lockdep.rst 5.0 KB

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  1. .. SPDX-License-Identifier: GPL-2.0
  2. ========================
  3. RCU and lockdep checking
  4. ========================
  5. All flavors of RCU have lockdep checking available, so that lockdep is
  6. aware of when each task enters and leaves any flavor of RCU read-side
  7. critical section. Each flavor of RCU is tracked separately (but note
  8. that this is not the case in 2.6.32 and earlier). This allows lockdep's
  9. tracking to include RCU state, which can sometimes help when debugging
  10. deadlocks and the like.
  11. In addition, RCU provides the following primitives that check lockdep's
  12. state::
  13. rcu_read_lock_held() for normal RCU.
  14. rcu_read_lock_bh_held() for RCU-bh.
  15. rcu_read_lock_sched_held() for RCU-sched.
  16. srcu_read_lock_held() for SRCU.
  17. These functions are conservative, and will therefore return 1 if they
  18. aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
  19. This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
  20. positives when lockdep is disabled.
  21. In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
  22. checking of rcu_dereference() primitives:
  23. rcu_dereference(p):
  24. Check for RCU read-side critical section.
  25. rcu_dereference_bh(p):
  26. Check for RCU-bh read-side critical section.
  27. rcu_dereference_sched(p):
  28. Check for RCU-sched read-side critical section.
  29. srcu_dereference(p, sp):
  30. Check for SRCU read-side critical section.
  31. rcu_dereference_check(p, c):
  32. Use explicit check expression "c" along with
  33. rcu_read_lock_held(). This is useful in code that is
  34. invoked by both RCU readers and updaters.
  35. rcu_dereference_bh_check(p, c):
  36. Use explicit check expression "c" along with
  37. rcu_read_lock_bh_held(). This is useful in code that
  38. is invoked by both RCU-bh readers and updaters.
  39. rcu_dereference_sched_check(p, c):
  40. Use explicit check expression "c" along with
  41. rcu_read_lock_sched_held(). This is useful in code that
  42. is invoked by both RCU-sched readers and updaters.
  43. srcu_dereference_check(p, c):
  44. Use explicit check expression "c" along with
  45. srcu_read_lock_held(). This is useful in code that
  46. is invoked by both SRCU readers and updaters.
  47. rcu_dereference_raw(p):
  48. Don't check. (Use sparingly, if at all.)
  49. rcu_dereference_protected(p, c):
  50. Use explicit check expression "c", and omit all barriers
  51. and compiler constraints. This is useful when the data
  52. structure cannot change, for example, in code that is
  53. invoked only by updaters.
  54. rcu_access_pointer(p):
  55. Return the value of the pointer and omit all barriers,
  56. but retain the compiler constraints that prevent duplicating
  57. or coalescsing. This is useful when when testing the
  58. value of the pointer itself, for example, against NULL.
  59. The rcu_dereference_check() check expression can be any boolean
  60. expression, but would normally include a lockdep expression. However,
  61. any boolean expression can be used. For a moderately ornate example,
  62. consider the following::
  63. file = rcu_dereference_check(fdt->fd[fd],
  64. lockdep_is_held(&files->file_lock) ||
  65. atomic_read(&files->count) == 1);
  66. This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
  67. and, if CONFIG_PROVE_RCU is configured, verifies that this expression
  68. is used in:
  69. 1. An RCU read-side critical section (implicit), or
  70. 2. with files->file_lock held, or
  71. 3. on an unshared files_struct.
  72. In case (1), the pointer is picked up in an RCU-safe manner for vanilla
  73. RCU read-side critical sections, in case (2) the ->file_lock prevents
  74. any change from taking place, and finally, in case (3) the current task
  75. is the only task accessing the file_struct, again preventing any change
  76. from taking place. If the above statement was invoked only from updater
  77. code, it could instead be written as follows::
  78. file = rcu_dereference_protected(fdt->fd[fd],
  79. lockdep_is_held(&files->file_lock) ||
  80. atomic_read(&files->count) == 1);
  81. This would verify cases #2 and #3 above, and furthermore lockdep would
  82. complain if this was used in an RCU read-side critical section unless one
  83. of these two cases held. Because rcu_dereference_protected() omits all
  84. barriers and compiler constraints, it generates better code than do the
  85. other flavors of rcu_dereference(). On the other hand, it is illegal
  86. to use rcu_dereference_protected() if either the RCU-protected pointer
  87. or the RCU-protected data that it points to can change concurrently.
  88. Like rcu_dereference(), when lockdep is enabled, RCU list and hlist
  89. traversal primitives check for being called from within an RCU read-side
  90. critical section. However, a lockdep expression can be passed to them
  91. as a additional optional argument. With this lockdep expression, these
  92. traversal primitives will complain only if the lockdep expression is
  93. false and they are called from outside any RCU read-side critical section.
  94. For example, the workqueue for_each_pwq() macro is intended to be used
  95. either within an RCU read-side critical section or with wq->mutex held.
  96. It is thus implemented as follows::
  97. #define for_each_pwq(pwq, wq)
  98. list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node,
  99. lock_is_held(&(wq->mutex).dep_map))