mcs_spinlock.h 3.7 KB

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  1. /* SPDX-License-Identifier: GPL-2.0 */
  2. /*
  3. * MCS lock defines
  4. *
  5. * This file contains the main data structure and API definitions of MCS lock.
  6. *
  7. * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
  8. * with the desirable properties of being fair, and with each cpu trying
  9. * to acquire the lock spinning on a local variable.
  10. * It avoids expensive cache bouncings that common test-and-set spin-lock
  11. * implementations incur.
  12. */
  13. #ifndef __LINUX_MCS_SPINLOCK_H
  14. #define __LINUX_MCS_SPINLOCK_H
  15. #include <asm/mcs_spinlock.h>
  16. struct mcs_spinlock {
  17. struct mcs_spinlock *next;
  18. int locked; /* 1 if lock acquired */
  19. int count; /* nesting count, see qspinlock.c */
  20. };
  21. #ifndef arch_mcs_spin_lock_contended
  22. /*
  23. * Using smp_cond_load_acquire() provides the acquire semantics
  24. * required so that subsequent operations happen after the
  25. * lock is acquired. Additionally, some architectures such as
  26. * ARM64 would like to do spin-waiting instead of purely
  27. * spinning, and smp_cond_load_acquire() provides that behavior.
  28. */
  29. #define arch_mcs_spin_lock_contended(l) \
  30. do { \
  31. smp_cond_load_acquire(l, VAL); \
  32. } while (0)
  33. #endif
  34. #ifndef arch_mcs_spin_unlock_contended
  35. /*
  36. * smp_store_release() provides a memory barrier to ensure all
  37. * operations in the critical section has been completed before
  38. * unlocking.
  39. */
  40. #define arch_mcs_spin_unlock_contended(l) \
  41. smp_store_release((l), 1)
  42. #endif
  43. /*
  44. * Note: the smp_load_acquire/smp_store_release pair is not
  45. * sufficient to form a full memory barrier across
  46. * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
  47. * For applications that need a full barrier across multiple cpus
  48. * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
  49. * used after mcs_lock.
  50. */
  51. /*
  52. * In order to acquire the lock, the caller should declare a local node and
  53. * pass a reference of the node to this function in addition to the lock.
  54. * If the lock has already been acquired, then this will proceed to spin
  55. * on this node->locked until the previous lock holder sets the node->locked
  56. * in mcs_spin_unlock().
  57. */
  58. static inline
  59. void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  60. {
  61. struct mcs_spinlock *prev;
  62. /* Init node */
  63. node->locked = 0;
  64. node->next = NULL;
  65. /*
  66. * We rely on the full barrier with global transitivity implied by the
  67. * below xchg() to order the initialization stores above against any
  68. * observation of @node. And to provide the ACQUIRE ordering associated
  69. * with a LOCK primitive.
  70. */
  71. prev = xchg(lock, node);
  72. if (likely(prev == NULL)) {
  73. /*
  74. * Lock acquired, don't need to set node->locked to 1. Threads
  75. * only spin on its own node->locked value for lock acquisition.
  76. * However, since this thread can immediately acquire the lock
  77. * and does not proceed to spin on its own node->locked, this
  78. * value won't be used. If a debug mode is needed to
  79. * audit lock status, then set node->locked value here.
  80. */
  81. return;
  82. }
  83. WRITE_ONCE(prev->next, node);
  84. /* Wait until the lock holder passes the lock down. */
  85. arch_mcs_spin_lock_contended(&node->locked);
  86. }
  87. /*
  88. * Releases the lock. The caller should pass in the corresponding node that
  89. * was used to acquire the lock.
  90. */
  91. static inline
  92. void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
  93. {
  94. struct mcs_spinlock *next = READ_ONCE(node->next);
  95. if (likely(!next)) {
  96. /*
  97. * Release the lock by setting it to NULL
  98. */
  99. if (likely(cmpxchg_release(lock, node, NULL) == node))
  100. return;
  101. /* Wait until the next pointer is set */
  102. while (!(next = READ_ONCE(node->next)))
  103. cpu_relax();
  104. }
  105. /* Pass lock to next waiter. */
  106. arch_mcs_spin_unlock_contended(&next->locked);
  107. }
  108. #endif /* __LINUX_MCS_SPINLOCK_H */