Question

Suppose I have two threads A and B that are both incrementing a ~global~ variable “count”. Each thread runs a for loop like this one:

for(int i=0; i<1000; i++)
    count++; //alternatively, count = count + 1;

i.e. each thread increments count 1000 times, and let’s say count starts at 0. Can there be sync issues in this case? Or will count correctly equal 2000 when the execution is finished? I guess since the statement “count = count + 1” may break down into TWO assembly instructions, there is potential for the other thread to be swapped in between these two instructions? Not sure. What do you think?

Answer 1

Count clearly needs to be protected with a mutex or other synchronization mechanism.

At a fundamental level, the count++ statment breaks down to:

load count into register
increment register
store count from register

A context switch could occur before/after any of those steps, leading to situations like:

Thread 1:  load count into register A (value = 0)
Thread 2:  load count into register B (value = 0)
Thread 1:  increment register A (value = 1)
Thread 1:  store count from register A (value = 1)
Thread 2:  increment register B (value = 1)
Thread 2:  store count from register B (value = 1)

As you can see, both threads completed one iteration of the loop, but the net result is that count was only incremented once.

You probably would also want to make count volatile to force loads & stores to go to memory, since a good optimizer would likely keep count in a register unless otherwise told.

Also, I would suggest that if this is all the work that’s going to be done in your threads, performance will dramatically drop from all the mutex locking/unlocking required to keep it consistent. Threads should have much bigger work units to perform.