Does const mean thread-safe?

Tuesday, 14 March 2017

There was a discussion recently on the cpplang slack about whether const meant thread-safe.

As with everything in life, it depends. In some ways, yes it does. In others, it does not. Read on for the details.

What do we mean by "thread-safe"?

If someone says something is "thread-safe", then it is important to define what that means. Here is an incomplete list of some things people might mean when they say something is "thread safe".

• Calling foo(x) on one thread and foo(y) on a second thread concurrently is OK.
• Calling foo(x_i) on any number of threads concurrently is OK, provided each x_i is different.
• Calling foo(x) on a specific number of threads concurrently is OK.
• Calling foo(x) on any number of threads concurrently is OK.
• Calling foo(x) on one thread and bar(x) on another thread concurrently is OK.
• Calling foo(x) on one thread and bar(x) on any number of threads concurrently is OK.

Which one we mean in a given circumstance is important. For example, a concurrent queue might be a Single-Producer, Single-Consumer queue (SPSC), in which case it is safe for one thread to push a value while another pops a value, but if two threads try and push values then things go wrong. Alternatively, it might be a Multi-Producer, Single-Consumer queue (MPSC), which allows multiple threads to push values, but only one to pop values. Both of these are "thread safe", but the meaning is subtly different.

Before we look at what sort of thread safety we're after, let's just define what it means to be "OK".

Data races

At the basic level, when we say an operation is "OK" from a thread-safety point of view, we mean it has defined behaviour, and there are no data races, since data races lead to undefined behaviour.

From the C++ Standard perspective, a data race is where there are 2 operations that access the same memory location, such that neither happens-before the other, at least one of them modifies that memory location, and at least one of them is not an atomic operation.

An operation is thus "thread safe" with respect to the set of threads we wish to perform the operation concurrently if:

• none of the threads modify a memory location accessed by any of the other threads, or
• all accesses performed by the threads to memory locations which are modified by one or more of the threads are atomic, or
• the threads use suitable synchronization to ensure that there are happens-before operations between all modifications, and any other accesses to the modified memory locations.

So: what sort of thread-safety are we looking for from const objects, and why?

Do as ints do

A good rule of thumb for choosing behaviour for a class in C++ is "do as ints do".

With regard to thread safety, ints are simple:

• Any number of threads may read a given int concurrently
• If any thread modifies a given int, no other threads may access that int for reading or writing concurrently.

This follows naturally from the definition of a data race, since ints cannot do anything special to provide synchronization or atomic operations.

If you have an int, and more than one thread that wants to access it, if any of those threads wants to modify it then you need external synchronization. Typically you'll use a mutex for the external synchronization, but other mechanisms can work too.

If your int is const, (or you have const int& that references it, or const int* that points to it) then you can't modify it.

What does that mean for your class? In a well-designed class, the const member functions do not modify the state of the object. It is "logically" immutable when accessed exclusively through const member functions. On the other hand, if you use a non-const member function then you are potentially modifying the state of the object. So far, so good: this is what ints do with regard to reading and modifying.

To do what ints do with respect to thread safety, we need to ensure that it is OK to call any const member functions concurrently on any number of threads. For many classes this is trivially achieved: if you don't modify the internal representation of the object in any way, you're home dry.

Consider an employee class that stores basic information about an employee, such as their name, employee ID and so forth. The natural implementation of const member functions will just read the members, perform some simple manipulation on the values, and return. Nothing is modified.

class employee{
    std::string first_name;
    std::string last_name;
    // other data
public:
    std::string get_full_name() const{
        return last_name + ", " + first_name;
    }
    // other member functions
}

Provided that reading from a const std::string and appending it to another string is OK, employee::get_full_name is OK to be called from any number of threads concurrently.

You only have to do something special to "do as ints do" if you modify the internal state in your const member function, e.g. to keep a tally of calls, or cache calculation values, or similar things which modify the internal state without modifying the externally-visible state. Of course, you would also need to add some synchronization if you were modifying externally-visible state in your const member function, but we've already decided that's not a good plan.

In employee::get_full_name, we're relying on the thread-safety of std::string to get us through. Is that OK? Can we rely on that?

Thread-safety in the C++ Standard Library

The C++ Standard Library itself sticks to the "do as ints do" rule. This is spelled out in the section on Data race avoidance (res.on.data.races). Specifically,

A C++ standard library function shall not directly or indirectly modify objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments, including this.

and

Implementations may share their own internal objects between threads if the objects are not visible to users and are protected against data races.

This means that if you have a const std::string& referencing an object, then any calls to member functions on it must not modify the object, and any shared internal state must be protected against data races. The same applies if it is passed to any other function in the C++ Standard Library.

However, if you have a std::string& referencing the same object, then you must ensure that all accesses to that object must be synchronized externally, otherwise you may trigger a data race.

Our employee::get_full_name function is thus as thread-safe as an int: concurrent reads are OK, but any modifications will require external synchronization for all concurrent accesses.

There are two little words in the first paragraph quoted above which have a surprising consequence: "or indirectly".

Indirect Accesses

If you have two const std::vector<X>s, vx and vy, then calling standard library functions on those objects must not modify any objects accessible by other threads, otherwise we've violated the requirements from the "data race avoidance" section quoted above, since those objects would be "indirectly" modified by the function.

This means that any operations on the X objects within those containers that are performed by the operations we do on the vectors must also refrain from modifying any objects accessible by other threads. For example, the expression vx==vy compares each of the elements in turn. These comparisons must thus not modify any objects accessible by other threads. Likewise, std::for_each(vx.begin(),vx.end(),foo) must not modify any objects accessible by other threads.

This pretty much boils down to a requirement that if you use your class with the C++ Standard Library, then const operations on your class must be safe if called from multiple threads. There is no such requirement for non-const operations, or combinations of const and non-const operations.

You may of course decide that your class is going to allow concurrent modifications (even if that is by using a mutex to restrict accesses internally), but that is up to you. A class designed for concurrent access, such as a concurrent queue, will need to have the internal synchronization; a value class such as our employee is unlikely to need it.

Summary

Do as ints do: concurrent calls to const member functions on your class must be OK, but you are not required to ensure thread-safety if there are also concurrent calls to non-const member functions.

The C++ Standard Library sticks to this rule itself, and requires it of your code. In most cases, it's trivial to ensure; it's only classes with complex const member functions that modify internal state that may need synchronization added.

Posted by Anthony Williams
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Tags: cplusplus, const, concurrency
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