Is a mutex necessary when only inc/decrementing a variable?

问题

I have a struct with some integers field, like

struct s {
    int a;
    int b;
    int c;
    int max;
};
struct s mystruct = {0, 0, 0, 0}; // Global var

And then I have N threads that sometimes have to do things like ++ or -- on the first three fields, and sometimes have to read them and then update the max field. Do I have to use a mutex here? And if yes, is it needed only when reading/updating max or always? Why? If I just increment or decrement the first three fields, does it matter if a thread runs before another?

回答1:

You need to synchronize the access to the variables if the operations you are performing are not atomic. Consider a scenario of a single variable a having initial value of, say, 5. Now we have two threads T1 and T2, both want to increment it. How increment operation is broken down, in case it is not atomic (just an example, it could be some other way)?

1) Read a value into a temp location.
2) Increment and write the temp location back into a.

Now consider a scenario T1 is performing this operation first, and then pre-empted by T2 after completing step 1 only:

T1: 1) Read a into temp1 => temp1=5
T2: 1) Read a into temp2 => temp2=5
T2: 2) Write temp2+1 into a => a=6
T1: 2) Write temp1+1 into a => a=6

So the final value of a will be 6 and not 7 as you would expect.

回答2:

The universal rule of concurrency is this: If a memory location is accessed concurrently in a non-atomic way, and not all accesses are reads, then the accesses must be ordered. Ordering can be imposed either by serializing access through locking a mutex, or by some lower-level operation like an ordered atomic access or an ordered fence.

The only time you're allowed to have unordered accesses if all of the accesses are reads. E.g. multiple threads can read a (non-atomic) constant without ordering.

回答3:

If you have multiple threads updating global data then you should have mutex. You should use mutex in both cases in read and write. Only reading constant data have no issue.

If you avoid to use the mutex you will not get desired result because other threads will not get proper value. Have look on this When should we use mutex ?

Have look on example.

来源：https://stackoverflow.com/questions/37569571/is-a-mutex-necessary-when-only-inc-decrementing-a-variable