Readers Writer Problem

Readers writer problem is another example of a classic synchronization problem. There are many variants of this problem, one of which is examined below.

Statement

There is a shared resource which should be accessed by multiple processes. There are two types of processes in this context. They are reader and writer. Any number of readers can read from the shared resource simultaneously, but only one writer can write to the shared resource. When a writer is writing data to the resource, no other process can access the resource. A writer cannot write to the resource if there are non zero number of readers accessing the resource at that time.

The Solution

From the above problem statement, it is evident that readers have higher priority than writer. If a writer wants to write to the resource, it must wait until there are no readers currently accessing that resource.

Here, we use one mutex m and a semaphore w. An integer variable read_count is used to maintain the number of readers currently accessing the resource. The variable read_count is initialized to 0. A value of 1 is given initially to m and w.

Instead of having the process to acquire lock on the shared resource, we use the mutex m to make the process to acquire and release lock whenever it is updating the read_count variable.

The code for the writer process looks like this:

while(TRUE) 
{
    wait(w);
    
   /* perform the write operation */
   
   signal(w);
}

And, the code for the reader process looks like this:

while(TRUE) 
{
    //acquire lock
    wait(m);
    read_count++;
    if(read_count == 1)
        wait(w);
    
    //release lock  
    signal(m);  
    
    /* perform the reading operation */
    
    // acquire lock
    wait(m);   
    read_count--;
    if(read_count == 0)
        signal(w);
        
    // release lock
    signal(m);  
} 

Code explaination

• As seen above in the code for the writer, the writer just waits on the w semaphore until it gets a chance to write to the resource.
• After performing the write operation, it increments w so that the next writer can access the resource.
• On the other hand, in the code for the reader, the lock is acquired whenever the read_count is updated by a process.
• When a reader wants to access the resource, first it increments the read_count value, then accesses the resource and then decrements the read_count value.
• The semaphore w is used by the first reader which enters the critical section and the last reader which exits the critical section.
• The reason for this is, when the first readers enters the critical section, the writer is blocked from the resource. Only new readers can access the resource now.
• Similarly, when the last reader exits the critical section, it signals the writer using the w semaphore because there are zero readers now and a writer can have the chance to access the resource.