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 /* SPDX-License-Identifier: BSD-2-Clause */
 
 /**
  * @file
  *
  * @ingroup RTEMSScoreSMPLock
  *
  * @brief This header file provides the interfaces of the
  *   @ref RTEMSScoreSMPLock related to sequence locks.
  */
 
 /*
  * Copyright (c) 2016 embedded brains GmbH & Co. KG
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
 #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[out] 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[out] 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[out] 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[out] 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[in, out] 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 The read operation need not be retried.
  */
 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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