elf/tlsdeschtab.h

   1 /* Hash table for TLS descriptors.
   2    Copyright (C) 2005-2018 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Alexandre Oliva  <aoliva@redhat.com>
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 #ifndef TLSDESCHTAB_H
  21 # define TLSDESCHTAB_H 1
  22
  23 #include <atomic.h>
  24
  25 # ifdef SHARED
  26
  27 #  include <inline-hashtab.h>
  28
  29 inline static int
  30 hash_tlsdesc (void *p)
  31 {
  32   struct tlsdesc_dynamic_arg *td = p;
  33
  34   /* We know all entries are for the same module, so ti_offset is the
  35      only distinguishing entry.  */
  36   return td->tlsinfo.ti_offset;
  37 }
  38
  39 inline static int
  40 eq_tlsdesc (void *p, void *q)
  41 {
  42   struct tlsdesc_dynamic_arg *tdp = p, *tdq = q;
  43
  44   return tdp->tlsinfo.ti_offset == tdq->tlsinfo.ti_offset;
  45 }
  46
  47 inline static size_t
  48 map_generation (struct link_map *map)
  49 {
  50   size_t idx = map->l_tls_modid;
  51   struct dtv_slotinfo_list *listp = GL(dl_tls_dtv_slotinfo_list);
  52
  53   /* Find the place in the dtv slotinfo list.  */
  54   do
  55     {
  56       /* Does it fit in the array of this list element?  */
  57       if (idx < listp->len)
  58         {
  59           /* We should never get here for a module in static TLS, so
  60              we can assume that, if the generation count is zero, we
  61              still haven't determined the generation count for this
  62              module.  */
  63           if (listp->slotinfo[idx].map == map && listp->slotinfo[idx].gen)
  64             return listp->slotinfo[idx].gen;
  65           else
  66             break;
  67         }
  68       idx -= listp->len;
  69       listp = listp->next;
  70     }
  71   while (listp != NULL);
  72
  73   /* If we get to this point, the module still hasn't been assigned an
  74      entry in the dtv slotinfo data structures, and it will when we're
  75      done with relocations.  At that point, the module will get a
  76      generation number that is one past the current generation, so
  77      return exactly that.  */
  78   return GL(dl_tls_generation) + 1;
  79 }
  80
  81 void *
  82 _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
  83 {
  84   struct hashtab *ht;
  85   void **entry;
  86   struct tlsdesc_dynamic_arg *td, test;
  87
  88   /* FIXME: We could use a per-map lock here, but is it worth it?  */
  89   __rtld_lock_lock_recursive (GL(dl_load_lock));
  90
  91   ht = map->l_mach.tlsdesc_table;
  92   if (! ht)
  93     {
  94       ht = htab_create ();
  95       if (! ht)
  96         {
  97           __rtld_lock_unlock_recursive (GL(dl_load_lock));
  98           return 0;
  99         }
 100       map->l_mach.tlsdesc_table = ht;
 101     }
 102
 103   test.tlsinfo.ti_module = map->l_tls_modid;
 104   test.tlsinfo.ti_offset = ti_offset;
 105   entry = htab_find_slot (ht, &test, 1, hash_tlsdesc, eq_tlsdesc);
 106   if (! entry)
 107     {
 108       __rtld_lock_unlock_recursive (GL(dl_load_lock));
 109       return 0;
 110     }
 111
 112   if (*entry)
 113     {
 114       td = *entry;
 115       __rtld_lock_unlock_recursive (GL(dl_load_lock));
 116       return td;
 117     }
 118
 119   *entry = td = malloc (sizeof (struct tlsdesc_dynamic_arg));
 120   /* This may be higher than the map's generation, but it doesn't
 121      matter much.  Worst case, we'll have one extra DTV update per
 122      thread.  */
 123   td->gen_count = map_generation (map);
 124   td->tlsinfo = test.tlsinfo;
 125
 126   __rtld_lock_unlock_recursive (GL(dl_load_lock));
 127   return td;
 128 }
 129
 130 # endif /* SHARED */
 131
 132 /* The idea of the following two functions is to stop multiple threads
 133    from attempting to resolve the same TLS descriptor without busy
 134    waiting.  Ideally, we should be able to release the lock right
 135    after changing td->entry, and then using say a condition variable
 136    or a futex wake to wake up any waiting threads, but let's try to
 137    avoid introducing such dependencies.  */
 138
 139 static int
 140 __attribute__ ((unused))
 141 _dl_tlsdesc_resolve_early_return_p (struct tlsdesc volatile *td, void *caller)
 142 {
 143   if (caller != atomic_load_relaxed (&td->entry))
 144     return 1;
 145
 146   __rtld_lock_lock_recursive (GL(dl_load_lock));
 147   if (caller != atomic_load_relaxed (&td->entry))
 148     {
 149       __rtld_lock_unlock_recursive (GL(dl_load_lock));
 150       return 1;
 151     }
 152
 153   atomic_store_relaxed (&td->entry, _dl_tlsdesc_resolve_hold);
 154
 155   return 0;
 156 }
 157
 158 static void
 159 __attribute__ ((unused))
 160 _dl_tlsdesc_wake_up_held_fixups (void)
 161 {
 162   __rtld_lock_unlock_recursive (GL(dl_load_lock));
 163 }
 164
 165 #endif