mutex-xchg.h 3.8 KB

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  1. /*
  2. * include/asm-generic/mutex-xchg.h
  3. *
  4. * Generic implementation of the mutex fastpath, based on xchg().
  5. *
  6. * NOTE: An xchg based implementation might be less optimal than an atomic
  7. * decrement/increment based implementation. If your architecture
  8. * has a reasonable atomic dec/inc then you should probably use
  9. * asm-generic/mutex-dec.h instead, or you could open-code an
  10. * optimized version in asm/mutex.h.
  11. */
  12. #ifndef _ASM_GENERIC_MUTEX_XCHG_H
  13. #define _ASM_GENERIC_MUTEX_XCHG_H
  14. /**
  15. * __mutex_fastpath_lock - try to take the lock by moving the count
  16. * from 1 to a 0 value
  17. * @count: pointer of type atomic_t
  18. * @fail_fn: function to call if the original value was not 1
  19. *
  20. * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  21. * wasn't 1 originally. This function MUST leave the value lower than 1
  22. * even when the "1" assertion wasn't true.
  23. */
  24. static inline void
  25. __mutex_fastpath_lock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
  26. {
  27. if (unlikely(atomic_xchg(count, 0) != 1))
  28. fail_fn(count);
  29. else
  30. smp_mb();
  31. }
  32. /**
  33. * __mutex_fastpath_lock_retval - try to take the lock by moving the count
  34. * from 1 to a 0 value
  35. * @count: pointer of type atomic_t
  36. * @fail_fn: function to call if the original value was not 1
  37. *
  38. * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
  39. * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
  40. * or anything the slow path function returns
  41. */
  42. static inline int
  43. __mutex_fastpath_lock_retval(atomic_t *count, fastcall int (*fail_fn)(atomic_t *))
  44. {
  45. if (unlikely(atomic_xchg(count, 0) != 1))
  46. return fail_fn(count);
  47. else {
  48. smp_mb();
  49. return 0;
  50. }
  51. }
  52. /**
  53. * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
  54. * @count: pointer of type atomic_t
  55. * @fail_fn: function to call if the original value was not 0
  56. *
  57. * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
  58. * In the failure case, this function is allowed to either set the value to
  59. * 1, or to set it to a value lower than one.
  60. * If the implementation sets it to a value of lower than one, the
  61. * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
  62. * to return 0 otherwise.
  63. */
  64. static inline void
  65. __mutex_fastpath_unlock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
  66. {
  67. smp_mb();
  68. if (unlikely(atomic_xchg(count, 1) != 0))
  69. fail_fn(count);
  70. }
  71. #define __mutex_slowpath_needs_to_unlock() 0
  72. /**
  73. * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
  74. *
  75. * @count: pointer of type atomic_t
  76. * @fail_fn: spinlock based trylock implementation
  77. *
  78. * Change the count from 1 to a value lower than 1, and return 0 (failure)
  79. * if it wasn't 1 originally, or return 1 (success) otherwise. This function
  80. * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
  81. * Additionally, if the value was < 0 originally, this function must not leave
  82. * it to 0 on failure.
  83. *
  84. * If the architecture has no effective trylock variant, it should call the
  85. * <fail_fn> spinlock-based trylock variant unconditionally.
  86. */
  87. static inline int
  88. __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
  89. {
  90. int prev = atomic_xchg(count, 0);
  91. if (unlikely(prev < 0)) {
  92. /*
  93. * The lock was marked contended so we must restore that
  94. * state. If while doing so we get back a prev value of 1
  95. * then we just own it.
  96. *
  97. * [ In the rare case of the mutex going to 1, to 0, to -1
  98. * and then back to 0 in this few-instructions window,
  99. * this has the potential to trigger the slowpath for the
  100. * owner's unlock path needlessly, but that's not a problem
  101. * in practice. ]
  102. */
  103. prev = atomic_xchg(count, prev);
  104. if (prev < 0)
  105. prev = 0;
  106. }
  107. smp_mb();
  108. return prev;
  109. }
  110. #endif