include/urcu/static/pointer.h

   1 #ifndef _URCU_POINTER_STATIC_H
   2 #define _URCU_POINTER_STATIC_H
   3
   4 /*
   5  * urcu/static/pointer.h
   6  *
   7  * Userspace RCU header. Operations on pointers.
   8  *
   9  * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
  10  * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
  11  *
  12  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  13  * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
  14  *
  15  * This library is free software; you can redistribute it and/or
  16  * modify it under the terms of the GNU Lesser General Public
  17  * License as published by the Free Software Foundation; either
  18  * version 2.1 of the License, or (at your option) any later version.
  19  *
  20  * This library is distributed in the hope that it will be useful,
  21  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  22  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  23  * Lesser General Public License for more details.
  24  *
  25  * You should have received a copy of the GNU Lesser General Public
  26  * License along with this library; if not, write to the Free Software
  27  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  28  *
  29  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  30  */
  31
  32 #include <urcu/compiler.h>
  33 #include <urcu/arch.h>
  34 #include <urcu/system.h>
  35 #include <urcu/uatomic.h>
  36
  37 #ifdef __cplusplus
  38 extern "C" {
  39 #endif
  40
  41 /**
  42  * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
  43  * into a RCU read-side critical section. The pointer can later be safely
  44  * dereferenced within the critical section.
  45  *
  46  * This ensures that the pointer copy is invariant thorough the whole critical
  47  * section.
  48  *
  49  * Inserts memory barriers on architectures that require them (currently only
  50  * Alpha) and documents which pointers are protected by RCU.
  51  *
  52  * With C standards prior to C11/C++11, the compiler memory barrier in
  53  * CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
  54  * VSS: Value Speculation Scheduling) does not perform the data read
  55  * before the pointer read by speculating the value of the pointer.
  56  * Correct ordering is ensured because the pointer is read as a volatile
  57  * access. This acts as a global side-effect operation, which forbids
  58  * reordering of dependent memory operations.
  59  *
  60  * With C standards C11/C++11, concerns about dependency-breaking
  61  * optimizations are taken care of by the "memory_order_consume" atomic
  62  * load.
  63  *
  64  * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
  65  * volatile access to implement rcu_dereference rather than
  66  * memory_order_consume load from the C11/C++11 standards.
  67  *
  68  * This may improve performance on weakly-ordered architectures where
  69  * the compiler implements memory_order_consume as a
  70  * memory_order_acquire, which is stricter than required by the
  71  * standard.
  72  *
  73  * Note that using volatile accesses for rcu_dereference may cause
  74  * LTO to generate incorrectly ordered code starting from C11/C++11.
  75  *
  76  * Should match rcu_assign_pointer() or rcu_xchg_pointer().
  77  *
  78  * This macro is less than 10 lines long.  The intent is that this macro
  79  * meets the 10-line criterion in LGPL, allowing this function to be
  80  * expanded directly in non-LGPL code.
  81  */
  82
  83 #ifdef URCU_DEREFERENCE_USE_VOLATILE
  84 # define __rcu_dereference(p)   CMM_LOAD_SHARED(p)
  85 #else
  86 # if defined (__cplusplus)
  87 #  if __cplusplus >= 201103L
  88 #   include <atomic>
  89 #   define __rcu_dereference(p) ((std::atomic<__typeof__(p)>)(p)).load(std::memory_order_consume)
  90 #  else
  91 #   define __rcu_dereference(p) CMM_LOAD_SHARED(x)
  92 #  endif
  93 # else
  94 #  if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
  95 #   include <stdatomic.h>
  96 #   define __rcu_dereference(p) atomic_load_explicit(&(p), memory_order_consume)
  97 #  else
  98 #   define __rcu_dereference(p) CMM_LOAD_SHARED(p)
  99 #  endif
 100 # endif
 101 #endif
 102
 103 #define _rcu_dereference(p)                                             \
 104                                 __extension__                           \
 105                                 ({                                      \
 106                                 __typeof__(p) _________p1 = __rcu_dereference(p); \
 107                                 cmm_smp_read_barrier_depends();         \
 108                                 (_________p1);                          \
 109                                 })
 110
 111 /**
 112  * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
 113  * is as expected by "old". If succeeds, returns the previous pointer to the
 114  * data structure, which can be safely freed after waiting for a quiescent state
 115  * using synchronize_rcu(). If fails (unexpected value), returns old (which
 116  * should not be freed !).
 117  *
 118  * uatomic_cmpxchg() acts as both release and acquire barriers.
 119  *
 120  * This macro is less than 10 lines long.  The intent is that this macro
 121  * meets the 10-line criterion in LGPL, allowing this function to be
 122  * expanded directly in non-LGPL code.
 123  */
 124 #define _rcu_cmpxchg_pointer(p, old, _new)                              \
 125         __extension__                                                   \
 126         ({                                                              \
 127                 __typeof__(*p) _________pold = (old);                   \
 128                 __typeof__(*p) _________pnew = (_new);                  \
 129                 uatomic_cmpxchg(p, _________pold, _________pnew);       \
 130         })
 131
 132 /**
 133  * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
 134  * pointer to the data structure, which can be safely freed after waiting for a
 135  * quiescent state using synchronize_rcu().
 136  *
 137  * uatomic_xchg() acts as both release and acquire barriers.
 138  *
 139  * This macro is less than 10 lines long.  The intent is that this macro
 140  * meets the 10-line criterion in LGPL, allowing this function to be
 141  * expanded directly in non-LGPL code.
 142  */
 143 #define _rcu_xchg_pointer(p, v)                         \
 144         __extension__                                   \
 145         ({                                              \
 146                 __typeof__(*p) _________pv = (v);       \
 147                 uatomic_xchg(p, _________pv);           \
 148         })
 149
 150
 151 #define _rcu_set_pointer(p, v)                          \
 152         do {                                            \
 153                 __typeof__(*p) _________pv = (v);       \
 154                 if (!__builtin_constant_p(v) ||         \
 155                     ((v) != NULL))                      \
 156                         cmm_wmb();                              \
 157                 uatomic_set(p, _________pv);            \
 158         } while (0)
 159
 160 /**
 161  * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
 162  * meant to be read by RCU read-side critical sections. Returns the assigned
 163  * value.
 164  *
 165  * Documents which pointers will be dereferenced by RCU read-side critical
 166  * sections and adds the required memory barriers on architectures requiring
 167  * them. It also makes sure the compiler does not reorder code initializing the
 168  * data structure before its publication.
 169  *
 170  * Should match rcu_dereference().
 171  *
 172  * This macro is less than 10 lines long.  The intent is that this macro
 173  * meets the 10-line criterion in LGPL, allowing this function to be
 174  * expanded directly in non-LGPL code.
 175  */
 176 #define _rcu_assign_pointer(p, v)       _rcu_set_pointer(&(p), v)
 177
 178 #ifdef __cplusplus
 179 }
 180 #endif
 181
 182 #endif /* _URCU_POINTER_STATIC_H */