I’m trying to write a bounded lockless single-producer, single-consumer queue, however, the order of elements inserted into the queue is not the same as the order of the elements removed from the queue. The test code in main.cpp adds 100,000,000 sequential ints to the queue in one thread, and pops the values in another, checking that the values are sequential.
Currently, the when running the program, it prints “Oh no! last value was: 219718 but this value is 220231” (the number values differ between program runs). The range of the numbers seems to depend on yield_frequency in writer_thread, lower values of yield_frequency cause the test to (often, but not always) fail at a number in the 100s.
(The code is a wip so there’s probably some code that doesn’t match c++ best practices)
spsc_queue.hpp
#pragma once
#include <memory>
#include <utility>
#include <bit>
#include <atomic>
#include <optional>
template <typename T> class ChannelReader;
template <typename T> class ChannelWriter;
// instances of this class cannot be created directly, make_queue returns a pair of ChannelReader and ChannelWriter instances that wrap the queue to prevent reads (or writes) from multiple threads
template <typename T>
class SpscQueue {
friend ChannelReader<T>;
friend ChannelWriter<T>;
public:
static std::pair<ChannelReader<T>, ChannelWriter<T>> make_queue(size_t size) {
if (!std::__has_single_bit(size)) throw "";
std::shared_ptr<SpscQueue<T>> queue_ptr(new SpscQueue<T>(size));
return std::make_pair(ChannelReader<T>(queue_ptr), ChannelWriter<T>(queue_ptr));
}
~SpscQueue() {
delete[] data;
}
private:
SpscQueue (size_t _size): data(new T[_size]()), size(_size) {}
alignas(64) T* data;
const size_t size;
alignas(64) std::atomic<size_t> read_idx{0};
alignas(64) std::atomic<size_t> write_idx{0};
};
// wrapper class that allows ONE thread to read from a SPSC queue
template <typename T>
class ChannelReader {
friend SpscQueue<T>;
public:
// attempt to read from the queue, returning the value if successful, and std::nullopt otherwise
std::optional<T> try_get_next() {
size_t read_idx = queue->read_idx;
size_t write_idx = cached_write_idx;
if (write_idx <= read_idx) {
write_idx = queue->write_idx;
cached_write_idx = write_idx;
}
if (write_idx <= read_idx) {
return std::nullopt;
} else {
queue->read_idx++;
return std::move(queue->data[read_idx % queue->size]);
}
}
ChannelReader(const ChannelReader&) = delete;
ChannelReader& operator=(const ChannelReader&) = delete;
ChannelReader(ChannelReader&&) = default;
ChannelReader& operator=(ChannelReader&&) = default;
private:
ChannelReader(std::shared_ptr<SpscQueue<T>> ptr): queue(ptr) {}
std::shared_ptr<SpscQueue<T>> queue;
alignas(64) size_t cached_write_idx{0};
};
// wrapper class that allows ONE thread to write to a SPSC queue
template <typename T>
class ChannelWriter {
friend SpscQueue<T>;
public:
// attempt to write to the queue, returns true if the write was successful, and false otherwise
bool try_write_next(const T& obj) {
size_t read_idx = cached_read_idx;
size_t write_idx = queue->write_idx;
if (write_idx >= read_idx + queue->size) {
read_idx = queue->read_idx;
cached_read_idx = read_idx;
}
if (write_idx >= read_idx + queue->size) {
return false;
} else {
queue->data[write_idx % queue->size] = obj;
++queue->write_idx;
return true;
}
}
ChannelWriter(const ChannelWriter&) = delete;
ChannelWriter& operator=(const ChannelWriter&) = delete;
ChannelWriter(ChannelWriter&&) = default;
ChannelWriter& operator=(ChannelWriter&&) = default;
private:
ChannelWriter(std::shared_ptr<SpscQueue<T>> ptr): queue(ptr) {}
std::shared_ptr<SpscQueue<T>> queue;
alignas(64) size_t cached_read_idx{0};
};
main.cpp
#include <atomic>
#include <chrono>
#include <iostream>
#include <thread>
#include "../include/spsc_queue.hpp"
namespace chrono = std::chrono;
int main() {
std::atomic<bool> latch{false};
auto [reader, writer] = SpscQueue<int>::make_queue(512);
std::thread reader_thread([_reader = std::move(reader), &latch]() mutable {
latch.wait(false);
std::cerr << "Starting reader...n";
int last_val = -1;
while (last_val != 100'000'000 - 1) {
if (auto data = _reader.try_get_next()) {
if (*data != last_val + 1) {
std::cerr << "Oh no! last value was: " << last_val
<< " but this value is " << *data << 'n';
std::exit(1);
}
last_val = *data;
// std::cerr << last_val << " Readn";
}
}
});
std::thread writer_thread([_writer = std::move(writer), &latch]() mutable {
latch.wait(false);
std::cerr << "Starting writer...n";
for (int i = 0; i < 100'000'000; ++i) {
for (int j = 0; !_writer.try_write_next(i); ++j) {
constexpr int yield_frequency = 1 << 0;
if (j % yield_frequency)
std::this_thread::yield();
}
// std::cerr << "writer wrote value" << i << 'n';
// if (i == 10'000'000) std::exit(1);
}
});
std::cout << "Start" << std::endl;
{
auto start = chrono::steady_clock::now();
latch = true;
latch.notify_all();
reader_thread.join();
writer_thread.join();
auto finish = chrono::steady_clock::now();
auto elapsed_seconds =
chrono::duration_cast<chrono::duration<double>>(finish - start).count();
std::cout << elapsed_seconds << std::endl;
}
std::cout << "End" << std::endl;
}
11
220231 - 219718 = 513 and the circular buffer size is 512, so the writer and the reader are racing over that last value …. you are incrementing the read index before reading the value, the writer could overwrite the value that the reader is going to read.
queue->read_idx++; // tell writer data is already read
// writer modifies data here
return std::move(queue->data[read_idx % queue->size]); // read new data
the solution is to read the value before you do the increment, same as you do when writing, first write the value then increment the write index.
auto result = std::move(queue->data[read_idx % queue->size]);
++queue->read_idx;
return result;
1