elf/tlsdeschtab.h

   1 /* Hash table for TLS descriptors.
   2    Copyright (C) 2005-2017 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 internal_function
  83 _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
  84 {
  85   struct hashtab *ht;
  86   void **entry;
  87   struct tlsdesc_dynamic_arg *td, test;
  88
  89   /* FIXME: We could use a per-map lock here, but is it worth it?  */
  90   __rtld_lock_lock_recursive (GL(dl_load_lock));
  91
  92   ht = map->l_mach.tlsdesc_table;
  93   if (! ht)
  94     {
  95       ht = htab_create ();
  96       if (! ht)
  97         {
  98           __rtld_lock_unlock_recursive (GL(dl_load_lock));
  99           return 0;
 100         }
 101       map->l_mach.tlsdesc_table = ht;
 102     }
 103
 104   test.tlsinfo.ti_module = map->l_tls_modid;
 105   test.tlsinfo.ti_offset = ti_offset;
 106   entry = htab_find_slot (ht, &test, 1, hash_tlsdesc, eq_tlsdesc);
 107   if (! entry)
 108     {
 109       __rtld_lock_unlock_recursive (GL(dl_load_lock));
 110       return 0;
 111     }
 112
 113   if (*entry)
 114     {
 115       td = *entry;
 116       __rtld_lock_unlock_recursive (GL(dl_load_lock));
 117       return td;
 118     }
 119
 120   *entry = td = malloc (sizeof (struct tlsdesc_dynamic_arg));
 121   /* This may be higher than the map's generation, but it doesn't
 122      matter much.  Worst case, we'll have one extra DTV update per
 123      thread.  */
 124   td->gen_count = map_generation (map);
 125   td->tlsinfo = test.tlsinfo;
 126
 127   __rtld_lock_unlock_recursive (GL(dl_load_lock));
 128   return td;
 129 }
 130
 131 # endif /* SHARED */
 132
 133 /* The idea of the following two functions is to stop multiple threads
 134    from attempting to resolve the same TLS descriptor without busy
 135    waiting.  Ideally, we should be able to release the lock right
 136    after changing td->entry, and then using say a condition variable
 137    or a futex wake to wake up any waiting threads, but let's try to
 138    avoid introducing such dependencies.  */
 139
 140 static int
 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 _dl_tlsdesc_wake_up_held_fixups (void)
 160 {
 161   __rtld_lock_unlock_recursive (GL(dl_load_lock));
 162 }
 163
 164 #endif