# A thread safe task queue implementation using my own lock_guard in C++

As an exercise in multithreaded programming, I am trying to implement a basic FIFO task queue. For this, I also implement simplified version of my lock_guard, from STL. Here is the code:

#include <mutex>
#include <iostream>
#include <chrono>
#include <optional>

namespace cho {
template <typename M>
struct lock_guard {
lock_guard() noexcept = default;
lock_guard(M& m) noexcept {
mtx = &m;
mtx->lock();
}

lock_guard(lock_guard const& rhs) = delete;
lock_guard(lock_guard && rhs) = delete;
lock_guard& operator=(lock_guard const& rhs) = delete;
lock_guard& operator=(lock_guard && rhs) = delete;

~lock_guard() {
mtx->unlock();
}
private:
M *mtx = nullptr;
};

template <typename T>
struct queue {
queue() noexcept = default;
~queue() {
queue_node *cur = head;
while(cur) {
queue_node *tmp = cur;
cur = cur->next;
delete tmp;
}
}

void push(T const& elem) {
queue_node *node = new queue_node{};
T *new_elem = new T(elem);
node->data = *new_elem;
node->next = nullptr;
one_elem_mtx.lock();
lock_guard<std::mutex> grd(push_mtx);
tail->next = node;
} else {
}
tail = node;
one_elem_mtx.unlock();
}

auto pop() {
return std::optional<T>{std::nullopt};
}
one_elem_mtx.lock();
lock_guard<std::mutex> grd(pop_mtx);
queue_node *c = head;
delete c;
one_elem_mtx.unlock();
return ret;
}
private:
struct queue_node {
T data = T{};
queue_node *next = nullptr;
};
queue_node *head = nullptr;
queue_node *tail = nullptr;
std::mutex push_mtx = std::mutex{};
std::mutex pop_mtx = std::mutex{};
std::mutex one_elem_mtx = std::mutex{};
};
}

void operator()(char const* msg) {
std::cout << "Log: " << msg << "#" << log_id++<< "\n" << std::flush;
}
private:
static std::size_t log_id;
};
std::size_t log_task::log_id = 0;

int main() {
while(true) {
using namespace std::chrono_literals;
}
});
while(true) {
using namespace std::chrono_literals;
}
});

while(true) {
using namespace std::chrono_literals;
std::optional<log_task> t = log_queue.pop();
if(t)
t.value()("log_message");
}
});
while(true) {
using namespace std::chrono_literals;
std::optional<log_task> t = log_queue.pop();
if(t)
t.value()("log_message");
}
});

return 0;
}


lock_guard and queue implementation lives in my cho namespace. For queue, I only provide basic push and pop, not a production-ready implementation by any means. Then, I implement a logger to be used as test functor in my task queue, called log_task. My queue push and pop operations are guarded by mutexes to avoid messing queue pointers. I want to be able to push tasks to the queue from multiple threads and consume those tasks from multiple threads as well. For this reason, I hold two different mutexes: push_mtx and pop_mtx. On the other hand, I realized that if there is only one element, they can still mess the values since head and tail points to the same node. I added another one_elem_mtx that needs to be locked just on this occasion.

1. What are code smells you can point? I am, I don't know why, not happy with using three mutexes and feel like there is something wrong there.

2. I played with delays in sleep_for() to try different values and my tests are working fine. But still, can you see any chance of deadlock or livelock?

3. I've seen many implementations on other questions that use only one mutex. So when one thread pushes, popping threads need to wait as well and vice versa. This seemed like a performance penalty to me but I am not sure. Does my approach indicate a bad design and should I abandon concurrent push/pop?

# lock_guard

• The destructor never checks if mtx == nullptr, which will cause problems if lock_guard got default-constructed.

• The lock_guard(M&) constructor cannot be noexcept, as lock on a standard library compatible mutex class is allowed to throw.

However, it could be made conditionally noexcept in case M::lock is noexcept itself

 lock_guard(M& m) noexcept(noexcept(std::declval<M>().lock())) : mtx{&m} { mtx->lock(); }

• lock_guard(M&) should be made explicit in order to prevent unwanted conversions.

Any specific reasons why std::lock_guard or std::unique_lock couldn't be used?

# queue

• Memory leak: push(const T&) never deletes new_elem.

Actually, why allocate new_elem at all? The whole first 4 lines could be simplified to:

 auto node = new queue_node{ elem, nullptr };

• push(const T&) and pop() may cause undefined behavior by calling one_elem_mtx.unlock() if it hasn't been locked before.

• pop() doesn't update tail in case the last element got removed and lets it dangle instead.

This will cause undefined behavior upon the next call to push(const T&).

This also means that one_elem_mtx will not be locked in calls to push(const T&) or pop(), since head != tail while the queue is empty again.

• Race condition: one_elem_mtx needs to be acquired after the lock on pop_mtx/push_mtx.

Reason: Assume we have a queue with two elements in it, and two threads A and B who both want to execute pop. Thread A executes until just after if(head == tail) (which of course right now evaluates to false) and then gets interrupted by the OS. Thread B runs pop() to completion in the meantime, leaving the queue at one object. Now, assuming the missing tail update mentioned above gets added, we have a potential data race on access to tail if another Thread C were to run push(const T&).

• Thread starvation: Once the race condition is fixed, if pop() gets called much more frequently than push(const T&), threads waiting on pop() might starve threads trying to push(const T&) from getting access to one_elem_mtx. Maybe make pop a blocking operation (using a std::condition_variable to notify if new elements got inserted)?

• For a production ready queue, you might want to think about adding a maximum capacity (so the queue doesn't grow too large if elements get added faster than they get removed).

• An overload push(T&&) might be nice.

# log_task

• operator()(const char*) is not thread safe: access to the static log_id isn't synchronized. Maybe add a lock, or make log_id a std::atomic<std::size_t>?

# main

• Waiting for some hundred milliseconds on such a simple task that might otherwise take nanoseconds (or microseconds at most) to execute? This might make the whole "test" obsolete, especially since the chance of finding race conditions is pretty much zero.

• t.value() checks again whether the std::optional contains a value (and throws an exception if not). Since at that point t has already been checked for emptiness, why not use *t to access the value directly?

# General stuff

• Any specific reason for not using std::unique_ptr?

• auto could be used more often.

• There's no way to stop the program other than forcefully terminating the process. This might surprise some people trying it.

# Q & A

1. See above.

2. I can't see any right now.

3. It is a performance penalty, yes, though that might not matter if push and pop don't get called too frequently (e.g. because task creation and task execution take some time). It also makes reasoning about the whole set of operations a lot easier: If all access to head and tail is secured behind one single mutex, there are some advantages:

• No race condition on either can occur.

• No deadlock based upon locking order can occur (unless you make recursive calls).

• No special care needed for edge cases (empty/full queue, or off by one element).

This makes it much easier to verify and trust that such a queue is working correctly.

Now, if this performance penalty is too much, often mutexes get skipped entirely in favor of lock-free queues. These require some more expert knowledge, though.

• The reason to avoid std::lock_guard and std::unique_lock` was to implement those myself as an exercise. Thanks for the points mentioned. – meguli Aug 15 '18 at 18:40