DataObjectLockFree.hpp

/***************************************************************************
  tag: Peter Soetens  Mon Jan 19 14:11:26 CET 2004  DataObjectLockFree.hpp

                        DataObjectLockFree.hpp -  description
                           -------------------
    begin                : Mon January 19 2004
    copyright            : (C) 2004 Peter Soetens
    email                : peter.soetens@mech.kuleuven.ac.be

 ***************************************************************************
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#ifndef CORELIB_DATAOBJECT_LOCK_FREE_HPP
#define CORELIB_DATAOBJECT_LOCK_FREE_HPP


#include "../os/oro_arch.h"
#include "DataObjectInterface.hpp"

namespace RTT
{ namespace base {

    template<class T>
    class DataObjectLockFree
        : public DataObjectInterface<T>
    {
    public:
        typedef T DataType;

        const unsigned int MAX_THREADS; // = 2
    private:
        const unsigned int BUF_LEN; // = MAX_THREADS+2

        struct DataBuf {
            DataBuf()
                : data(), counter(), next()
            {
                oro_atomic_set(&counter, 0);
            }
            DataType data; mutable oro_atomic_t counter; DataBuf* next;
        };

        typedef DataBuf* volatile VolPtrType;
        typedef DataBuf  ValueType;
        typedef DataBuf* PtrType;

        VolPtrType read_ptr;
        VolPtrType write_ptr;

        DataBuf* data;
    public:

        DataObjectLockFree( const T& initial_value = T(), unsigned int max_threads = 2 )
            : MAX_THREADS(max_threads), BUF_LEN( max_threads + 2),
              read_ptr(0),
              write_ptr(0)
        {
            data = new DataBuf[BUF_LEN];
            read_ptr = &data[0];
            write_ptr = &data[1];
            data_sample(initial_value);
        }

        ~DataObjectLockFree() {
            delete[] data;
        }

        virtual DataType Get() const {DataType cache; Get(cache); return cache; }

        virtual void Get( DataType& pull ) const
        {
            PtrType reading;
            // loop to combine Read/Modify of counter
            // This avoids a race condition where read_ptr
            // could become write_ptr ( then we would read corrupted data).
            do {
                reading = read_ptr;            // copy buffer location
                oro_atomic_inc(&reading->counter); // lock buffer, no more writes
                // XXX smp_mb
                if ( reading != read_ptr )     // if read_ptr changed,
                    oro_atomic_dec(&reading->counter); // better to start over.
                else
                    break;
            } while ( true );
            // from here on we are sure that 'reading'
            // is a valid buffer to read from.
            pull = reading->data;               // takes some time
            // XXX smp_mb
            oro_atomic_dec(&reading->counter);       // release buffer
        }

        virtual void Set( const DataType& push )
        {
            // writeout in any case
            write_ptr->data = push;
            PtrType wrote_ptr = write_ptr;
            // if next field is occupied (by read_ptr or counter),
            // go to next and check again...
            while ( oro_atomic_read( &write_ptr->next->counter ) != 0 || write_ptr->next == read_ptr )
                {
                    write_ptr = write_ptr->next;
                    if (write_ptr == wrote_ptr)
                        return; // nothing found, to many readers !
                }

            // we will be able to move, so replace read_ptr
            read_ptr  = wrote_ptr;
            write_ptr = write_ptr->next; // we checked this in the while loop
        }

        virtual void data_sample( const DataType& sample ) {
            // prepare the buffer.
            for (unsigned int i = 0; i < BUF_LEN-1; ++i) {
                data[i].data = sample;
                data[i].next = &data[i+1];
            }
            data[BUF_LEN-1].data = sample;
            data[BUF_LEN-1].next = &data[0];
        }
    };
}}

#endif