1 | /** |
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2 | * @file |
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3 | * |
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4 | * @ingroup ScoreSMPLock |
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5 | * |
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6 | * @brief SMP Lock API |
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7 | */ |
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8 | |
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9 | /* |
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10 | * Copyright (c) 2016 embedded brains GmbH |
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11 | * |
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12 | * The license and distribution terms for this file may be |
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13 | * found in the file LICENSE in this distribution or at |
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14 | * http://www.rtems.org/license/LICENSE. |
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15 | */ |
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16 | |
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17 | #ifndef _RTEMS_SCORE_SMPLOCKSEQ_H |
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18 | #define _RTEMS_SCORE_SMPLOCKSEQ_H |
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19 | |
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20 | #include <rtems/score/cpuopts.h> |
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21 | |
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22 | #if defined(RTEMS_SMP) |
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23 | |
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24 | #include <rtems/score/assert.h> |
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25 | #include <rtems/score/atomic.h> |
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26 | |
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27 | #ifdef __cplusplus |
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28 | extern "C" { |
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29 | #endif /* __cplusplus */ |
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30 | |
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31 | /** |
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32 | * @addtogroup ScoreSMPLock |
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33 | * |
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34 | * @{ |
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35 | */ |
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36 | |
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37 | /** |
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38 | * @brief SMP sequence lock control. |
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39 | * |
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40 | * The sequence lock offers a consistent data set for readers in the presence |
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41 | * of at most one concurrent writer. Due to the read-modify-write operation in |
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42 | * _SMP_sequence_lock_Read_retry() the data corresponding to the last written |
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43 | * sequence number is observed. To allow multiple writers an additional SMP |
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44 | * lock is necessary to serialize writes. |
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45 | * |
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46 | * See also Hans-J. Boehm, HP Laboratories, |
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47 | * "Can Seqlocks Get Along With Programming Language Memory Models?", |
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48 | * http://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf |
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49 | */ |
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50 | typedef struct { |
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51 | /** |
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52 | * @brief The sequence number. |
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53 | * |
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54 | * An odd value indicates that a write is in progress. |
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55 | */ |
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56 | Atomic_Uint sequence; |
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57 | } SMP_sequence_lock_Control; |
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58 | |
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59 | /** |
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60 | * @brief SMP sequence lock control initializer for static initialization. |
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61 | */ |
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62 | #define SMP_SEQUENCE_LOCK_INITIALIZER { ATOMIC_INITIALIZER_UINT( 0 ) } |
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63 | |
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64 | /** |
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65 | * @brief Initializes an SMP sequence lock. |
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66 | * |
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67 | * Concurrent initialization leads to unpredictable results. |
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68 | * |
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69 | * @param lock The SMP sequence lock control. |
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70 | */ |
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71 | static inline void _SMP_sequence_lock_Initialize( SMP_sequence_lock_Control *lock ) |
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72 | { |
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73 | _Atomic_Init_uint( &lock->sequence, 0 ); |
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74 | } |
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75 | |
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76 | /** |
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77 | * @brief Destroys an SMP sequence lock. |
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78 | * |
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79 | * Concurrent destruction leads to unpredictable results. |
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80 | * |
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81 | * @param lock The SMP sequence lock control. |
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82 | */ |
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83 | static inline void _SMP_sequence_lock_Destroy( SMP_sequence_lock_Control *lock ) |
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84 | { |
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85 | (void) lock; |
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86 | } |
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87 | |
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88 | /** |
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89 | * @brief Begins an SMP sequence lock write operation. |
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90 | * |
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91 | * This function will not disable interrupts. The caller must ensure that the |
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92 | * current thread of execution is not interrupted indefinite since this would |
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93 | * starve readers. |
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94 | * |
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95 | * @param lock The SMP sequence lock control. |
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96 | * |
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97 | * @return The current sequence number. |
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98 | */ |
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99 | static inline unsigned int _SMP_sequence_lock_Write_begin( |
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100 | SMP_sequence_lock_Control *lock |
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101 | ) |
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102 | { |
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103 | unsigned int seq; |
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104 | |
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105 | seq = _Atomic_Load_uint( &lock->sequence, ATOMIC_ORDER_RELAXED ); |
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106 | _Assert( seq % 2 == 0 ); |
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107 | |
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108 | _Atomic_Store_uint( &lock->sequence, seq + 1, ATOMIC_ORDER_RELAXED ); |
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109 | |
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110 | /* There is no atomic store with acquire/release semantics */ |
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111 | _Atomic_Fence( ATOMIC_ORDER_ACQ_REL ); |
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112 | |
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113 | return seq; |
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114 | } |
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115 | |
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116 | /** |
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117 | * @brief Ends an SMP sequence lock write operation. |
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118 | * |
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119 | * @param lock The SMP sequence lock control. |
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120 | * @param seq The sequence number returned by _SMP_sequence_lock_Write_begin(). |
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121 | */ |
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122 | static inline void _SMP_sequence_lock_Write_end( |
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123 | SMP_sequence_lock_Control *lock, |
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124 | unsigned int seq |
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125 | ) |
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126 | { |
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127 | _Atomic_Store_uint( &lock->sequence, seq + 2, ATOMIC_ORDER_RELEASE ); |
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128 | } |
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129 | |
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130 | /** |
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131 | * @brief Begins an SMP sequence lock read operation. |
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132 | * |
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133 | * This function will not disable interrupts. |
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134 | * |
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135 | * @param lock The SMP sequence lock control. |
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136 | * |
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137 | * @return The current sequence number. |
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138 | */ |
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139 | static inline unsigned int _SMP_sequence_lock_Read_begin( |
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140 | const SMP_sequence_lock_Control *lock |
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141 | ) |
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142 | { |
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143 | return _Atomic_Load_uint( &lock->sequence, ATOMIC_ORDER_ACQUIRE ); |
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144 | } |
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145 | |
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146 | /** |
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147 | * @brief Ends an SMP sequence lock read operation and indicates if a retry is |
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148 | * necessary. |
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149 | * |
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150 | * @param lock The SMP sequence lock control. |
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151 | * @param seq The sequence number returned by _SMP_sequence_lock_Read_begin(). |
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152 | * |
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153 | * @retval true The read operation must be retried with a call to |
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154 | * _SMP_sequence_lock_Read_begin(). |
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155 | * @retval false Otherwise. |
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156 | */ |
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157 | static inline bool _SMP_sequence_lock_Read_retry( |
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158 | SMP_sequence_lock_Control *lock, |
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159 | unsigned int seq |
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160 | ) |
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161 | { |
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162 | unsigned int seq2; |
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163 | |
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164 | seq2 = _Atomic_Fetch_add_uint( &lock->sequence, 0, ATOMIC_ORDER_RELEASE ); |
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165 | return seq != seq2 || seq % 2 != 0; |
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166 | } |
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167 | |
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168 | /**@}*/ |
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169 | |
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170 | #ifdef __cplusplus |
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171 | } |
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172 | #endif /* __cplusplus */ |
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173 | |
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174 | #endif /* RTEMS_SMP */ |
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175 | |
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176 | #endif /* _RTEMS_SCORE_SMPLOCKSEQ_H */ |
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