pthread_once.c 2.7 KB

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  1. /* Copyright (C) 2003, 2004, 2007 Free Software Foundation, Inc.
  2. This file is part of the GNU C Library.
  3. The GNU C Library is free software; you can redistribute it and/or
  4. modify it under the terms of the GNU Lesser General Public
  5. License as published by the Free Software Foundation; either
  6. version 2.1 of the License, or (at your option) any later version.
  7. The GNU C Library is distributed in the hope that it will be useful,
  8. but WITHOUT ANY WARRANTY; without even the implied warranty of
  9. MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  10. Lesser General Public License for more details.
  11. You should have received a copy of the GNU Lesser General Public
  12. License along with the GNU C Library; if not, see
  13. <http://www.gnu.org/licenses/>. */
  14. #include "pthreadP.h"
  15. #include <lowlevellock.h>
  16. unsigned long int __fork_generation attribute_hidden;
  17. static void
  18. clear_once_control (void *arg)
  19. {
  20. pthread_once_t *once_control = (pthread_once_t *) arg;
  21. *once_control = 0;
  22. lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  23. }
  24. int
  25. attribute_protected
  26. __pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
  27. {
  28. for (;;)
  29. {
  30. int oldval;
  31. int newval;
  32. int tmp;
  33. /* Pseudo code:
  34. newval = __fork_generation | 1;
  35. oldval = *once_control;
  36. if ((oldval & 2) == 0)
  37. *once_control = newval;
  38. Do this atomically.
  39. */
  40. newval = __fork_generation | 1;
  41. __asm__ __volatile__ (
  42. "1: ldl_l %0, %2\n"
  43. " and %0, 2, %1\n"
  44. " bne %1, 2f\n"
  45. " mov %3, %1\n"
  46. " stl_c %1, %2\n"
  47. " beq %1, 1b\n"
  48. "2: mb"
  49. : "=&r" (oldval), "=&r" (tmp), "=m" (*once_control)
  50. : "r" (newval), "m" (*once_control));
  51. /* Check if the initializer has already been done. */
  52. if ((oldval & 2) != 0)
  53. return 0;
  54. /* Check if another thread already runs the initializer. */
  55. if ((oldval & 1) == 0)
  56. break;
  57. /* Check whether the initializer execution was interrupted by a fork. */
  58. if (oldval != newval)
  59. break;
  60. /* Same generation, some other thread was faster. Wait. */
  61. lll_futex_wait (once_control, oldval, LLL_PRIVATE);
  62. }
  63. /* This thread is the first here. Do the initialization.
  64. Register a cleanup handler so that in case the thread gets
  65. interrupted the initialization can be restarted. */
  66. pthread_cleanup_push (clear_once_control, once_control);
  67. init_routine ();
  68. pthread_cleanup_pop (0);
  69. /* Add one to *once_control to take the bottom 2 bits from 01 to 10. */
  70. atomic_increment (once_control);
  71. /* Wake up all other threads. */
  72. lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  73. return 0;
  74. }
  75. weak_alias (__pthread_once, pthread_once)
  76. strong_alias (__pthread_once, __pthread_once_internal)