Compare and Exchange in C++: Mastering Atomic Operations for Concurrency

In the realm of concurrent programming with C++, managing shared data safely across multiple threads is paramount. C++ provides powerful tools to handle these challenges, and among them, the compare_exchange_weak and compare_exchange_strong operations of std::atomic stand out as fundamental building blocks. These operations are crucial for implementing lock-free algorithms and ensuring data integrity in multithreaded applications. This article delves into the intricacies of Compare And Exchange, exploring their functionality, differences, and practical applications in modern C++ development.

Understanding Atomic Compare and Exchange Operations

At the heart of concurrent programming lies the concept of atomicity. An atomic operation is indivisible; it executes as a single, uninterruptible unit. This is essential when multiple threads access shared resources to prevent race conditions and data corruption. std::atomic in C++ provides a way to work with primitive types and objects atomically. Within std::atomic, the compare_exchange_weak and compare_exchange_strong methods offer a sophisticated way to perform conditional updates on atomic variables.

The core idea behind compare and exchange is to atomically compare the current value of an atomic variable with an expected value. If they match, the atomic variable is updated with a new desired value. If they don’t match, the expected value is updated with the current value of the atomic variable. This operation is performed as a single atomic step, guaranteeing consistency even under heavy concurrent access.

compare_exchange_weak vs. compare_exchange_strong: Navigating the Nuances

Both compare_exchange_weak and compare_exchange_strong aim to achieve the same goal: atomically update a value based on a comparison. However, a critical difference lies in their behavior regarding spurious failures.

compare_exchange_strong guarantees that the operation will fail (return false) only if the current value of the atomic variable is genuinely different from the expected value. If it fails, it means another thread has successfully modified the atomic variable in the interim.

compare_exchange_weak, on the other hand, is allowed to exhibit “spurious failures”. This means that even if the current value of the atomic variable is equal to the expected value, compare_exchange_weak might still fail and return false. This can happen due to factors like processor limitations or optimization strategies at the hardware level. The key takeaway is that a weak compare-and-exchange might fail even when it could have theoretically succeeded.

So, why use compare_exchange_weak if it can spuriously fail? The answer lies in performance. On certain architectures, compare_exchange_weak can be implemented more efficiently than compare_exchange_strong. In scenarios where the compare-and-exchange is within a loop, the potential for spurious failures is handled by the loop’s retry mechanism. In such cases, using compare_exchange_weak can lead to better overall performance, as the occasional spurious failure is less costly than the potentially higher overhead of compare_exchange_strong on some systems.

When should you choose one over the other?

• Use compare_exchange_strong when you need a guaranteed comparison and exchange. If it fails, you know for certain that the value has changed. This is often simpler to reason about and suitable for most general cases.
• Use compare_exchange_weak when performance is critical and you are performing the compare-and-exchange in a loop. The loop naturally handles spurious failures by retrying the operation. It’s essential to ensure your algorithm can tolerate these potential extra iterations.

Memory Ordering in Compare and Exchange

Both compare_exchange_weak and compare_exchange_strong operations accept memory ordering arguments. Memory ordering specifies how memory accesses are ordered relative to other atomic operations and non-atomic memory accesses, particularly across different threads or processors. Understanding memory ordering is crucial for ensuring correctness in concurrent programs.

The compare_exchange operations can take up to two memory order arguments:

• success: This memory order applies to the read-modify-write operation when the comparison is successful (i.e., the exchange happens).
• failure: This memory order applies to the load operation when the comparison fails (i.e., the value is loaded into expected).

If only one order argument is provided, it’s used for both success and failure cases, with a slight modification for the failure case in certain scenarios (as detailed in the original documentation regarding std::memory_order_acquire and std::memory_order_release).

Choosing the correct memory order is vital for ensuring data consistency and avoiding issues like data races and unexpected behavior in concurrent applications. Common memory orders include:

• std::memory_order_relaxed: The most relaxed ordering. Only atomicity is guaranteed, no ordering constraints are imposed.
• std::memory_order_acquire: Ensures that loads happen-before subsequent accesses in the current thread.
• std::memory_order_release: Ensures that prior writes happen-before the release operation becomes visible to other threads.
• std::memory_order_acq_rel: Combines acquire and release semantics for read-modify-write operations.
• std::memory_order_seq_cst: (Sequentially consistent) The strongest ordering. Guarantees a total ordering of all sequentially consistent operations across all threads. This is the default if no memory order is explicitly specified.

Choosing the appropriate memory order depends on the specific synchronization requirements of your concurrent algorithm. Overly strong memory orders can introduce unnecessary performance overhead, while too weak orders can lead to data races and incorrect program behavior.

Parameters and Return Value

Let’s break down the parameters and return value of the compare_exchange operations:

Parameters:

• expected: This is a reference to a variable of the same type as the atomic variable. Before the operation, expected holds the value you expect the atomic variable to have. If the compare-and-exchange is successful, expected is unchanged (conceptually, although it might be written to and read back in some implementations). If it fails, expected is updated to hold the current value of the atomic variable. This allows you to retry the operation with the updated expected value.
• desired: This is the new value you want to store in the atomic variable if the comparison is successful.
• success (optional): Memory order to apply on successful exchange.
• failure (optional): Memory order to apply on failed comparison (load into expected).
• order (optional): Memory order to apply to both success and failure cases (if success and failure are not specified). Defaults to std::memory_order_seq_cst.

Return Value:

• bool: The function returns true if the compare and exchange operation was successful (i.e., the atomic variable’s value was updated to desired). It returns false otherwise (either due to a genuine value mismatch or a spurious failure in the case of compare_exchange_weak).

Practical Applications and Examples

Compare and exchange operations are fundamental for building various lock-free data structures and synchronization primitives. Let’s examine the provided examples and expand on their significance.

Lock-Free Stack Example

The first example demonstrates a lock-free stack implementation using compare_exchange_weak.

#include <atomic>

template<typename T>
struct node {
    T data;
    node* next;
    node(const T& data) : data(data), next(nullptr) {}
};

template<typename T>
class stack {
    std::atomic<node<T>*> head;
public:
    void push(const T& data) {
        node<T>* new_node = new node<T>(data);

        // put the current value of head into new_node->next
        new_node->next = head.load(std::memory_order_relaxed);

        // now make new_node the new head, but if the head
        // is no longer what's stored in new_node->next
        // (some other thread must have inserted a node just now)
        // then put that new head into new_node->next and try again
        while (!head.compare_exchange_weak(new_node->next, new_node,
                                           std::memory_order_release,
                                           std::memory_order_relaxed));
        // the body of the loop is empty
    }
};

int main() {
    stack<int> s;
    s.push(1);
    s.push(2);
    s.push(3);
}

In this example, the push operation attempts to atomically update the head of the stack.

1. A new node is created.
2. new_node->next is set to the current head (loaded with memory_order_relaxed).
3. The compare_exchange_weak operation is used in a loop:
   • It attempts to compare head with new_node->next.
   • If they are equal, it means no other thread has modified the head in the meantime. The exchange succeeds, setting head to new_node (using memory_order_release to make the update visible to other threads). The loop terminates.
   • If they are not equal (or a spurious failure occurs), it means another thread has modified the head. new_node->next is updated with the current head value. The loop retries the compare-and-exchange operation.

This loop ensures that even if multiple threads try to push onto the stack concurrently, only one will successfully update the head at a time, maintaining the stack’s integrity without explicit locks.

Demonstrating Value Exchange

The second example showcases how compare_exchange_strong modifies either the atomic variable or the expected variable.

#include <iostream>
#include <atomic>

std::atomic<int> ai;

int tst_val = 4;
int new_val = 5;
bool exchanged = false;

void valsout() {
    std::cout << "ai = " << ai << " tst_val = " << tst_val << " new_val = " << new_val << " exchanged = " << std::boolalpha << exchanged << 'n';
}

int main() {
    ai = 3;
    valsout();

    // tst_val != ai ==> tst_val is modified
    exchanged = ai.compare_exchange_strong(tst_val, new_val);
    valsout();

    // tst_val == ai ==> ai is modified
    exchanged = ai.compare_exchange_strong(tst_val, new_val);
    valsout();
}

Output:

ai = 3 tst_val = 4 new_val = 5 exchanged = false
ai = 3 tst_val = 3 new_val = 5 exchanged = false
ai = 5 tst_val = 3 new_val = 5 exchanged = true

This example clearly illustrates the behavior:

1. Initially, ai is 3, and tst_val is 4. compare_exchange_strong fails because ai (3) is not equal to tst_val (4). tst_val is updated to the current value of ai (which is 3), and exchanged is false.
2. In the second call, tst_val is now 3, which is equal to ai. compare_exchange_strong succeeds. ai is updated to new_val (5), and exchanged becomes true.

This example highlights the dual nature of compare_exchange: it not only attempts to update the atomic variable but also provides feedback by updating the expected value when the comparison fails, which is crucial for retry logic in algorithms.

Conclusion

compare_exchange_weak and compare_exchange_strong are powerful atomic operations in C++ that enable the construction of efficient and correct concurrent programs. Understanding their subtle differences, especially regarding spurious failures and memory ordering, is essential for leveraging their full potential. These operations are the bedrock of lock-free programming, allowing developers to create highly scalable and performant concurrent data structures and algorithms, critical in today’s multithreaded computing landscape. By mastering compare and exchange, C++ developers can write robust and efficient concurrent applications that effectively utilize modern hardware.