include/urcu/static/pointer.h

   1 // SPDX-FileCopyrightText: 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   2 // SPDX-FileCopyrightText: 2009 Paul E. McKenney, IBM Corporation.
   3 //
   4 // SPDX-License-Identifier: LGPL-2.1-or-later
   5
   6 #ifndef _URCU_POINTER_STATIC_H
   7 #define _URCU_POINTER_STATIC_H
   8
   9 /*
  10  * Userspace RCU header. Operations on pointers.
  11  *
  12  * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
  13  * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
  14  *
  15  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  16  */
  17
  18 #include <urcu/compiler.h>
  19 #include <urcu/arch.h>
  20 #include <urcu/system.h>
  21 #include <urcu/uatomic.h>
  22
  23 #ifdef __cplusplus
  24 extern "C" {
  25 #endif
  26
  27 /**
  28  * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
  29  * into a RCU read-side critical section. The pointer can later be safely
  30  * dereferenced within the critical section.
  31  *
  32  * This ensures that the pointer copy is invariant thorough the whole critical
  33  * section.
  34  *
  35  * Inserts memory barriers on architectures that require them (currently only
  36  * Alpha) and documents which pointers are protected by RCU.
  37  *
  38  * With C standards prior to C11/C++11, the compiler memory barrier in
  39  * CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
  40  * VSS: Value Speculation Scheduling) does not perform the data read
  41  * before the pointer read by speculating the value of the pointer.
  42  * Correct ordering is ensured because the pointer is read as a volatile
  43  * access. This acts as a global side-effect operation, which forbids
  44  * reordering of dependent memory operations.
  45  *
  46  * With C standards C11/C++11, concerns about dependency-breaking
  47  * optimizations are taken care of by the "memory_order_consume" atomic
  48  * load.
  49  *
  50  * Use the gcc __atomic_load() rather than C11/C++11 atomic load
  51  * explicit because the pointer used as input argument is a pointer,
  52  * not an _Atomic type as required by C11/C++11.
  53  *
  54  * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
  55  * volatile access to implement rcu_dereference rather than
  56  * memory_order_consume load from the C11/C++11 standards.
  57  *
  58  * This may improve performance on weakly-ordered architectures where
  59  * the compiler implements memory_order_consume as a
  60  * memory_order_acquire, which is stricter than required by the
  61  * standard.
  62  *
  63  * Note that using volatile accesses for rcu_dereference may cause
  64  * LTO to generate incorrectly ordered code starting from C11/C++11.
  65  *
  66  * Should match rcu_assign_pointer() or rcu_xchg_pointer().
  67  *
  68  * This macro is less than 10 lines long.  The intent is that this macro
  69  * meets the 10-line criterion in LGPL, allowing this function to be
  70  * expanded directly in non-LGPL code.
  71  */
  72
  73 #if !defined (URCU_DEREFERENCE_USE_VOLATILE) &&         \
  74         ((defined (__cplusplus) && __cplusplus >= 201103L) ||   \
  75         (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
  76 # define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
  77 #endif
  78
  79 /*
  80  * If p is const (the pointer itself, not what it points to), using
  81  * __typeof__(p) would declare a const variable, leading to
  82  * -Wincompatible-pointer-types errors.  Using the statement expression
  83  * makes it an rvalue and gets rid of the const-ness.
  84  */
  85 # define _rcu_dereference(p)                    \
  86         uatomic_load(&(p), CMM_CONSUME)
  87 /**
  88  * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
  89  * is as expected by "old". If succeeds, returns the previous pointer to the
  90  * data structure, which can be safely freed after waiting for a quiescent state
  91  * using synchronize_rcu(). If fails (unexpected value), returns old (which
  92  * should not be freed !).
  93  *
  94  * uatomic_cmpxchg() acts as both release and acquire barriers on success.
  95  *
  96  * This macro is less than 10 lines long.  The intent is that this macro
  97  * meets the 10-line criterion in LGPL, allowing this function to be
  98  * expanded directly in non-LGPL code.
  99  */
 100 #define _rcu_cmpxchg_pointer(p, old, _new)                              \
 101         __extension__                                                   \
 102         ({                                                              \
 103                 __typeof__(*p) _________pold = (old);                   \
 104                 __typeof__(*p) _________pnew = (_new);                  \
 105                 uatomic_cmpxchg_mo(p, _________pold, _________pnew,     \
 106                                    CMM_SEQ_CST, CMM_RELAXED);           \
 107         });
 108
 109 /**
 110  * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
 111  * pointer to the data structure, which can be safely freed after waiting for a
 112  * quiescent state using synchronize_rcu().
 113  *
 114  * uatomic_xchg() acts as both release and acquire barriers.
 115  *
 116  * This macro is less than 10 lines long.  The intent is that this macro
 117  * meets the 10-line criterion in LGPL, allowing this function to be
 118  * expanded directly in non-LGPL code.
 119  */
 120 #define _rcu_xchg_pointer(p, v)                         \
 121         __extension__                                   \
 122         ({                                              \
 123                 __typeof__(*p) _________pv = (v);       \
 124                 uatomic_xchg_mo(p, _________pv,         \
 125                                 CMM_SEQ_CST);           \
 126         })
 127
 128
 129 #define _rcu_set_pointer(p, v)                                          \
 130         do {                                                            \
 131                 __typeof__(*p) _________pv = (v);                       \
 132                 uatomic_store(p, _________pv,                           \
 133                         __builtin_constant_p(v) && (v) == NULL ?        \
 134                         CMM_RELAXED : CMM_RELEASE);                     \
 135         } while (0)
 136
 137 /**
 138  * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
 139  * meant to be read by RCU read-side critical sections. Returns the assigned
 140  * value.
 141  *
 142  * Documents which pointers will be dereferenced by RCU read-side critical
 143  * sections and adds the required memory barriers on architectures requiring
 144  * them. It also makes sure the compiler does not reorder code initializing the
 145  * data structure before its publication.
 146  *
 147  * Should match rcu_dereference().
 148  *
 149  * This macro is less than 10 lines long.  The intent is that this macro
 150  * meets the 10-line criterion in LGPL, allowing this function to be
 151  * expanded directly in non-LGPL code.
 152  */
 153 #define _rcu_assign_pointer(p, v)       _rcu_set_pointer(&(p), v)
 154
 155 #ifdef __cplusplus
 156 }
 157 #endif
 158
 159 #endif /* _URCU_POINTER_STATIC_H */