lib/ovs-atomic.h

   1 /*
   2  * Copyright (c) 2013 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #ifndef OVS_ATOMIC_H
  18 #define OVS_ATOMIC_H 1
  19
  20 /* Atomic operations.
  21  *
  22  * This library implements atomic operations with an API based on the one
  23  * defined in C11.  It includes multiple implementations for compilers and
  24  * libraries with varying degrees of built-in support for C11, including a
  25  * fallback implementation for systems that have pthreads but no other support
  26  * for atomics.
  27  *
  28  * This comment describes the common features of all the implementations.
  29  *
  30  *
  31  * Types
  32  * =====
  33  *
  34  * The following atomic types are supported as typedefs for atomic versions of
  35  * the listed ordinary types:
  36  *
  37  *     ordinary type            atomic version
  38  *     -------------------      ----------------------
  39  *     bool                     atomic_bool
  40  *
  41  *     char                     atomic_char
  42  *     signed char              atomic_schar
  43  *     unsigned char            atomic_uchar
  44  *
  45  *     short                    atomic_short
  46  *     unsigned short           atomic_ushort
  47  *
  48  *     int                      atomic_int
  49  *     unsigned int             atomic_uint
  50  *
  51  *     long                     atomic_long
  52  *     unsigned long            atomic_ulong
  53  *
  54  *     long long                atomic_llong
  55  *     unsigned long long       atomic_ullong
  56  *
  57  *     size_t                   atomic_size_t
  58  *     ptrdiff_t                atomic_ptrdiff_t
  59  *
  60  *     intmax_t                 atomic_intmax_t
  61  *     uintmax_t                atomic_uintmax_t
  62  *
  63  *     intptr_t                 atomic_intptr_t
  64  *     uintptr_t                atomic_uintptr_t
  65  *
  66  *     uint8_t                  atomic_uint8_t     (*)
  67  *     uint16_t                 atomic_uint16_t    (*)
  68  *     uint32_t                 atomic_uint32_t    (*)
  69  *     int8_t                   atomic_int8_t      (*)
  70  *     int16_t                  atomic_int16_t     (*)
  71  *     int32_t                  atomic_int32_t     (*)
  72  *     uint64_t                 atomic_uint64_t    (*)
  73  *     int64_t                  atomic_int64_t     (*)
  74  *
  75  *     (*) Not specified by C11.
  76  *
  77  * The atomic version of a type doesn't necessarily have the same size or
  78  * representation as the ordinary version; for example, atomic_int might be a
  79  * typedef for a struct that also includes a mutex.  The range of an atomic
  80  * type does match the range of the corresponding ordinary type.
  81  *
  82  * C11 says that one may use the _Atomic keyword in place of the typedef name,
  83  * e.g. "_Atomic int" instead of "atomic_int".  This library doesn't support
  84  * that.
  85  *
  86  *
  87  * Life Cycle
  88  * ==========
  89  *
  90  * To initialize an atomic variable at its point of definition, use
  91  * ATOMIC_VAR_INIT:
  92  *
  93  *     static atomic_int ai = ATOMIC_VAR_INIT(123);
  94  *
  95  * To initialize an atomic variable in code, use atomic_init():
  96  *
  97  *     static atomic_int ai;
  98  * ...
  99  *     atomic_init(&ai, 123);
 100  *
 101  * C11 does not hav an destruction function for atomic types, but some
 102  * implementations of the OVS atomics do need them.  Thus, the following
 103  * function is provided for destroying non-static atomic objects (A is any
 104  * atomic type):
 105  *
 106  *     void atomic_destroy(A *object);
 107  *
 108  *         Destroys 'object'.
 109  *
 110  *
 111  * Barriers
 112  * ========
 113  *
 114  * enum memory_order specifies the strictness of a memory barrier.  It has the
 115  * following values:
 116  *
 117  *    memory_order_relaxed:
 118  *
 119  *        Compiler barrier only.  Does not imply any CPU memory ordering.
 120  *
 121  *    memory_order_acquire:
 122  *
 123  *        Memory accesses after an acquire barrier cannot be moved before the
 124  *        barrier.  Memory accesses before an acquire barrier *can* be moved
 125  *        after it.
 126  *
 127  *    memory_order_release:
 128  *
 129  *        Memory accesses before a release barrier cannot be moved after the
 130  *        barrier.  Memory accesses after a release barrier *can* be moved
 131  *        before it.
 132  *
 133  *    memory_order_acq_rel:
 134  *
 135  *        Memory accesses cannot be moved across an acquire-release barrier in
 136  *        either direction.
 137  *
 138  *    memory_order_seq_cst:
 139  *
 140  *        Prevents movement of memory accesses like an acquire-release barrier,
 141  *        but whereas acquire-release synchronizes cooperating threads,
 142  *        sequential-consistency synchronizes the whole system.
 143  *
 144  *    memory_order_consume:
 145  *
 146  *        A slight relaxation of memory_order_acquire.
 147  *
 148  * The following functions insert explicit barriers.  Most of the other atomic
 149  * functions also include barriers.
 150  *
 151  *     void atomic_thread_fence(memory_order order);
 152  *
 153  *         Inserts a barrier of the specified type.
 154  *
 155  *         For memory_order_relaxed, this is a no-op.
 156  *
 157  *     void atomic_signal_fence(memory_order order);
 158  *
 159  *         Inserts a barrier of the specified type, but only with respect to
 160  *         signal handlers in the same thread as the barrier.  This is
 161  *         basically a compiler optimization barrier, except for
 162  *         memory_order_relaxed, which is a no-op.
 163  *
 164  *
 165  * Atomic Operations
 166  * =================
 167  *
 168  * In this section, A is an atomic type and C is the corresponding non-atomic
 169  * type.
 170  *
 171  * The "store" primitives match C11:
 172  *
 173  *     void atomic_store(A *object, C value);
 174  *     void atomic_store_explicit(A *object, C value, memory_order);
 175  *
 176  *         Atomically stores 'value' into '*object', respecting the given
 177  *         memory order (or memory_order_seq_cst for atomic_store()).
 178  *
 179  * The following primitives differ from the C11 ones (and have different names)
 180  * because there does not appear to be a way to implement the standard
 181  * primitives in standard C:
 182  *
 183  *     void atomic_read(A *src, C *dst);
 184  *     void atomic_read_explicit(A *src, C *dst, memory_order);
 185  *
 186  *         Atomically loads a value from 'src', writing the value read into
 187  *         '*dst', respecting the given memory order (or memory_order_seq_cst
 188  *         for atomic_read()).
 189  *
 190  *     void atomic_add(A *rmw, C arg, C *orig);
 191  *     void atomic_sub(A *rmw, C arg, C *orig);
 192  *     void atomic_or(A *rmw, C arg, C *orig);
 193  *     void atomic_xor(A *rmw, C arg, C *orig);
 194  *     void atomic_and(A *rmw, C arg, C *orig);
 195  *     void atomic_add_explicit(A *rmw, C arg, C *orig, memory_order);
 196  *     void atomic_sub_explicit(A *rmw, C arg, C *orig, memory_order);
 197  *     void atomic_or_explicit(A *rmw, C arg, C *orig, memory_order);
 198  *     void atomic_xor_explicit(A *rmw, C arg, C *orig, memory_order);
 199  *     void atomic_and_explicit(A *rmw, C arg, C *orig, memory_order);
 200  *
 201  *         Atomically applies the given operation, with 'arg' as the second
 202  *         operand, to '*rmw', and stores the original value of '*rmw' into
 203  *         '*orig', respecting the given memory order (or memory_order_seq_cst
 204  *         if none is specified).
 205  *
 206  *         The results are similar to those that would be obtained with +=, -=,
 207  *         |=, ^=, or |= on non-atomic types.
 208  *
 209  *
 210  * atomic_flag
 211  * ===========
 212  *
 213  * atomic_flag is a typedef for a type with two states, set and clear, that
 214  * provides atomic test-and-set functionality.
 215  *
 216  *
 217  * Life Cycle
 218  * ----------
 219  *
 220  * ATOMIC_FLAG_INIT is an initializer for atomic_flag.  The initial state is
 221  * "clear".
 222  *
 223  * C11 does not have an initialization or destruction function for atomic_flag,
 224  * because implementations should not need one (one may simply
 225  * atomic_flag_clear() an uninitialized atomic_flag), but some implementations
 226  * of the OVS atomics do need them.  Thus, the following two functions are
 227  * provided for initializing and destroying non-static atomic_flags:
 228  *
 229  *     void atomic_flag_init(volatile atomic_flag *object);
 230  *
 231  *         Initializes 'object'.  The initial state is "clear".
 232  *
 233  *     void atomic_flag_destroy(volatile atomic_flag *object);
 234  *
 235  *         Destroys 'object'.
 236  *
 237  *
 238  * Operations
 239  * ----------
 240  *
 241  * The following functions are available.
 242  *
 243  *     bool atomic_flag_test_and_set(atomic_flag *object)
 244  *     bool atomic_flag_test_and_set_explicit(atomic_flag *object,
 245  *                                            memory_order);
 246  *
 247  *         Atomically sets '*object', respsecting the given memory order (or
 248  *         memory_order_seq_cst for atomic_flag_test_and_set()).  Returns the
 249  *         previous value of the flag (false for clear, true for set).
 250  *
 251  *     void atomic_flag_clear(atomic_flag *object);
 252  *     void atomic_flag_clear_explicit(atomic_flag *object, memory_order);
 253  *
 254  *         Atomically clears '*object', respecting the given memory order (or
 255  *         memory_order_seq_cst for atomic_flag_clear()).
 256  */
 257
 258 #include <limits.h>
 259 #include <pthread.h>
 260 #include <stdbool.h>
 261 #include <stddef.h>
 262 #include <stdint.h>
 263 #include "compiler.h"
 264 #include "util.h"
 265
 266 #define IN_OVS_ATOMIC_H
 267     #if __CHECKER__
 268         /* sparse doesn't understand some GCC extensions we use. */
 269         #include "ovs-atomic-pthreads.h"
 270     #elif HAVE_STDATOMIC_H
 271         #include "ovs-atomic-c11.h"
 272     #elif __has_extension(c_atomic)
 273         #include "ovs-atomic-clang.h"
 274     #elif __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
 275         #include "ovs-atomic-gcc4.7+.h"
 276     #elif HAVE_GCC4_ATOMICS
 277         #include "ovs-atomic-gcc4+.h"
 278     #else
 279         #include "ovs-atomic-pthreads.h"
 280     #endif
 281 #undef IN_OVS_ATOMIC_H
 282
 283 #endif /* ovs-atomic.h */