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Your job in this homework is to modify read_condition() and write_condition() to satisfy the invariant condition that is specified in each of these parts

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Your job in this homework is to modify read_condition() and write_condition()

to satisfy the invariant condition that is specified in each of these

parts.

 

1.  The current code has a bug.  It deadlocks.  Therefore the invariants

    (read_condition and/or write_condition) must have a bug.  Fix the

    bug so that the deadlock disappears.  (You can choose any reasonable

    invariant consistent with multiple active readers or one active writer

    at a time.

    HINT:  To debug this, do:

      % make gdb

      (gdb) run

      (gdb) print num_readers

      (gdb) print num_readers_waiting

      (gdb) print ... # and so on for writers, etc.

    Then change the read_condition and/or write_condition functions to

    fix the bug that you are observing.

 

2.  Modify read_condition() and write_condition() to make this

      writer-preferred:  If any writer is waiting, then a reader

      should call pthread_cond_wait() and let the writer acquire

      the resource.

 

3.  Modify read_condition() and write_condition() to make this

      reader-preferred:  If any reader is waiting, then a writer

      should call pthread_cond_wait() and let the reader acquire

      the resource.

 

4.  Modify read_condition() and write_condition() to make this

      mostly-writer-preferred:  If all three readers are waiting,

      then a writer should call pthread_cond_wait()

      and let one of the readers acquire the resource.

      If there are fewer than three readers waiting, then the

      writer should be preferred.

 

5.  Neither semaphores nor condition variables guarantee that the

      the thread that has been waiting the longest will be woken up.

      The operating system is allowed to choose any thread to wake up.

      So, one thread could be starved if the operating system always

      prefers to wake up other threads.

        Let's fix this for the writers in the writer-preferred case

      (in case 2, above).  The solution is motivated by when people

      wait in line (in a queue).  They take a ticket and wait until

      their ticket number is called.

        So, to take the writer threads in order, add two global state variables:

          int writer_ticket_number = 0;

          int next_writer_ticket_to_be_served = 0;

      When a writer enters the critical section to acquire a resource,

      it should save the writer_ticket_number in a local variable, and then

      increment the global writer_ticket_number.  Just before a writer

      leaves the "acquire" critical section, it should increment the

      global next_writer_ticket_to_be_served, so that this is one more

      than the local writer_ticket_number of that writer thread.

        Finally, change the function write_condition() to be a function of one

      argument:

        int write_condition(int writer_ticket_number);

      The function should return 1 (true) only if this additional

      condition is also true:

        writer_ticket_number == next_writer_ticket_to_be_served

      And of course, the "while" loop (item b in the diagram in

      condition-variables.pdf), should be modified to call:

        write_condition(writer_ticket_number);