Mutex Flag Define

I defined 3 read flag and 2 write flag.

int reader[3], writer[2]; // reader/writer flag

Lock/Free Function

I define read lock function for the readers, the function wait until all the writer flags are off.

Then multilple reader can read concurrently, because it will not cause the reader get the wrong value.

I define write lock function for the writers, the function wait until all the read flags are off.

Then I defined additional write mutex to make the two writer write their value with out conflict with each other.

int mutex[2]; // maximum of 2 concurrent writer

int write_mutex(int id, int timeout) { //return -1 while timeout
    int time = 0;
    if( id = 1 ) {
        mutex[0] = 1;
        while(mutex[1]) {
            time++;
            if( time >= timeout ) {
                mutex[0] = 0;
                return -1;
            }
        }
    } else if ( id = 2 ) {
        mutex[1] = 1;
        while(mutex[0]) {
            time++;
            if( time >= timeout ) {
                mutex[1] = 0;
                return -1;;
            }
        }
    }

    return 0;
}

Priority

The one with lower priority should wait if there is a conflict,

and the one with higher priority should hold the flag until others turn off their flag.

So the code with lower priority should always like this:

while(reader[i]) {
    if( timeout++ < 1024 ) {
        continue;
    }
    timeout = 0;
    writer[ id - 1 ] = 0; // turn off flag
    while( timeout++ < 1024 ) ;
    writer[ id - 1 ] = 1; // turn on flag again
    timeout = 0;
}

Observation

1. We should define multiple level of mutex for the READ/WRITE functions.
2. The priority determines who should release their flag and wait when conflict happens.

The simulation is done by using BACI, get the source here.