pthreads: If I increment a global from two differe

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

Yes there can be sync issues in this case. You need to either protect the count variable with a mutex, or use a (usually platform specific) atomic operation.

Example using pthread mutexes

static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

for(int i=0; i<1000; i++) {
    pthread_mutex_lock(&mutex);
    count++;
    pthread_mutex_unlock(&mutex);
}

Using atomic ops

There is a prior discussion of platform specific atomic ops here: UNIX Portable Atomic Operations

If you only need to support GCC, this approach is straightforward. If you're supporting other compilers, you'll probably have to make some per-platform decisions.

Answer 2

Yes, there can be.

There is no guarantee that an increment itself is an atomic operation.

For example, if one thread reads the value for increment then gets swapped out, the other thread could come in and change it, then the first thread will write back the wrong value:

+-----+
|   0 | Value stored in memory (0).
+-----+
|   0 | Thread 1 reads value into register (r1 = 0).
+-----+
|   0 | Thread 2 reads value into register (r2 = 0).
+-----+
|   1 | Thread 2 increments r2 and writes back.
+-----+
|   1 | Thread 1 increments r1 and writes back.
+-----+

So you can see that, even though both threads have tried to increment the value, it's only increased by one. This is just one of the possible problems. It may even be that the write itself is not atomic and one thread may update only part of the value before being swapped out.

Use mutexes. That's what they're for.

Answer 3

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.