glibc/malloc/arena.c

/* Malloc implementation for multiple threads without lock contention.
   Copyright (C) 2001,2002,2003,2004,2005,2006 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,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#include <stdbool.h>

/* Compile-time constants.  */

#define HEAP_MIN_SIZE (32*1024)
#ifndef HEAP_MAX_SIZE
#define HEAP_MAX_SIZE (1024*1024) /* must be a power of two */
#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.  */


#ifndef THREAD_STATS
#define THREAD_STATS 0
#endif

/* If THREAD_STATS is non-zero, some statistics on mutex locking are
   computed.  */

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

#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.  Not used unless compiling with
   USE_ARENAS. */

typedef struct _heap_info {
  mstate ar_ptr; /* Arena for this heap. */
  struct _heap_info *prev; /* Previous heap. */
  size_t size;   /* Current size in bytes. */
  /* Make sure the following data is properly aligned, particularly
     that sizeof (heap_info) + 2 * SIZE_SZ is a multiple of
     MALLOG_ALIGNMENT. */
  char pad[-5 * 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;

#if THREAD_STATS
static int stat_n_heaps;
#define THREAD_STAT(x) x
#else
#define THREAD_STAT(x) do ; while(0)
#endif

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

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

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

#if USE_ARENAS

/* 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 { \
  Void_t *vptr = NULL; \
  ptr = (mstate)tsd_getspecific(arena_key, vptr); \
  if(ptr && !mutex_trylock(&ptr->mutex)) { \
    THREAD_STAT(++(ptr->stat_lock_direct)); \
  } else \
    ptr = arena_get2(ptr, (size)); \
} 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)

#else /* !USE_ARENAS */

/* There is only one arena, main_arena. */

#if THREAD_STATS
#define arena_get(ar_ptr, sz) do { \
  ar_ptr = &main_arena; \
  if(!mutex_trylock(&ar_ptr->mutex)) \
    ++(ar_ptr->stat_lock_direct); \
  else { \
    (void)mutex_lock(&ar_ptr->mutex); \
    ++(ar_ptr->stat_lock_wait); \
  } \
} while(0)
#else
#define arena_get(ar_ptr, sz) do { \
  ar_ptr = &main_arena; \
  (void)mutex_lock(&ar_ptr->mutex); \
} while(0)
#endif
#define arena_for_chunk(ptr) (&main_arena)

#endif /* USE_ARENAS */

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

#ifndef NO_THREADS

/* atfork support.  */

static __malloc_ptr_t (*save_malloc_hook) (size_t __size,
					   __const __malloc_ptr_t);
# if !defined _LIBC || !defined USE_TLS || (defined SHARED && !USE___THREAD)
static __malloc_ptr_t (*save_memalign_hook) (size_t __align, size_t __size,
					     __const __malloc_ptr_t);
# endif
static void           (*save_free_hook) (__malloc_ptr_t __ptr,
					 __const __malloc_ptr_t);
static Void_t*        save_arena;

/* Magic value for the thread-specific arena pointer when
   malloc_atfork() is in use.  */

#define ATFORK_ARENA_PTR ((Void_t*)-1)

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

static Void_t*
malloc_atfork(size_t sz, const Void_t *caller)
{
  Void_t *vptr = NULL;
  Void_t *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 public_mALLOc(sz);
  }
}

static void
free_atfork(Void_t* mem, const Void_t *caller)
{
  Void_t *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 HAVE_MMAP
  if (chunk_is_mmapped(p))                       /* release mmapped memory. */
  {
    munmap_chunk(p);
    return;
  }
#endif

  ar_ptr = arena_for_chunk(p);
  tsd_getspecific(arena_key, vptr);
  if(vptr != ATFORK_ARENA_PTR)
    (void)mutex_lock(&ar_ptr->mutex);
  _int_free(ar_ptr, mem);
  if(vptr != ATFORK_ARENA_PTR)
    (void)mutex_unlock(&ar_ptr->mutex);
}


/* 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_t *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;
#if defined _LIBC || defined MALLOC_HOOKS
  tsd_setspecific(arena_key, save_arena);
  __malloc_hook = save_malloc_hook;
  __free_hook = save_free_hook;
#endif
  for(ar_ptr = &main_arena;;) {
    mutex_init(&ar_ptr->mutex);
    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 /* !defined NO_THREADS */

/* Initialization routine. */
#ifdef _LIBC
#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;
}
#endif /* _LIBC */

/* Set up basic state so that _int_malloc et al can work.  */
static void
ptmalloc_init_minimal (void)
{
#if DEFAULT_TOP_PAD != 0
  mp_.top_pad        = DEFAULT_TOP_PAD;
#endif
  mp_.n_mmaps_max    = DEFAULT_MMAP_MAX;
  mp_.mmap_threshold = DEFAULT_MMAP_THRESHOLD;
  mp_.trim_threshold = DEFAULT_TRIM_THRESHOLD;
  mp_.pagesize       = malloc_getpagesize;
}


#ifdef _LIBC
# 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

# if defined SHARED && defined USE_TLS && !USE___THREAD
/* This is called by __pthread_initialize_minimal when it needs to use
   malloc to set up the TLS state.  We cannot do the full work of
   ptmalloc_init (below) until __pthread_initialize_minimal has finished,
   so it has to switch to using the special startup-time hooks while doing
   those allocations.  */
void
__libc_malloc_pthread_startup (bool first_time)
{
  if (first_time)
    {
      ptmalloc_init_minimal ();
      save_malloc_hook = __malloc_hook;
      save_memalign_hook = __memalign_hook;
      save_free_hook = __free_hook;
      __malloc_hook = malloc_starter;
      __memalign_hook = memalign_starter;
      __free_hook = free_starter;
    }
  else
    {
      __malloc_hook = save_malloc_hook;
      __memalign_hook = save_memalign_hook;
      __free_hook = save_free_hook;
    }
}
# endif
#endif

static void
ptmalloc_init (void)
{
#if __STD_C
  const char* s;
#else
  char* s;
#endif
  int secure = 0;

  if(__malloc_initialized >= 0) return;
  __malloc_initialized = 0;

#ifdef _LIBC
# if defined SHARED && defined USE_TLS && !USE___THREAD
  /* ptmalloc_init_minimal may already have been called via
     __libc_malloc_pthread_startup, above.  */
  if (mp_.pagesize == 0)
# endif
#endif
    ptmalloc_init_minimal();

#ifndef NO_THREADS
# if defined _LIBC && defined USE_TLS
  /* We know __pthread_initialize_minimal has already been called,
     and that is enough.  */
#   define NO_STARTER
# endif
# ifndef NO_STARTER
  /* With some threads implementations, creating thread-specific data
     or initializing a mutex may call malloc() itself.  Provide a
     simple starter version (realloc() won't work). */
  save_malloc_hook = __malloc_hook;
  save_memalign_hook = __memalign_hook;
  save_free_hook = __free_hook;
  __malloc_hook = malloc_starter;
  __memalign_hook = memalign_starter;
  __free_hook = free_starter;
#  ifdef _LIBC
  /* Initialize the pthreads interface. */
  if (__pthread_initialize != NULL)
    __pthread_initialize();
#  endif /* !defined _LIBC */
# endif	/* !defined NO_STARTER */
#endif /* !defined NO_THREADS */
  mutex_init(&main_arena.mutex);
  main_arena.next = &main_arena;

#if defined _LIBC && defined 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

  mutex_init(&list_lock);
  tsd_key_create(&arena_key, NULL);
  tsd_setspecific(arena_key, (Void_t *)&main_arena);
  thread_atfork(ptmalloc_lock_all, ptmalloc_unlock_all, ptmalloc_unlock_all2);
#ifndef NO_THREADS
# ifndef NO_STARTER
  __malloc_hook = save_malloc_hook;
  __memalign_hook = save_memalign_hook;
  __free_hook = save_free_hook;
# else
#  undef NO_STARTER
# endif
#endif
#ifdef _LIBC
  secure = __libc_enable_secure;
  s = NULL;
  if (__builtin_expect (_environ != NULL, 1))
    {
      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 (! secure)
		{
		  if (memcmp (envline, "TOP_PAD_", 8) == 0)
		    mALLOPt(M_TOP_PAD, atoi(&envline[9]));
		  else if (memcmp (envline, "PERTURB_", 8) == 0)
		    mALLOPt(M_PERTURB, atoi(&envline[9]));
		}
	      break;
	    case 9:
	      if (! secure && memcmp (envline, "MMAP_MAX_", 9) == 0)
		mALLOPt(M_MMAP_MAX, atoi(&envline[10]));
	      break;
	    case 15:
	      if (! secure)
		{
		  if (memcmp (envline, "TRIM_THRESHOLD_", 15) == 0)
		    mALLOPt(M_TRIM_THRESHOLD, atoi(&envline[16]));
		  else if (memcmp (envline, "MMAP_THRESHOLD_", 15) == 0)
		    mALLOPt(M_MMAP_THRESHOLD, atoi(&envline[16]));
		}
	      break;
	    default:
	      break;
	    }
	}
    }
#else
  if (! secure)
    {
      if((s = getenv("MALLOC_TRIM_THRESHOLD_")))
	mALLOPt(M_TRIM_THRESHOLD, atoi(s));
      if((s = getenv("MALLOC_TOP_PAD_")))
	mALLOPt(M_TOP_PAD, atoi(s));
      if((s = getenv("MALLOC_PERTURB_")))
	mALLOPt(M_PERTURB, atoi(s));
      if((s = getenv("MALLOC_MMAP_THRESHOLD_")))
	mALLOPt(M_MMAP_THRESHOLD, atoi(s));
      if((s = getenv("MALLOC_MMAP_MAX_")))
	mALLOPt(M_MMAP_MAX, atoi(s));
    }
  s = getenv("MALLOC_CHECK_");
#endif
  if(s && s[0]) {
    mALLOPt(M_CHECK_ACTION, (int)(s[0] - '0'));
    if (check_action != 0)
      __malloc_check_init();
  }
  if(__malloc_initialize_hook != NULL)
    (*__malloc_initialize_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 USE_ARENAS

#if MALLOC_DEBUG > 1

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

static void
#if __STD_C
dump_heap(heap_info *heap)
#else
dump_heap(heap) heap_info *heap;
#endif
{
  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
#if __STD_C
new_heap(size_t size, size_t top_pad)
#else
new_heap(size, top_pad) size_t size, top_pad;
#endif
{
  size_t page_mask = malloc_getpagesize - 1;
  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 = (size + page_mask) & ~page_mask;

  /* 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_PRIVATE|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_PRIVATE|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_PRIVATE|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;
  THREAD_STAT(stat_n_heaps++);
  return h;
}

/* Grow or shrink a heap.  size is automatically rounded up to a
   multiple of the page size if it is positive. */

static int
#if __STD_C
grow_heap(heap_info *h, long diff)
#else
grow_heap(h, diff) heap_info *h; long diff;
#endif
{
  size_t page_mask = malloc_getpagesize - 1;
  long new_size;

  if(diff >= 0) {
    diff = (diff + page_mask) & ~page_mask;
    new_size = (long)h->size + diff;
    if(new_size > HEAP_MAX_SIZE)
      return -1;
    if(mprotect((char *)h + h->size, diff, PROT_READ|PROT_WRITE) != 0)
      return -2;
  } else {
    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. */
    if((char *)MMAP((char *)h + new_size, -diff, PROT_NONE,
                    MAP_PRIVATE|MAP_FIXED) == (char *) MAP_FAILED)
      return -2;
    /*fprintf(stderr, "shrink %p %08lx\n", h, new_size);*/
  }
  h->size = new_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
#if __STD_C
heap_trim(heap_info *heap, size_t pad)
#else
heap_trim(heap, pad) heap_info *heap; size_t pad;
#endif
{
  mstate ar_ptr = heap->ar_ptr;
  unsigned long pagesz = mp_.pagesize;
  mchunkptr top_chunk = top(ar_ptr), p, bck, fwd;
  heap_info *prev_heap;
  long new_size, top_size, extra;

  /* Can this heap go away completely? */
  while(top_chunk == chunk_at_offset(heap, sizeof(*heap))) {
    prev_heap = heap->prev;
    p = chunk_at_offset(prev_heap, prev_heap->size - (MINSIZE-2*SIZE_SZ));
    assert(p->size == (0|PREV_INUSE)); /* must be fencepost */
    p = prev_chunk(p);
    new_size = chunksize(p) + (MINSIZE-2*SIZE_SZ);
    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;
    delete_heap(heap);
    heap = prev_heap;
    if(!prev_inuse(p)) { /* consolidate backward */
      p = prev_chunk(p);
      unlink(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);*/
  }
  top_size = chunksize(top_chunk);
  extra = ((top_size - pad - MINSIZE + (pagesz-1))/pagesz - 1) * pagesz;
  if(extra < (long)pagesz)
    return 0;
  /* Try to shrink. */
  if(grow_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;
#ifdef NO_THREADS
  if((unsigned long)(mp_.mmapped_mem + arena_mem + main_arena.system_mem) >
     mp_.max_total_mem)
    mp_.max_total_mem = mp_.mmapped_mem + arena_mem + main_arena.system_mem;
#endif

  /* 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);

  return a;
}

static mstate
internal_function
#if __STD_C
arena_get2(mstate a_tsd, size_t size)
#else
arena_get2(a_tsd, size) mstate a_tsd; size_t size;
#endif
{
  mstate a;

  if(!a_tsd)
    a = a_tsd = &main_arena;
  else {
    a = a_tsd->next;
    if(!a) {
      /* This can only happen while initializing the new arena. */
      (void)mutex_lock(&main_arena.mutex);
      THREAD_STAT(++(main_arena.stat_lock_wait));
      return &main_arena;
    }
  }

  /* Check the global, circularly linked list for available arenas. */
  bool retried = false;
 repeat:
  do {
    if(!mutex_trylock(&a->mutex)) {
      if (retried)
	(void)mutex_unlock(&list_lock);
      THREAD_STAT(++(a->stat_lock_loop));
      tsd_setspecific(arena_key, (Void_t *)a);
      return a;
    }
    a = a->next;
  } while(a != a_tsd);

  /* If not even the list_lock can be obtained, try again.  This can
     happen during `atfork', or for example on systems where thread
     creation makes it temporarily impossible to obtain _any_
     locks. */
  if(!retried && mutex_trylock(&list_lock)) {
    /* We will block to not run in a busy loop.  */
    (void)mutex_lock(&list_lock);

    /* Since we blocked there might be an arena available now.  */
    retried = true;
    a = a_tsd;
    goto repeat;
  }

  /* Nothing immediately available, so generate a new arena.  */
  a = _int_new_arena(size);
  if(a)
    {
      tsd_setspecific(arena_key, (Void_t *)a);
      mutex_init(&a->mutex);
      mutex_lock(&a->mutex); /* remember result */

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

      THREAD_STAT(++(a->stat_lock_loop));
    }
  (void)mutex_unlock(&list_lock);

  return a;
}

#endif /* USE_ARENAS */

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