Question

I want to know how race condition will happen during context switching, and where and how this happens.

I know about race condition can occur when accessing shared resource, I just need to understand it better. Can someone help me grasp this?

Answer 1

Here’s a classic example:

int global_int = 0;

void update () {
   ++ global_int;
   /* generated assembly is something like
       register = global_int
       increment register
       global_int = register
   */
}

Say the first thread starts running, calls update(), but gets interrupted (by a signal, context switch, whatever) in-between the second and third instructions. At this stage global_int==0 and register==1: it hasn’t saved the result yet.

Now suppose a second thread runs update() and completes, so global_int==1. The first thread resumes and saves register (which is 1) to global_int, yielding no change.

In this situation, global_int==1 after two calls to update() have completed. Anything which assumes that update() updates global_int will now be broken.

In general it is very hard to detect this problem by looking at code, you have to analyse the data and say to yourself “global_int is being accessed by different threads, I’d better guard it with a mutex”. If you try to get clever and worry about how the threads will access it so as to avoid the expense of a lock, you will probably get it wrong except in trivial cases.