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/**
* @file
*
* @ingroup RTEMSScoreSMPLock
*
* @brief SMP Lock API
*/
/*
* Copyright (c) 2016 embedded brains GmbH
*
* The license and distribution terms for this file may be
* found in the file LICENSE in this distribution or at
* http://www.rtems.org/license/LICENSE.
*/
#ifndef _RTEMS_SCORE_SMPLOCKSEQ_H
#define _RTEMS_SCORE_SMPLOCKSEQ_H
#include <rtems/score/cpuopts.h>
#if defined(RTEMS_SMP)
#include <rtems/score/assert.h>
#include <rtems/score/atomic.h>
#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */
/**
* @addtogroup RTEMSScoreSMPLock
*
* @{
*/
/**
* @brief SMP sequence lock control.
*
* The sequence lock offers a consistent data set for readers in the presence
* of at most one concurrent writer. Due to the read-modify-write operation in
* _SMP_sequence_lock_Read_retry() the data corresponding to the last written
* sequence number is observed. To allow multiple writers an additional SMP
* lock is necessary to serialize writes.
*
* See also Hans-J. Boehm, HP Laboratories,
* "Can Seqlocks Get Along With Programming Language Memory Models?",
* http://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf
*/
typedef struct {
/**
* @brief The sequence number.
*
* An odd value indicates that a write is in progress.
*/
Atomic_Uint sequence;
} SMP_sequence_lock_Control;
/**
* @brief SMP sequence lock control initializer for static initialization.
*/
#define SMP_SEQUENCE_LOCK_INITIALIZER { ATOMIC_INITIALIZER_UINT( 0 ) }
/**
* @brief Initializes an SMP sequence lock.
*
* Concurrent initialization leads to unpredictable results.
*
* @param lock The SMP sequence lock control.
*/
static inline void _SMP_sequence_lock_Initialize( SMP_sequence_lock_Control *lock )
{
_Atomic_Init_uint( &lock->sequence, 0 );
}
/**
* @brief Destroys an SMP sequence lock.
*
* Concurrent destruction leads to unpredictable results.
*
* @param lock The SMP sequence lock control.
*/
static inline void _SMP_sequence_lock_Destroy( SMP_sequence_lock_Control *lock )
{
(void) lock;
}
/**
* @brief Begins an SMP sequence lock write operation.
*
* This function will not disable interrupts. The caller must ensure that the
* current thread of execution is not interrupted indefinite since this would
* starve readers.
*
* @param lock The SMP sequence lock control.
*
* @return The current sequence number.
*/
static inline unsigned int _SMP_sequence_lock_Write_begin(
SMP_sequence_lock_Control *lock
)
{
unsigned int seq;
seq = _Atomic_Load_uint( &lock->sequence, ATOMIC_ORDER_RELAXED );
_Assert( seq % 2 == 0 );
_Atomic_Store_uint( &lock->sequence, seq + 1, ATOMIC_ORDER_RELAXED );
/* There is no atomic store with acquire/release semantics */
_Atomic_Fence( ATOMIC_ORDER_ACQ_REL );
return seq;
}
/**
* @brief Ends an SMP sequence lock write operation.
*
* @param lock The SMP sequence lock control.
* @param seq The sequence number returned by _SMP_sequence_lock_Write_begin().
*/
static inline void _SMP_sequence_lock_Write_end(
SMP_sequence_lock_Control *lock,
unsigned int seq
)
{
_Atomic_Store_uint( &lock->sequence, seq + 2, ATOMIC_ORDER_RELEASE );
}
/**
* @brief Begins an SMP sequence lock read operation.
*
* This function will not disable interrupts.
*
* @param lock The SMP sequence lock control.
*
* @return The current sequence number.
*/
static inline unsigned int _SMP_sequence_lock_Read_begin(
const SMP_sequence_lock_Control *lock
)
{
return _Atomic_Load_uint( &lock->sequence, ATOMIC_ORDER_ACQUIRE );
}
/**
* @brief Ends an SMP sequence lock read operation and indicates if a retry is
* necessary.
*
* @param lock The SMP sequence lock control.
* @param seq The sequence number returned by _SMP_sequence_lock_Read_begin().
*
* @retval true The read operation must be retried with a call to
* _SMP_sequence_lock_Read_begin().
* @retval false Otherwise.
*/
static inline bool _SMP_sequence_lock_Read_retry(
SMP_sequence_lock_Control *lock,
unsigned int seq
)
{
unsigned int seq2;
seq2 = _Atomic_Fetch_add_uint( &lock->sequence, 0, ATOMIC_ORDER_RELEASE );
return seq != seq2 || seq % 2 != 0;
}
/**@}*/
#ifdef __cplusplus
}
#endif /* __cplusplus */
#endif /* RTEMS_SMP */
#endif /* _RTEMS_SCORE_SMPLOCKSEQ_H */
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