[05b7eec] | 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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[64ed0bb3] | 24 | #include <rtems/score/assert.h> |
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[05b7eec] | 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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[64ed0bb3] | 106 | _Assert( seq % 2 == 0 ); |
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| 107 | |
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[05b7eec] | 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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