stdlib/cxa_thread_atexit_impl.c

   1 /* Register destructors for C++ TLS variables declared with thread_local.
   2    Copyright (C) 2013-2015 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 /* CONCURRENCY NOTES:
  20
  21    This documents concurrency for the non-POD TLS destructor registration,
  22    calling and destruction.  The functions __cxa_thread_atexit_impl,
  23    _dl_close_worker and __call_tls_dtors are the three main routines that may
  24    run concurrently and access shared data.  The shared data in all possible
  25    combinations of all three functions are the link map list, a link map for a
  26    DSO and the link map member l_tls_dtor_count.
  27
  28    __cxa_thread_atexit_impl acquires the dl_load_lock before accessing any
  29    shared state and hence multiple of its instances can safely execute
  30    concurrently.
  31
  32    _dl_close_worker acquires the dl_load_lock before accessing any shared state
  33    as well and hence can concurrently execute multiple of its own instances as
  34    well as those of __cxa_thread_atexit_impl safely.  Not all accesses to
  35    l_tls_dtor_count are protected by the dl_load_lock, so we need to
  36    synchronize using atomics.
  37
  38    __call_tls_dtors accesses the l_tls_dtor_count without taking the lock; it
  39    decrements the value by one.  It does not need the big lock because it does
  40    not access any other shared state except for the current DSO link map and
  41    its member l_tls_dtor_count.
  42
  43    Correspondingly, _dl_close_worker loads l_tls_dtor_count and if it is zero,
  44    unloads the DSO, thus deallocating the current link map.  This is the goal
  45    of maintaining l_tls_dtor_count - to unload the DSO and free resources if
  46    there are no pending destructors to be called.
  47
  48    We want to eliminate the inconsistent state where the DSO is unloaded in
  49    _dl_close_worker before it is used in __call_tls_dtors.  This could happen
  50    if __call_tls_dtors uses the link map after it sets l_tls_dtor_count to 0,
  51    since _dl_close_worker will conclude from the 0 l_tls_dtor_count value that
  52    it is safe to unload the DSO.  Hence, to ensure that this does not happen,
  53    the following conditions must be met:
  54
  55    1. In _dl_close_worker, the l_tls_dtor_count load happens before the DSO is
  56       unloaded and its link map is freed
  57    2. The link map dereference in __call_tls_dtors happens before the
  58       l_tls_dtor_count dereference.
  59
  60    To ensure this, the l_tls_dtor_count decrement in __call_tls_dtors should
  61    have release semantics and the load in _dl_close_worker should have acquire
  62    semantics.
  63
  64    Concurrent executions of __call_tls_dtors should only ensure that the value
  65    is accessed atomically; no reordering constraints need to be considered.
  66    Likewise for the increment of l_tls_dtor_count in __cxa_thread_atexit_impl.
  67
  68    There is still a possibility on concurrent execution of _dl_close_worker and
  69    __call_tls_dtors where _dl_close_worker reads the value of l_tls_dtor_count
  70    as 1, __call_tls_dtors decrements the value of l_tls_dtor_count but
  71    _dl_close_worker does not unload the DSO, having read the old value.  This
  72    is not very different from a case where __call_tls_dtors is called after
  73    _dl_close_worker on the DSO and hence is an accepted execution.  */
  74
  75 #include <stdlib.h>
  76 #include <ldsodefs.h>
  77
  78 typedef void (*dtor_func) (void *);
  79
  80 struct dtor_list
  81 {
  82   dtor_func func;
  83   void *obj;
  84   struct link_map *map;
  85   struct dtor_list *next;
  86 };
  87
  88 static __thread struct dtor_list *tls_dtor_list;
  89 static __thread void *dso_symbol_cache;
  90 static __thread struct link_map *lm_cache;
  91
  92 /* Register a destructor for TLS variables declared with the 'thread_local'
  93    keyword.  This function is only called from code generated by the C++
  94    compiler.  FUNC is the destructor function and OBJ is the object to be
  95    passed to the destructor.  DSO_SYMBOL is the __dso_handle symbol that each
  96    DSO has at a unique address in its map, added from crtbegin.o during the
  97    linking phase.  */
  98 int
  99 __cxa_thread_atexit_impl (dtor_func func, void *obj, void *dso_symbol)
 100 {
 101   /* Prepend.  */
 102   struct dtor_list *new = calloc (1, sizeof (struct dtor_list));
 103   new->func = func;
 104   new->obj = obj;
 105   new->next = tls_dtor_list;
 106   tls_dtor_list = new;
 107
 108   /* We have to acquire the big lock to prevent a racing dlclose from pulling
 109      our DSO from underneath us while we're setting up our destructor.  */
 110   __rtld_lock_lock_recursive (GL(dl_load_lock));
 111
 112   /* See if we already encountered the DSO.  */
 113   if (__glibc_unlikely (dso_symbol_cache != dso_symbol))
 114     {
 115       ElfW(Addr) caller = (ElfW(Addr)) dso_symbol;
 116
 117       struct link_map *l = _dl_find_dso_for_object (caller);
 118
 119       /* If the address is not recognized the call comes from the main
 120          program (we hope).  */
 121       lm_cache = l ? l : GL(dl_ns)[LM_ID_BASE]._ns_loaded;
 122     }
 123
 124   /* This increment may only be concurrently observed either by the decrement
 125      in __call_tls_dtors since the other l_tls_dtor_count access in
 126      _dl_close_worker is protected by the dl_load_lock.  The execution in
 127      __call_tls_dtors does not really depend on this value beyond the fact that
 128      it should be atomic, so Relaxed MO should be sufficient.  */
 129   atomic_fetch_add_relaxed (&lm_cache->l_tls_dtor_count, 1);
 130   __rtld_lock_unlock_recursive (GL(dl_load_lock));
 131
 132   new->map = lm_cache;
 133
 134   return 0;
 135 }
 136
 137 /* Call the destructors.  This is called either when a thread returns from the
 138    initial function or when the process exits via the exit function.  */
 139 void
 140 __call_tls_dtors (void)
 141 {
 142   while (tls_dtor_list)
 143     {
 144       struct dtor_list *cur = tls_dtor_list;
 145
 146       tls_dtor_list = tls_dtor_list->next;
 147       cur->func (cur->obj);
 148
 149       /* Ensure that the MAP dereference happens before
 150          l_tls_dtor_count decrement.  That way, we protect this access from a
 151          potential DSO unload in _dl_close_worker, which happens when
 152          l_tls_dtor_count is 0.  See CONCURRENCY NOTES for more detail.  */
 153       atomic_fetch_add_release (&cur->map->l_tls_dtor_count, -1);
 154       free (cur);
 155     }
 156 }
 157 libc_hidden_def (__call_tls_dtors)