pthread_rwlock_rdlock.c 2.9 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. Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
  4. The GNU C Library is free software; you can redistribute it and/or
  5. modify it under the terms of the GNU Lesser General Public
  6. License as published by the Free Software Foundation; either
  7. version 2.1 of the License, or (at your option) any later version.
  8. The GNU C Library is distributed in the hope that it will be useful,
  9. but WITHOUT ANY WARRANTY; without even the implied warranty of
  10. MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  11. Lesser General Public License for more details.
  12. You should have received a copy of the GNU Lesser General Public
  13. License along with the GNU C Library; if not, see
  14. <http://www.gnu.org/licenses/>. */
  15. #include <errno.h>
  16. #include <sysdep.h>
  17. #include <lowlevellock.h>
  18. #include <pthread.h>
  19. #include <pthreadP.h>
  20. /* Acquire read lock for RWLOCK. */
  21. int
  22. attribute_protected
  23. __pthread_rwlock_rdlock (
  24. pthread_rwlock_t *rwlock)
  25. {
  26. int result = 0;
  27. /* Make sure we are along. */
  28. lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  29. while (1)
  30. {
  31. /* Get the rwlock if there is no writer... */
  32. if (rwlock->__data.__writer == 0
  33. /* ...and if either no writer is waiting or we prefer readers. */
  34. && (!rwlock->__data.__nr_writers_queued
  35. || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  36. {
  37. /* Increment the reader counter. Avoid overflow. */
  38. if (__builtin_expect (++rwlock->__data.__nr_readers == 0, 0))
  39. {
  40. /* Overflow on number of readers. */
  41. --rwlock->__data.__nr_readers;
  42. result = EAGAIN;
  43. }
  44. break;
  45. }
  46. /* Make sure we are not holding the rwlock as a writer. This is
  47. a deadlock situation we recognize and report. */
  48. if (__builtin_expect (rwlock->__data.__writer
  49. == THREAD_GETMEM (THREAD_SELF, tid), 0))
  50. {
  51. result = EDEADLK;
  52. break;
  53. }
  54. /* Remember that we are a reader. */
  55. if (__builtin_expect (++rwlock->__data.__nr_readers_queued == 0, 0))
  56. {
  57. /* Overflow on number of queued readers. */
  58. --rwlock->__data.__nr_readers_queued;
  59. result = EAGAIN;
  60. break;
  61. }
  62. int waitval = rwlock->__data.__readers_wakeup;
  63. /* Free the lock. */
  64. lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  65. /* Wait for the writer to finish. */
  66. lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  67. rwlock->__data.__shared);
  68. /* Get the lock. */
  69. lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  70. --rwlock->__data.__nr_readers_queued;
  71. }
  72. /* We are done, free the lock. */
  73. lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  74. return result;
  75. }
  76. weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
  77. strong_alias (__pthread_rwlock_rdlock, __pthread_rwlock_rdlock_internal)