Implementing solution for readers-writers with readers preference

Question

I am trying to implement a solution for reader-writers problem with readers preference. Following is the problem statement:

Data is shared between one writer thread and multiple reader threads
Reader thread should wait if writer thread is accessing the shared data.
Reader thread should not wait if another reader thread is is accessing the shared data

I have come up with the following pseudo-code.

Can somebody tell me if the following solution is adequate and what are the issues with it ?

initialize()
{
    initialize semaphore with value 0
    initialize reader_active_count with value 1
}

writer()
{
    // Wait until reader thread makes the semaphore non-zero
    sem_wait(s)

    write data;

    sem_post(s)
   }

reader()
{
    lock(r1)
        reader_active_count ++;

        // Am I the first reader thread
        if (reader_active_count == 1)
        {
            // Wait until writer thread makes the semaphore non-zero
            sem_wait(s);
        }
    unlock(r1)


    read data


    lock(r1)
        reader_active_count--;
        // Am I the last reader thread
        if(reader_active_count == 0)
        {
            // Increment the sempahore to a non-zero value
            sem_post(s);
        }
    unlock(r1)
}

Jim Mischel Jim Mischel · Accepted Answer · 2014-04-10T23:02:57

Although it looks like your code will prevent multiple writers, and will prevent concurrent write and read access while allowing multiple reads, it won't prefer readers over writers.

Consider the case when thread 1 is writing and thread 2 comes along, also wanting to write. Then thread 3 wants to read.

Thread 1 will release the semaphore, making it non-zero. If your thread scheduler is fair (i.e. strictly FIFO), then thread 2 will acquire the semaphore next because it was the first in line. If your scheduler is not strictly FIFO, either one of the threads can be the next to acquire the semaphore.

If you want to favor readers over writers, then you need some way to give readers higher priority in the queue.

Implementing solution for readers-writers with readers preference

1 Answers