = 1;
static mstate free_list;

/* Mapped memory in non-main arenas (reliable only for NO_THREADS). */
static unsigned long arena_mem;

/* Already initialized? */
int __malloc_initialized = -1;

/**************************************************************************/


/* arena_get() acquires an arena and locks the corresponding mutex.
   First, try the one last locked successfully by this thread.  (This
   is the common case and handled with a macro for speed.)  Then, loop
   once over the circularly linked list of arenas.  If no arena is
   readily available, create a new one.  In this latter case, `size'
   is just a hint as to how much memory will be required immediately
   in the new arena. */

#define arena_get(ptr, size) do { \
      arena_lookup (ptr);						      \
      arena_lock (ptr, size);						      \
  } while (0)

#define arena_lookup(ptr) do { \
      void *vptr = NULL;						      \
      ptr = (mstate) tsd_getspecific (arena_key, vptr);			      \
  } while (0)

#define arena_lock(ptr, size) do {					      \
      if (ptr && !arena_is_corrupt (ptr))				      \
        (void) mutex_lock (&ptr->mutex);				      \
      else								      \
        ptr = arena_get2 (ptr, (size), NULL);				      \
  } while (0)

/* find the heap and corresponding arena for a given ptr */

#define heap_for_ptr(ptr) \
  ((heap_info *) ((unsigned long) (ptr) & ~(HEAP_MAX_SIZE - 1)))
#define arena_for_chunk(ptr) \
  (chunk_non_main_arena (ptr) ? heap_for_ptr (ptr)->ar_ptr : &main_arena)


/**************************************************************************/

#ifndef NO_THREADS

/* atfork support.  */

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.  */

# define ATFORK_ARENA_PTR ((void *) -1)

/* The following hooks are used while the `atfork' handling mechanism
   is active. */

static void *
malloc_atfork (size_t sz, const void *caller)
{
  void *vptr = NULL;
  void *victim;

  tsd_getspecific (arena_key, vptr);
  if (vptr == ATFORK_ARENA_PTR)
    {
      /* We are the only thread that may allocate at all.  */
      if (save_malloc_hook != malloc_check)
        {
          return _int_malloc (&main_arena, sz);
        }
      else
        {
          if (top_check () < 0)
            return 0;

          victim = _int_malloc (&main_arena, sz + 1);
          return mem2mem_check (victim, sz);
        }
    }
  else
    {
      /* Suspend the thread until the `atfork' handlers have completed.
         By that time, the hooks will have been reset as well, so that
         mALLOc() can be used again. */
      (void) mutex_lock (&list_lock);
      (void) mutex_unlock (&list_lock);
      return __libc_malloc (sz);
    }
}

static void
free_atfork (void *mem, const void *caller)
{
  void *vptr = NULL;
  mstate ar_ptr;
  mchunkptr p;                          /* chunk corresponding to mem */

  if (mem == 0)                              /* free(0) has no effect */
    return;

  p = mem2chunk (mem);         /* do not bother to replicate free_check here */

  if (chunk_is_mmapped (p))                       /* release mmapped memory. */
    {
      munmap_chunk (p);
      return;
    }

  ar_ptr = arena_for_chunk (p);
  tsd_getspecific (arena_key, vptr);
  _int_free (ar_ptr, p, vptr == ATFORK_ARENA_PTR);
}


/* 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). */

static void
ptmalloc_lock_all (void)
{
  mstate ar_ptr;

  if (__malloc_initialized < 1)
    return;

  if (mutex_trylock (&list_lock))
    {
      void *my_arena;
      tsd_getspecific (arena_key, my_arena);
      if (my_arena == ATFORK_ARENA_PTR)
        /* This is the same thread which already locks the global list.
           Just bump the counter.  */
        goto out;

      /* This thread has to wait its turn.  */
      (void) mutex_lock (&list_lock);
    }
  for (ar_ptr = &main_arena;; )
    {
      (void) mutex_lock (&ar_ptr->mutex);
      ar_ptr = ar_ptr->next;
      if (ar_ptr == &main_arena)
        break;
    }
  save_malloc_hook = __malloc_hook;
  save_free_hook = __free_hook;
  __malloc_hook = malloc_atfork;
  __free_hook = free_atfork;
  /* Only the current thread may perform malloc/free calls now. */
  tsd_getspecific (arena_key, save_arena);
  tsd_setspecific (arena_key, ATFORK_ARENA_PTR);
out:
  ++atfork_recursive_cntr;
}

static void
ptmalloc_unlock_all (void)
{
  mstate ar_ptr;

  if (__malloc_initialized < 1)
    return;

  if (--atfork_recursive_cntr != 0)
    return;

  tsd_setspecific (arena_key, save_arena);
  __malloc_hook = save_malloc_hook;
  __free_hook = save_free_hook;
  for (ar_ptr = &main_arena;; )
    {
      (void) mutex_unlock (&ar_ptr->mutex);
      ar_ptr = ar_ptr->next;
      if (ar_ptr == &main_arena)
        break;
    }
  (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)
{
  mstate ar_ptr;

  if (__malloc_initialized < 1)
    return;

  tsd_setspecific (arena_key, save_arena);
  __malloc_hook = save_malloc_hook;
  __free_hook = save_free_hook;
  free_list = NULL;
  for (ar_ptr = &main_arena;; )
    {
      mutex_init (&ar_ptr->mutex);
      if (ar_ptr != save_arena)
        {
          ar_ptr->next_free = free_list;
          free_list = ar_ptr;
        }
      ar_ptr = ar_ptr->next;
      if (ar_ptr == &main_arena)
        break;
    }
  mutex_init (&list_lock);
  atfork_recursive_cntr = 0;
}

# else

#  define ptmalloc_unlock_all2 ptmalloc_unlock_all
# endif
#endif  /* !NO_THREADS */

/* Initialization routine. */
#include <string.h>
extern char **_environ;

static char *
internal_function
next_env_entry (char ***position)
{
  char **current = *position;
  char *result = NULL;

  while (*current != NULL)
    {
      if (__builtin_expect ((*current)[0] == 'M', 0)
          && (*current)[1] == 'A'
          && (*current)[2] == 'L'
          && (*current)[3] == 'L'
          && (*current)[4] == 'O'
          && (*current)[5] == 'C'
          && (*current)[6] == '_')
        {
          result = &(*current)[7];

          /* Save current position for next visit.  */
          *position = ++current;

          break;
        }

      ++current;
    }

  return result;
}


#ifdef SHARED
static void *
__failing_morecore (ptrdiff_t d)
{
  return (void *) MORECORE_FAILURE;
}

extern struct dl_open_hook *_dl_open_hook;
libc_hidden_proto (_dl_open_hook);
#endif

static void
ptmalloc_init (void)
{
  if (__malloc_initialized >= 0)
    return;

  __malloc_initialized = 0;

#ifdef SHARED
  /* In case this libc copy is in a non-default namespace, never use brk.
     Likewise if dlopened from statically linked program.  */
  Dl_info di;
  struct link_map *l;

  if (_dl_open_hook != NULL
      || (_dl_addr (ptmalloc_init, &di, &l, NULL) != 0
          && l->l_ns != LM_ID_BASE))
    __morecore = __failing_morecore;
#endif

  tsd_key_create (&arena_key, NULL);
  tsd_setspecific (arena_key, (void *) &main_arena);
  thread_atfork (ptmalloc_lock_all, ptmalloc_unlock_all, ptmalloc_unlock_all2);
  const char *s = NULL;
  if (__glibc_likely (_environ != NULL))
    {
      char **runp = _environ;
      char *envline;

      while (__builtin_expect ((envline = next_env_entry (&runp)) != NULL,
                               0))
        {
          size_t len = strcspn (envline, "=");

          if (envline[len] != '=')
            /* This is a "MALLOC_" variable at the end of the string
               without a '=' character.  Ignore it since otherwise we
               will access invalid memory below.  */
            continue;

          switch (len)
            {
            case 6:
              if (memcmp (envline, "CHECK_", 6) == 0)
                s = &envline[7];
              break;
            case 8:
              if (!__builtin_expect (__libc_enable_secure, 0))
                {
                  if (memcmp (envline, "TOP_PAD_", 8) == 0)
                    __libc_mallopt (M_TOP_PAD, atoi (&envline[9]));
                  else if (memcmp (envline, "PERTURB_", 8) == 0)
                    __libc_mallopt (M_PERTURB, atoi (&envline[9]));
                }
              break;
            case 9:
              if (!__builtin_expect (__libc_enable_secure, 0))
                {
                  if (memcmp (envline, "MMAP_MAX_", 9) == 0)
                    __libc_mallopt (M_MMAP_MAX, atoi (&envline[10]));
                  else if (memcmp (envline, "ARENA_MAX", 9) == 0)
                    __libc_mallopt (M_ARENA_MAX, atoi (&envline[10]));
                }
              break;
            case 10:
              if (!__builtin_expect (__libc_enable_secure, 0))
                {
                  if (memcmp (envline, "ARENA_TEST", 10) == 0)
                    __libc_mallopt (M_ARENA_TEST, atoi (&envline[11]));
                }
              break;
            case 15:
              if (!__builtin_expect (__libc_enable_secure, 0))
                {
                  if (memcmp (envline, "TRIM_THRESHOLD_", 15) == 0)
                    __libc_mallopt (M_TRIM_THRESHOLD, atoi (&envline[16]));
                  else if (memcmp (envline, "MMAP_THRESHOLD_", 15) == 0)
                    __libc_mallopt (M_MMAP_THRESHOLD, atoi (&envline[16]));
                }
              break;
            default:
              break;
            }
        }
    }
  if (s && s[0])
    {
      __libc_mallopt (M_CHECK_ACTION, (int) (s[0] - '0'));
      if (check_action != 0)
        __malloc_check_init ();
    }
  void (*hook) (void) = atomic_forced_read (__malloc_initialize_hook);
  if (hook != NULL)
    (*hook)();
  __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

/* Print the complete contents of a single heap to stderr. */

static void
dump_heap (heap_info *heap)
{
  char *ptr;
  mchunkptr p;

  fprintf (stderr, "Heap %p, size %10lx:\n", heap, (long) heap->size);
  ptr = (heap->ar_ptr != (mstate) (heap + 1)) ?
        (char *) (heap + 1) : (char *) (heap + 1) + sizeof (struct malloc_state);
  p = (mchunkptr) (((unsigned long) ptr + MALLOC_ALIGN_MASK) &
                   ~MALLOC_ALIGN_MASK);
  for (;; )
    {
      fprintf (stderr, "chunk %p size %10lx", p, (long) p->size);
      if (p == top (heap->ar_ptr))
        {
          fprintf (stderr, " (top)\n");
          break;
        }
      else if (p->size == (0 | PREV_INUSE))
        {
          fprintf (stderr, " (fence)\n");
          break;
        }
      fprintf (stderr, "\n");
      p = next_chunk (p);
    }
}
#endif /* MALLOC_DEBUG > 1 */

/* If consecutive mmap (0, HEAP_MAX_SIZE << 1, ...) calls return decreasing
   addresses as opposed to increasing, new_heap would badly fragment the
   address space.  In that case remember the second HEAP_MAX_SIZE part
   aligned to HEAP_MAX_SIZE from last mmap (0, HEAP_MAX_SIZE << 1, ...)
   call (if it is already aligned) and try to reuse it next time.  We need
   no locking for it, as kernel ensures the atomicity for us - worst case
   we'll call mmap (addr, HEAP_MAX_SIZE, ...) for some value of addr in
   multiple threads, but only one will succeed.  */
static char *aligned_heap_area;

/* Create a new heap.  size is automatically rounded up to a multiple
   of the page size. */

static heap_info *
internal_function
new_heap (size_t size, size_t top_pad)
{
  size_t pagesize = GLRO (dl_pagesize);
  char *p1, *p2;
  unsigned long ul;
  heap_info *h;

  if (size + top_pad < HEAP_MIN_SIZE)
    size = HEAP_MIN_SIZE;
  else if (size + top_pad <= HEAP_MAX_SIZE)
    size += top_pad;
  else if (size > HEAP_MAX_SIZE)
    return 0;
  else
    size = HEAP_MAX_SIZE;
  size = ALIGN_UP (size, pagesize);

  /* A memory region aligned to a multiple of HEAP_MAX_SIZE is needed.
     No swap space needs to be reserved for the following large
     mapping (on Linux, this is the case for all non-writable mappings
     anyway). */
  p2 = MAP_FAILED;
  if (aligned_heap_area)
    {
      p2 = (char *) MMAP (aligned_heap_area, HEAP_MAX_SIZE, PROT_NONE,
                          MAP_NORESERVE);
      aligned_heap_area = NULL;
      if (p2 != MAP_FAILED && ((unsigned long) p2 & (HEAP_MAX_SIZE - 1)))
        {
          __munmap (p2, HEAP_MAX_SIZE);
          p2 = MAP_FAILED;
        }
    }
  if (p2 == MAP_FAILED)
    {
      p1 = (char *) MMAP (0, HEAP_MAX_SIZE << 1, PROT_NONE, MAP_NORESERVE);
      if (p1 != MAP_FAILED)
        {
          p2 = (char *) (((unsigned long) p1 + (HEAP_MAX_SIZE - 1))
                         & ~(HEAP_MAX_SIZE - 1));
          ul = p2 - p1;
          if (ul)
            __munmap (p1, ul);
          else
            aligned_heap_area = p2 + HEAP_MAX_SIZE;
          __munmap (p2 + HEAP_MAX_SIZE, HEAP_MAX_SIZE - ul);
        }
      else
        {
          /* Try to take the chance that an allocation of only HEAP_MAX_SIZE
             is already aligned. */
          p2 = (char *) MMAP (0, HEAP_MAX_SIZE, PROT_NONE, MAP_NORESERVE);
          if (p2 == MAP_FAILED)
            return 0;

          if ((unsigned long) p2 & (HEAP_MAX_SIZE - 1))
            {
              __munmap (p2, HEAP_MAX_SIZE);
              return 0;
            }
        }
    }
  if (__mprotect (p2, size, PROT_READ | PROT_WRITE) != 0)
    {
      __munmap (p2, HEAP_MAX_SIZE);
      return 0;
    }
  h = (heap_info *) p2;
  h->size = size;
  h->mprotect_size = size;
  LIBC_PROBE (memory_heap_new, 2, h, h->size);
  return h;
}

/* Grow a heap.  size is automatically rounded up to a
   multiple of the page size. */

static int
grow_heap (heap_info *h, long diff)
{
  size_t pagesize = GLRO (dl_pagesize);
  long new_size;

  diff = ALIGN_UP (diff, pagesize);
  new_size = (long) h->size + diff;
  if ((unsigned long) new_size > (unsigned long) HEAP_MAX_SIZE)
    return -1;

  if ((unsigned long) new_size > h->mprotect_size)
    {
      if (__mprotect ((char *) h + h->mprotect_size,
                      (unsigned long) new_size - h->mprotect_size,
                      PROT_READ | PROT_WRITE) != 0)
        return -2;

      h->mprotect_size = new_size;
    }

  h->size = new_size;
  LIBC_PROBE (memory_heap_more, 2, h, h->size);
  return 0;
}

/* Shrink a heap.  */

static int
shrink_heap (heap_info *h, long diff)
{
  long new_size;

  new_size = (long) h->size - diff;
  if (new_size < (long) sizeof (*h))
    return -1;

  /* Try to re-map the extra heap space freshly to save memory, and make it
     inaccessible.  See malloc-sysdep.h to know when this is true.  */
  if (__glibc_unlikely (check_may_shrink_heap ()))
    {
      if ((char *) MMAP ((char *) h + new_size, diff, PROT_NONE,
                         MAP_FIXED) == (char *) MAP_FAILED)
        return -2;

      h->mprotect_size = new_size;
    }
  else
    __madvise ((char *) h + new_size, diff, MADV_DONTNEED);
  /*fprintf(stderr, "shrink %p %08lx\n", h, new_size);*/

  h->size = new_size;
  LIBC_PROBE (memory_heap_less, 2, h, h->size);
  return 0;
}

/* Delete a heap. */

#define delete_heap(heap) \
  do {									      \
      if ((char *) (heap) + HEAP_MAX_SIZE == aligned_heap_area)		      \
        aligned_heap_area = NULL;					      \
      __munmap ((char *) (heap), HEAP_MAX_SIZE);			      \
    } while (0)

static int
internal_function
heap_trim (heap_info *heap, size_t pad)
{
  mstate ar_ptr = heap->ar_ptr;
  unsigned long pagesz = GLRO (dl_pagesize);
  mchunkptr top_chunk = top (ar_ptr), p, bck, fwd;
  heap_info *prev_heap;
  long new_size, top_size, top_area, extra, prev_size, misalign;

  /* Can this heap go away completely? */
  while (top_chunk == chunk_at_offset (heap, sizeof (*heap)))
    {
      prev_heap = heap->prev;
      prev_size = prev_heap->size - (MINSIZE - 2 * SIZE_SZ);
      p = chunk_at_offset (prev_heap, prev_size);
      /* fencepost must be properly aligned.  */
      misalign = ((long) p) & MALLOC_ALIGN_MASK;
      p = chunk_at_offset (prev_heap, prev_size - misalign);
      assert (p->size == (0 | PREV_INUSE)); /* must be fencepost */
      p = prev_chunk (p);
      new_size = chunksize (p) + (MINSIZE - 2 * SIZE_SZ) + misalign;
      assert (new_size > 0 && new_size < (long) (2 * MINSIZE));
      if (!prev_inuse (p))
        new_size += p->prev_size;
      assert (new_size > 0 && new_size < HEAP_MAX_SIZE);
      if (new_size + (HEAP_MAX_SIZE - prev_heap->size) < pad + MINSIZE + pagesz)
        break;
      ar_ptr->system_mem -= heap->size;
      arena_mem -= heap->size;
      LIBC_PROBE (memory_heap_free, 2, heap, heap->size);
      delete_heap (heap);
      heap = prev_heap;
      if (!prev_inuse (p)) /* consolidate backward */
        {
          p = prev_chunk (p);
          unlink (ar_ptr, p, bck, fwd);
        }
      assert (((unsigned long) ((char *) p + new_size) & (pagesz - 1)) == 0);
      assert (((char *) p + new_size) == ((char *) heap + heap->size));
      top (ar_ptr) = top_chunk = p;
      set_head (top_chunk, new_size | PREV_INUSE);
      /*check_chunk(ar_ptr, top_chunk);*/
    }

  /* Uses similar logic for per-thread arenas as the main arena with systrim
     by preserving the top pad and at least a page.  */
  top_size = chunksize (top_chunk);
  top_area = top_size - MINSIZE - 1;
  if (top_area < 0 || (size_t) top_area <= pad)
    return 0;

  extra = ALIGN_DOWN(top_area - pad, pagesz);
  if ((unsigned long) extra < mp_.trim_threshold)
    return 0;

  /* Try to shrink. */
  if (shrink_heap (heap, extra) != 0)
    return 0;

  ar_ptr->system_mem -= extra;
  arena_mem -= extra;

  /* Success. Adjust top accordingly. */
  set_head (top_chunk, (top_size - extra) | PREV_INUSE);
  /*check_chunk(ar_ptr, top_chunk);*/
  return 1;
}

/* Create a new arena with initial size "size".  */

static mstate
_int_new_arena (size_t size)
{
  mstate a;
  heap_info *h;
  char *ptr;
  unsigned long misalign;

  h = new_heap (size + (sizeof (*h) + sizeof (*a) + MALLOC_ALIGNMENT),
                mp_.top_pad);
  if (!h)
    {
      /* Maybe size is too large to fit in a single heap.  So, just try
         to create a minimally-sized arena and let _int_malloc() attempt
         to deal with the large request via mmap_chunk().  */
      h = new_heap (sizeof (*h) + sizeof (*a) + MALLOC_ALIGNMENT, mp_.top_pad);
      if (!h)
        return 0;
    }
  a = h->ar_ptr = (mstate) (h + 1);
  malloc_init_state (a);
  /*a->next = NULL;*/
  a->system_mem = a->max_system_mem = h->size;
  arena_mem += h->size;

  /* Set up the top chunk, with proper alignment. */
  ptr = (char *) (a + 1);
  misalign = (unsigned long) chunk2mem (ptr) & MALLOC_ALIGN_MASK;
  if (misalign > 0)
    ptr += MALLOC_ALIGNMENT - misalign;
  top (a) = (mchunkptr) ptr;
  set_head (top (a), (((char *) h + h->size) - ptr) | PREV_INUSE);

  LIBC_PROBE (memory_arena_new, 2, a, size);
  tsd_setspecific (arena_key, (void *) a);
  mutex_init (&a->mutex);
  (void) mutex_lock (&a->mutex);

  (void) mutex_lock (&list_lock);

  /* Add the new arena to the global list.  */
  a->next = main_arena.next;
  atomic_write_barrier ();
  main_arena.next = a;

  (void) mutex_unlock (&list_lock);

  return a;
}


static mstate
get_free_list (void)
{
  mstate result = free_list;
  if (result != NULL)
    {
      (void) mutex_lock (&list_lock);
      result = free_list;
      if (result != NULL)
        free_list = result->next_free;
      (void) mutex_unlock (&list_lock);

      if (result != NULL)
        {
          LIBC_PROBE (memory_arena_reuse_free_list, 1, result);
          (void) mutex_lock (&result->mutex);
          tsd_setspecific (arena_key, (void *) result);
        }
    }

  return result;
}

/* Lock and return an arena that can be reused for memory allocation.
   Avoid AVOID_ARENA as we have already failed to allocate memory in
   it and it is currently locked.  */
static mstate
reused_arena (mstate avoid_arena)
{
  mstate result;
  static mstate next_to_use;
  if (next_to_use == NULL)
    next_to_use = &main_arena;

  result = next_to_use;
  do
    {
      if (!arena_is_corrupt (result) && !mutex_trylock (&result->mutex))
        goto out;

      result = result->next;
    }
  while (result != next_to_use);

  /* Avoid AVOID_ARENA as we have already failed to allocate memory
     in that arena and it is currently locked.   */
  if (result == avoid_arena)
    result = result->next;

  /* Make sure that the arena we get is not corrupted.  */
  mstate begin = result;
  while (arena_is_corrupt (result) || result == avoid_arena)
    {
      result = result->next;
      if (result == begin)
	break;
    }

  /* We could not find any arena that was either not corrupted or not the one
     we wanted to avoid.  */
  if (result == begin || result == avoid_arena)
    return NULL;

  /* No arena available without contention.  Wait for the next in line.  */
  LIBC_PROBE (memory_arena_reuse_wait, 3, &result->mutex, result, avoid_arena);
  (void) mutex_lock (&result->mutex);

out:
  LIBC_PROBE (memory_arena_reuse, 2, result, avoid_arena);
  tsd_setspecific (arena_key, (void *) result);
  next_to_use = result->next;

  return result;
}

static mstate
internal_function
arena_get2 (mstate a_tsd, size_t size, mstate avoid_arena)
{
  mstate a;

  static size_t narenas_limit;

  a = get_free_list ();
  if (a == NULL)
    {
      /* Nothing immediately available, so generate a new arena.  */
      if (narenas_limit == 0)
        {
          if (mp_.arena_max != 0)
            narenas_limit = mp_.arena_max;
          else if (narenas > mp_.arena_test)
            {
              int n = __get_nprocs ();

              if (n >= 1)
                narenas_limit = NARENAS_FROM_NCORES (n);
              else
                /* We have no information about the system.  Assume two
                   cores.  */
                narenas_limit = NARENAS_FROM_NCORES (2);
            }
        }
    repeat:;
      size_t n = narenas;
      /* NB: the following depends on the fact that (size_t)0 - 1 is a
         very large number and that the underflow is OK.  If arena_max
         is set the value of arena_test is irrelevant.  If arena_test
         is set but narenas is not yet larger or equal to arena_test
         narenas_limit is 0.  There is no possibility for narenas to
         be too big for the test to always fail since there is not
         enough address space to create that many arenas.  */
      if (__glibc_unlikely (n <= narenas_limit - 1))
        {
          if (catomic_compare_and_exchange_bool_acq (&narenas, n + 1, n))
            goto repeat;
          a = _int_new_arena (size);
	  if (__glibc_unlikely (a == NULL))
            catomic_decrement (&narenas);
        }
      else
        a = reused_arena (avoid_arena);
    }
  return a;
}

/* If we don't have the main arena, then maybe the failure is due to running
   out of mmapped areas, so we can try allocating on the main arena.
   Otherwise, it is likely that sbrk() has failed and there is still a chance
   to mmap(), so try one of the other arenas.  */
static mstate
arena_get_retry (mstate ar_ptr, size_t bytes)
{
  LIBC_PROBE (memory_arena_retry, 2, bytes, ar_ptr);
  if (ar_ptr != &main_arena)
    {
      (void) mutex_unlock (&ar_ptr->mutex);
      ar_ptr = &main_arena;
      (void) mutex_lock (&ar_ptr->mutex);
    }
  else
    {
      /* Grab ar_ptr->next prior to releasing its lock.  */
      mstate prev = ar_ptr->next ? ar_ptr : 0;
      (void) mutex_unlock (&ar_ptr->mutex);
      ar_ptr = arena_get2 (prev, bytes, ar_ptr);
    }

  return ar_ptr;
}

static void __attribute__ ((section ("__libc_thread_freeres_fn")))
arena_thread_freeres (void)
{
  void *vptr = NULL;
  mstate a = tsd_getspecific (arena_key, vptr);
  tsd_setspecific (arena_key, NULL);

  if (a != NULL)
    {
      (void) mutex_lock (&list_lock);
      a->next_free = free_list;
      free_list = a;
      (void) mutex_unlock (&list_lock);
    }
}
text_set_element (__libc_thread_subfreeres, arena_thread_freeres);

/*
 * Local variables:
 * c-basic-offset: 2
 * End:
 */