Do I really need mutex lock in this case?

Question

Consider we have three thread, bool status_flag[500] array, and working situations as follow :

1. Two threads only writing in status_flag array at different index. while third thread is only reading at any index.

2. All three thread writing at different index. While all three threads reading at any index.

In writing operation we are just setting the flag never reset it again.

status_flag [i] = true;

In reading operation we are doing something like that :

    for(;;){  //spinning to get flag true
       if(status_flag [i] == true){
           //do_something ;
           break;
       }
    }

What happen if compiler optimize (branch prediction) code?

I have read lot about lock but still having confusion to conclude result. Please help me to conclude.

Answer 1

POSIX is quite clear on this:

Applications shall ensure that access to any memory location by more than one thread of control (threads or processes) is restricted such that no thread of control can read or modify a memory location while another thread of control may be modifying it.

So without locking you're not allowed to read memory that some other thread may be writing. Furthermore, that part of POSIX describes which function will synchronize the memory between the threads. Before both threads have called any of the functions listed in there, you have no guarantee that the changes made by one thread will be visible to the other thread.

Answer 2

If all the threads are operating on different index value, then you do not need a lock. Basically it is equivalent to using different variables. In your code , the value of the variable i is not set or modified. So it is reading only a particular index of flag. And for writing you are using different index, in this case no need to use lock.