I have an very delicate deadlock bug when I use the shared_ptr with custom Deleter and boost::share_mutex:
First there is an WorkManager class to manage Work object:
class Work;
class WorkManager {
public:
static WorkManager& instance() {
static WorkManager instance;
return instance;
}
std::shared_ptr<Work> get(int id);
std::shared_ptr<Work> create(int id);
void update(unsigned int diff);
private:
void _remove(int id, Work* t);
WorkManager();
mutable boost::shared_mutex _mutex;
std::map<int, std::weak_ptr<Work>> _works;
};
void WorkManager::_remove(int id, Work* t)
{
BOOST_ASSERT(t);
std::lock_guard<boost::shared_mutex> lock(_mutex);
_works.erase(id);
delete t;
}
std::shared_ptr<Work> WorkManager::get(int id) {
boost::shared_lock<boost::shared_mutex> lock(_mutex);
auto iter = _works.find(id);
if(iter == _works.end())
return nullptr;
return iter->second.lock();
}
std::shared_ptr<Work> WorkManager::create(int id) {
Work* t = new Work(id);
std::shared_ptr<Work> res;
res.reset(t, std::bind(&WorkManager::_remove, this, t->id(), std::placeholders::_1));
std::lock_guard<boost::shared_mutex> lock(_mutex);
_works[res->id()] = res;
return res;
}
void WorkManager::update(unsigned int diff) {
boost::shared_lock<boost::shared_mutex> lock(_mutex);
for(auto& t : _works)
{
if(std::shared_ptr<Work> work = t.second.lock())
work->update(diff);
}
}
WorkManager::update() is triggered by a timer, called at a fixed interval like 100ms. Because WorkManager doesn't hold shared_ptr<Work> but only weak_ptr<Work>, after work->update(diff); returns the work may be destroyed because there may be no shared_ptr<Work> holding this particular work any more. Then the WorkManager::_remove() will be called via the custom deleter for the shared_ptr<Work>. Notice the update thread already hold a read lock, in _remove(), it needs to acquire a write lock. However it must release the read lock before it can acquire this write lock. So this thread is dead.
This problem means that I cannot write any Manager's function do things like:
boost::shared_lock<boost::shared_mutex> lock(_mutex);
for(auto& t : _works)
{
if(std::shared_ptr<Work> w = t.second.lock())
w->do_something();
}
How to solve this? I figure out a solution is:
std::vector<std::shared_ptr<Work>> tmp;
{
boost::shared_lock<boost::shared_mutex> lock(_mutex);
for(auto& t : _works)
{
if(std::shared_ptr<Work> w = t.second.lock())
{
w->do_something();
tmp.push_back(w); //prevent w destroy before the read lock unlock
}
}
}
However this is error-prone, if someone forget this trick.
Related
Is there an standard way to add a std::packaged_task to an existing thread? There's a nontrivial amount of overhead that must happen before the task is run, so I want to do that once, then keep the thread running and waiting for tasks to execute. I want to be able to use futures so I can optionally get the result of the task and catch exceptions.
My pre-C++11 implementation requires my tasks to inherit from an abstract base class with a Run() method (a bit of a pain, can't use lambdas), and having a std::deque collection of those that I add to in the main thread and dequeue from in the worker thread. I have to protect that collection from simultaneous access and provide a signal to the worker thread that there's something to do so it isn't spinning or sleeping. Enqueing something returns a "result" object with a synchronization object to wait for the task to complete, and a result value. It all works well but it's time for an upgrade if there's something better.
Here is a toy thread pool:
template<class T>
struct threaded_queue {
using lock = std::unique_lock<std::mutex>;
void push_back( T t ) {
{
lock l(m);
data.push_back(std::move(t));
}
cv.notify_one();
}
boost::optional<T> pop_front() {
lock l(m);
cv.wait(l, [this]{ return abort || !data.empty(); } );
if (abort) return {};
auto r = std::move(data.back());
data.pop_back();
return std::move(r);
}
void terminate() {
{
lock l(m);
abort = true;
data.clear();
}
cv.notify_all();
}
~threaded_queue()
{
terminate();
}
private:
std::mutex m;
std::deque<T> data;
std::condition_variable cv;
bool abort = false;
};
struct thread_pool {
thread_pool( std::size_t n = 1 ) { start_thread(n); }
thread_pool( thread_pool&& ) = delete;
thread_pool& operator=( thread_pool&& ) = delete;
~thread_pool() = default; // or `{ terminate(); }` if you want to abandon some tasks
template<class F, class R=std::result_of_t<F&()>>
std::future<R> queue_task( F task ) {
std::packaged_task<R()> p(std::move(task));
auto r = p.get_future();
tasks.push_back( std::move(p) );
return r;
}
template<class F, class R=std::result_of_t<F&()>>
std::future<R> run_task( F task ) {
if (threads_active() >= total_threads()) {
start_thread();
}
return queue_task( std::move(task) );
}
void terminate() {
tasks.terminate();
}
std::size_t threads_active() const {
return active;
}
std::size_t total_threads() const {
return threads.size();
}
void clear_threads() {
terminate();
threads.clear();
}
void start_thread( std::size_t n = 1 ) {
while(n-->0) {
threads.push_back(
std::async( std::launch::async,
[this]{
while(auto task = tasks.pop_front()) {
++active;
try{
(*task)();
} catch(...) {
--active;
throw;
}
--active;
}
}
)
);
}
}
private:
std::vector<std::future<void>> threads;
threaded_queue<std::packaged_task<void()>> tasks;
std::atomic<std::size_t> active;
};
copied from another answer of mine.
A thread_pool with 1 thread matches your description pretty much.
The above is only a toy, a real thread pool I'd replace the std::packaged_task<void()> with a move_only_function<void()>, which is all I use it for. (A packaged_task<void()> can hold a packaged_task<R()> amusingly, if inefficiencly).
You will have to reason about shutdown and make a plan. The above code locks up if you try to shut it down without first clearing the threads.
I'm struggling to implement a thread-safe reference-counted queue. The idea is that I have a number of tasks that each maintain a shared_ptr to a task manager that owns the queue. Here is a minimal implementation that should encounter that same issue:
#include <condition_variable>
#include <deque>
#include <functional>
#include <iostream>
#include <memory>
#include <mutex>
#include <thread>
namespace {
class TaskManager;
struct Task {
std::function<void()> f;
std::shared_ptr<TaskManager> manager;
};
class Queue {
public:
Queue()
: _queue()
, _mutex()
, _cv()
, _running(true)
, _thread([this]() { sweepQueue(); })
{
}
~Queue() { close(); }
void close() noexcept
{
try {
{
std::lock_guard<std::mutex> lock(_mutex);
if (!_running) {
return;
}
_running = false;
}
_cv.notify_one();
_thread.join();
} catch (...) {
std::cerr << "An error occurred while closing the queue\n";
}
}
void push(Task&& task)
{
std::unique_lock<std::mutex> lock(_mutex);
_queue.emplace_back(std::move(task));
lock.unlock();
_cv.notify_one();
}
private:
void sweepQueue() noexcept
{
while (true) {
try {
std::unique_lock<std::mutex> lock(_mutex);
_cv.wait(lock, [this] { return !_running || !_queue.empty(); });
if (!_running && _queue.empty()) {
return;
}
if (!_queue.empty()) {
const auto task = _queue.front();
_queue.pop_front();
task.f();
}
} catch (...) {
std::cerr << "An error occurred while sweeping the queue\n";
}
}
}
std::deque<Task> _queue;
std::mutex _mutex;
std::condition_variable _cv;
bool _running;
std::thread _thread;
};
class TaskManager : public std::enable_shared_from_this<TaskManager> {
public:
void addTask(std::function<void()> f)
{
_queue.push({ f, shared_from_this() });
}
private:
Queue _queue;
};
} // anonymous namespace
int main(void)
{
const auto manager = std::make_shared<TaskManager>();
manager->addTask([]() { std::cout << "Hello world\n"; });
}
The problem I find is that on rare occasions, the queue will try to invoke its own destructor within the sweepQueue method. Upon further inspection, it seems that the reference count on the TaskManager hits zero once the last task is dequeued. How can I safely maintain the reference count without invoking the destructor?
Update: The example does not clarify the need for the std::shared_ptr<TaskManager> within Task. Here is an example use case that should illustrate the need for this seemingly unnecessary ownership cycle.
std::unique_ptr<Task> task;
{
const auto manager = std::make_shared<TaskManager>();
task = std::make_unique<Task>(someFunc, manager);
}
// Guarantees manager is not destroyed while task is still in scope.
The ownership hierarchy here is TaskManager owns Queue and Queue owns Tasks. Tasks maintaining a shared pointer to TaskManager create an ownership cycle which does not seem to serve a useful purpose here.
This is the ownership what is root of the problem here. A Queue is owned by TaskManager, so that Queue can have a plain pointer to TaskManager and pass that pointer to Task in sweepQueue. You do not need std::shared_pointer<TaskManager> in Task at all here.
I'd refactor the queue from the thread first.
But to fix your problem:
struct am_I_alive {
explicit operator bool() const { return m_ptr.lock(); }
private:
std::weak_ptr<void> m_ptr;
};
struct lifetime_tracker {
am_I_alive track_lifetime() {
if (!m_ptr) m_ptr = std::make_shared<bool>(true);
return {m_ptr};
}
lifetime_tracker() = default;
lifetime_tracker(lifetime_tracker const&) {} // do nothing, don't copy
lifetime_tracker& operator=(lifetime_tracker const&){ return *this; }
private:
std::shared_ptr<void> m_ptr;
};
this is a little utility to detect if we have been deleted. It is useful in any code that calls an arbitrary callback whose side effect could include delete(this).
Privately inherit your Queue from it.
Then split popping the task from running it.
std::optional<Task> get_task() {
std::unique_lock<std::mutex> lock(_mutex);
_cv.wait(lock, [this] { return !_running || !_queue.empty(); });
if (!_running && _queue.empty()) {
return {}; // end
}
auto task = _queue.front();
_queue.pop_front();
return task;
}
void sweepQueue() noexcept
{
while (true) {
try {
auto task = get_task();
if (!task) return;
// we are alive here
auto alive = track_lifetime();
try {
(*task).f();
} catch(...) {
std::cerr << "An error occurred while running a task\n";
}
task={};
// we could be deleted here
if (!alive)
return; // this was deleted, get out of here
}
} catch (...) {
std::cerr << "An error occurred while sweeping the queue\n";
}
}
}
and now you are safe.
After that you need to deal with the thread problem.
The thread problem is that you need your code to destroy the thread from within the thread it is running. At the same time, you also need to guarantee that the thread has terminated before main ends.
These are not compatible.
To fix that, you need to create a thread owning pool that doesn't have your "keep alive" semantics, and get your thread from there.
These threads don't delete themselves; instead, they return themselves to that pool for reuse by another client.
At shutdown, those threads are blocked on to ensure you don't have code running elsewhere that hasn't halted before the end of main.
To write such a pool without your inverted dependency mess, split the queue part of your code off. This queue owns no thread.
template<class T>
struct threadsafe_queue {
void push(T);
std::optional<T> pop(); // returns empty if thread is aborted
void abort();
~threadsafe_queue();
private:
std::mutex m;
std::condition_variable v;
std::deque<T> data;
bool aborted = false;
};
then a simple thread pool:
struct thread_pool {
template<class F>
std::future<std::result_of_t<F&()>> enqueue( F&& f );
template<class F>
std::future<std::result_of_t<F&()>> thread_off_now( F&& f ); // starts a thread if there aren't any free
void abort();
void start_thread( std::size_t n = 1 );
std::size_t count_threads() const;
~thread_pool();
private:
threadsafe_queue< std::function<void()> > tasks;
std::vector< std::thread > threads;
static void thread_loop( thread_pool* pool );
};
make a thread pool singleton. Get your threads for your queue from thread_off_now method, guaranteeing you a thread that (when you are done with it) can be recycled, and whose lifetime is handled by someone else.
But really, you should instead be thinking with ownership in mind. The idea that tasks and task queues mutually own each other is a mess.
If someone disposes of a task queue, it is probably a good idea to abandon the tasks instead of persisting it magically and silently.
Which is what my simple thread pool does.
There is a shared_mutex class planned for C++17. And shared_timed_mutex already in C++14. (Who knows why they came in that order, but whatever.) Then there is a recursive_mutex and a recursive_timed_mutex since C++11. What I need is a shared_recursive_mutex. Did I miss something in the standard or do I have to wait another three years for a standardized version of that?
If there is currently no such facility, what would be a simple (first priority) and efficient (2nd priority) implementation of such a feature using standard C++ only?
Recursive property of the mutex operates with the term "owner", which in case of a shared_mutex is not well-defined: several threads may have .lock_shared() called at the same time.
Assuming "owner" to be a thread which calls .lock() (not .lock_shared()!), an implementation of the recursive shared mutex can be simply derived from shared_mutex:
class shared_recursive_mutex: public shared_mutex
{
public:
void lock(void) {
std::thread::id this_id = std::this_thread::get_id();
if(owner == this_id) {
// recursive locking
count++;
}
else {
// normal locking
shared_mutex::lock();
owner = this_id;
count = 1;
}
}
void unlock(void) {
if(count > 1) {
// recursive unlocking
count--;
}
else {
// normal unlocking
owner = std::thread::id();
count = 0;
shared_mutex::unlock();
}
}
private:
std::atomic<std::thread::id> owner;
int count;
};
The field .owner needs to be declared as atomic, because in the .lock() method this field is checked without a protection from the concurrent access.
If you want to recursively call .lock_shared() method, you need to maintain a map of owners, and accesses to that map should be protected with some additional mutex.
Allowing a thread with active .lock() to call .lock_shared() makes implementation to be more complex.
Finally, allowing a thread to advance locking from .lock_shared() to .lock() is no-no, as it leads to possible deadlock when two threads attempt to perform that advancing.
Again, semantic of recursive shared mutex would be very fragile, so it is better to not use it at all.
If you are on Linux / POSIX platform, you are in luck because C++ mutexes are modelled after POSIX ones. The POSIX ones provide more features, including being recursive, process shared and more. And wrapping POSIX primitives into C++ classes is straight-forward.
Good entry point into POSIX threads documentation.
Here is a quick thread-safety wrapper around a type T:
template<class T, class Lock>
struct lock_guarded {
Lock l;
T* t;
T* operator->()&&{ return t; }
template<class Arg>
auto operator[](Arg&&arg)&&
-> decltype(std::declval<T&>()[std::declval<Arg>()])
{
return (*t)[std::forward<Arg>(arg)];
}
T& operator*()&&{ return *t; }
};
constexpr struct emplace_t {} emplace {};
template<class T>
struct mutex_guarded {
lock_guarded<T, std::unique_lock<std::mutex>>
get_locked() {
return {{m},&t};
}
lock_guarded<T const, std::unique_lock<std::mutex>>
get_locked() const {
return {{m},&t};
}
lock_guarded<T, std::unique_lock<std::mutex>>
operator->() {
return get_locked();
}
lock_guarded<T const, std::unique_lock<std::mutex>>
operator->() const {
return get_locked();
}
template<class F>
std::result_of_t<F(T&)>
operator->*(F&& f) {
return std::forward<F>(f)(*get_locked());
}
template<class F>
std::result_of_t<F(T const&)>
operator->*(F&& f) const {
return std::forward<F>(f)(*get_locked());
}
template<class...Args>
mutex_guarded(emplace_t, Args&&...args):
t(std::forward<Args>(args)...)
{}
mutex_guarded(mutex_guarded&& o):
t( std::move(*o.get_locked()) )
{}
mutex_guarded(mutex_guarded const& o):
t( *o.get_locked() )
{}
mutex_guarded() = default;
~mutex_guarded() = default;
mutex_guarded& operator=(mutex_guarded&& o)
{
T tmp = std::move(o.get_locked());
*get_locked() = std::move(tmp);
return *this;
}
mutex_guarded& operator=(mutex_guarded const& o):
{
T tmp = o.get_locked();
*get_locked() = std::move(tmp);
return *this;
}
private:
std::mutex m;
T t;
};
You can use either:
mutex_guarded<std::vector<int>> guarded;
auto s0 = guarded->size();
auto s1 = guarded->*[](auto&&e){return e.size();};
both do roughly the same thing, and the object guarded is only accessed when the mutex is locked.
Stealing from #tsyvarev 's answer (with some minor changes) we get:
class shared_recursive_mutex
{
std::shared_mutex m
public:
void lock(void) {
std::thread::id this_id = std::this_thread::get_id();
if(owner == this_id) {
// recursive locking
++count;
} else {
// normal locking
m.lock();
owner = this_id;
count = 1;
}
}
void unlock(void) {
if(count > 1) {
// recursive unlocking
count--;
} else {
// normal unlocking
owner = std::thread::id();
count = 0;
m.unlock();
}
}
void lock_shared() {
std::thread::id this_id = std::this_thread::get_id();
if (shared_counts->count(this_id)) {
++(shared_count.get_locked()[this_id]);
} else {
m.lock_shared();
shared_count.get_locked()[this_id] = 1;
}
}
void unlock_shared() {
std::thread::id this_id = std::this_thread::get_id();
auto it = shared_count->find(this_id);
if (it->second > 1) {
--(it->second);
} else {
shared_count->erase(it);
m.unlock_shared();
}
}
private:
std::atomic<std::thread::id> owner;
std::atomic<std::size_t> count;
mutex_guarded<std::map<std::thread::id, std::size_t>> shared_counts;
};
try_lock and try_lock_shared left as an exercise.
Both lock and unlock shared lock the mutex twice (this is safe, as the branches are really about "is this thread in control of the mutex", and another thread cannot change that answer from "no" to "yes" or vice versa). You could do it with one lock with ->* instead of ->, which would make it faster (at the cost of some complexity in the logic).
The above does not support having an exclusive lock, then a shared lock. That is tricky. It cannot support having a shared lock, then upgrading to an unique lock, because that is basically impossible to stop it from deadlocking when 2 threads try that.
That last issue may be why recursive shared mutexes are a bad idea.
It is possible to construct a shared recursive mutex using existing primitives. I don't recommend doing it, though.
It isn't simple, and wrapping the existing POSIX implementation (or whatever is native to your platform) is very likely to be more efficient.
If you do decide to write your own implementation, making it efficient still depends on platform-specific details, so you're either writing an interface with a different implementation for each platform, or you're selecting a platform and could just as easily use the native (POSIX or whatever) facilities instead.
I'm certainly not going to provide a sample recursive read/write lock implementation, because it's a wholly unreasonable amount of work for an Stack Overflow answer.
Sharing my implementation, no promises
recursive_shared_mutex.h
#ifndef _RECURSIVE_SHARED_MUTEX_H
#define _RECURSIVE_SHARED_MUTEX_H
#include <thread>
#include <mutex>
#include <map>
struct recursive_shared_mutex
{
public:
recursive_shared_mutex() :
m_mtx{}, m_exclusive_thread_id{}, m_exclusive_count{ 0 }, m_shared_locks{}
{}
void lock();
bool try_lock();
void unlock();
void lock_shared();
bool try_lock_shared();
void unlock_shared();
recursive_shared_mutex(const recursive_shared_mutex&) = delete;
recursive_shared_mutex& operator=(const recursive_shared_mutex&) = delete;
private:
inline bool is_exclusive_locked()
{
return m_exclusive_count > 0;
}
inline bool is_shared_locked()
{
return m_shared_locks.size() > 0;
}
inline bool can_exclusively_lock()
{
return can_start_exclusive_lock() || can_increment_exclusive_lock();
}
inline bool can_start_exclusive_lock()
{
return !is_exclusive_locked() && (!is_shared_locked() || is_shared_locked_only_on_this_thread());
}
inline bool can_increment_exclusive_lock()
{
return is_exclusive_locked_on_this_thread();
}
inline bool can_lock_shared()
{
return !is_exclusive_locked() || is_exclusive_locked_on_this_thread();
}
inline bool is_shared_locked_only_on_this_thread()
{
return is_shared_locked_only_on_thread(std::this_thread::get_id());
}
inline bool is_shared_locked_only_on_thread(std::thread::id id)
{
return m_shared_locks.size() == 1 && m_shared_locks.find(id) != m_shared_locks.end();
}
inline bool is_exclusive_locked_on_this_thread()
{
return is_exclusive_locked_on_thread(std::this_thread::get_id());
}
inline bool is_exclusive_locked_on_thread(std::thread::id id)
{
return m_exclusive_count > 0 && m_exclusive_thread_id == id;
}
inline void start_exclusive_lock()
{
m_exclusive_thread_id = std::this_thread::get_id();
m_exclusive_count++;
}
inline void increment_exclusive_lock()
{
m_exclusive_count++;
}
inline void decrement_exclusive_lock()
{
if (m_exclusive_count == 0)
{
throw std::logic_error("Not exclusively locked, cannot exclusively unlock");
}
if (m_exclusive_thread_id == std::this_thread::get_id())
{
m_exclusive_count--;
}
else
{
throw std::logic_error("Calling exclusively unlock from the wrong thread");
}
}
inline void increment_shared_lock()
{
increment_shared_lock(std::this_thread::get_id());
}
inline void increment_shared_lock(std::thread::id id)
{
if (m_shared_locks.find(id) == m_shared_locks.end())
{
m_shared_locks[id] = 1;
}
else
{
m_shared_locks[id] += 1;
}
}
inline void decrement_shared_lock()
{
decrement_shared_lock(std::this_thread::get_id());
}
inline void decrement_shared_lock(std::thread::id id)
{
if (m_shared_locks.size() == 0)
{
throw std::logic_error("Not shared locked, cannot shared unlock");
}
if (m_shared_locks.find(id) == m_shared_locks.end())
{
throw std::logic_error("Calling shared unlock from the wrong thread");
}
else
{
if (m_shared_locks[id] == 1)
{
m_shared_locks.erase(id);
}
else
{
m_shared_locks[id] -= 1;
}
}
}
std::mutex m_mtx;
std::thread::id m_exclusive_thread_id;
size_t m_exclusive_count;
std::map<std::thread::id, size_t> m_shared_locks;
std::condition_variable m_cond_var;
};
#endif
recursive_shared_mutex.cpp
#include "recursive_shared_mutex.h"
#include <condition_variable>
void recursive_shared_mutex::lock()
{
std::unique_lock sync_lock(m_mtx);
m_cond_var.wait(sync_lock, [this] { return can_exclusively_lock(); });
if (is_exclusive_locked_on_this_thread())
{
increment_exclusive_lock();
}
else
{
start_exclusive_lock();
}
}
bool recursive_shared_mutex::try_lock()
{
std::unique_lock sync_lock(m_mtx);
if (can_increment_exclusive_lock())
{
increment_exclusive_lock();
return true;
}
if (can_start_exclusive_lock())
{
start_exclusive_lock();
return true;
}
return false;
}
void recursive_shared_mutex::unlock()
{
{
std::unique_lock sync_lock(m_mtx);
decrement_exclusive_lock();
}
m_cond_var.notify_all();
}
void recursive_shared_mutex::lock_shared()
{
std::unique_lock sync_lock(m_mtx);
m_cond_var.wait(sync_lock, [this] { return can_lock_shared(); });
increment_shared_lock();
}
bool recursive_shared_mutex::try_lock_shared()
{
std::unique_lock sync_lock(m_mtx);
if (can_lock_shared())
{
increment_shared_lock();
return true;
}
return false;
}
void recursive_shared_mutex::unlock_shared()
{
{
std::unique_lock sync_lock(m_mtx);
decrement_shared_lock();
}
m_cond_var.notify_all();
}
If a thread owns a shared lock, it may also obtain an exclusive lock without giving up it's shared lock. (This of course requires no other thread currently has a shared or exclusive lock)
Vice-versa, a thread which owns an exclusive lock may obtain a shared lock.
Interestingly, these properties also allow locks to be upgradable/downgradable.
Temporarily upgrading lock:
recusrive_shared_mutex mtx;
foo bar;
mtx.lock_shared();
if (bar.read() == x)
{
mtx.lock();
bar.write(y);
mtx.unlock();
}
mtx.unlock_shared();
Downgrading from an exclusive lock to a shared lock
recusrive_shared_mutex mtx;
foo bar;
mtx.lock();
bar.write(x);
mtx.lock_shared();
mtx.unlock();
while (bar.read() != y)
{
// Something
}
mtx.unlock_shared();
I searched for a C++ read-write-lock and came across this related question. We did need exactly such a shared_recursive_mutex to control access to our "database" class from multiple threads. So, for completeness: If you are looking for another implementation example (like I was), you may want to consider this link too: shared_recursive_mutex implementation using C++17 (on github).
Features
C++17
Single Header
Dependency-free
It has a disadvantage though: static thread_local members specialized for a PhantomType class via template. So, you can't really use this shared_recursive_mutex in multiple separate instances of the same (PhantomType) class. Try it, if that is no restriction for you.
The following implementation supports first having a unique_lock and then acquiring an additional shared_lock in the same thread:
#include <shared_mutex>
#include <thread>
class recursive_shared_mutex: public std::shared_mutex {
public:
void lock() {
if (owner_ != std::this_thread::get_id()) {
std::shared_mutex::lock();
owner_ = std::this_thread::get_id();
}
++count_;
}
void unlock() {
--count_;
if (count_ == 0) {
owner_ = std::thread::id();
std::shared_mutex::unlock();
}
}
void lock_shared() {
if (owner_ != std::this_thread::get_id()) {
std::shared_mutex::lock_shared();
}
}
void unlock_shared() {
if (owner_ != std::this_thread::get_id()) {
std::shared_mutex::unlock_shared();
}
}
private:
std::atomic<std::thread::id> owner_;
std::atomic_uint32_t count_ = 0;
};
Okay this meight be a bit off for Stack but ill try to keep it as short as possible.
I got thread which takes tasks out of a list and executes them. Simple as this (The worker class has its own thread and runs doTask m_thread(&Worker::doTask, this)):
void Worker::doTask()
{
while (m_running)
{
auto task = m_tasks.pop_front();
task->execute();
if (task->isContinuous())
m_tasks.push_pack(task);
}
}
The list itself is/should be threadsafe:
Header:
class TaskQueue
{
public:
void push_pack(std::shared_ptr<Task> t);
std::shared_ptr<Task> pop_front();
private:
std::list<std::shared_ptr<Task>> m_tasks;
std::condition_variable m_cond;
std::mutex m_mutex;
void TaskQueue::push_pack(std::shared_ptr<Task> t)
}
Impls of the importand part:
void TaskQueue::push_pack(std::shared_ptr<Task> t)
{
m_tasks.push_back(t);
//notify that there is one more task, so one thread can work now
m_cond.notify_one();
}
std::shared_ptr<Task> TaskQueue::pop_front()
{
//regular lock so noone else acces this area now
std::unique_lock<std::mutex> lock(m_mutex);
while (m_tasks.size() == 0)
m_cond.wait(lock);
auto task = m_tasks.front();
m_tasks.pop_front();
return task;
}
last but not least the tasks:
class Task
{
public:
virtual ~Task()
{
}
virtual void execute() = 0;
virtual bool isContinuous()
{
return false;
};
};
So if i try to add this Task:
class NetworkRequestTask:public Task
{
public:
NetworkRequestTask(TaskQueue &q);
~NetworkRequestTask();
void execute() override;
bool isContinuous() override;
private:
TaskQueue &m_tasks;
};
Impl:
NetworkRequestTask::NetworkRequestTask(TaskQueue& q): m_tasks(q)
{
}
NetworkRequestTask::~NetworkRequestTask()
{
}
void NetworkRequestTask::execute()
{
while(dosomething)
{
//do something here
}
}
bool NetworkRequestTask::isContinuous()
{
return true;
}
Main:
int main(int argc, char* argv[])
{
TaskQueue tasks;
tasks.push_pack(std::make_shared<NetworkRequestTask>(tasks));
}
it gets into a bad state:
Expression: list iterator not derefercable
I am Confused. This only happens if i override continouse and this only happens at this task. If i add the queue to a other continouse task as reference it does not get into that bad state.
So whats going wrong here and more importand what have i done wrong?
As from the comments, i already tried to lock the push_back method which did not change anything to the behaviour. (You can exchange it for a regular mutex it doesnt matter.)
void TaskQueue::push_pack(std::shared_ptr<Task> t)
{
std::lock_guard<SpinLock> lock(m_spin);
m_tasks.push_back(t);
//notify that there is one more task, so one thread can work now
m_cond.notify_one();
}
I have a stl map which I would like to be synchronized across several threads. Currently I have...
Function A (Modifies map)
void Modify(std::string value)
{
pthread_mutex_lock(&map_mutex);
my_map[value] = value;
pthread_mutex_unlock(&map_mutex);
}
Function B (Reads map)
std::string Read(std::string key)
{
std::string value;
pthread_mutex_lock(&map_mutex);
std::map<std::string, std::string>::iterator it = my_map.find(key);
pthread_mutex_unlock(&map_mutex);
if(it != my_map.end())
{
return it->second;
}
else
{
return "DNE";
}
}
This is synchronized across all threads, due to the mutex. However, I have to lock the mutex in Function B even though it is not modifying the map at all. Is there a way to lock the my_map object itself in function A, and not lock it in function B while keeping thread synchronization. This way, all instances/calls of Function B continue to run freely, so long as Function A is not being run?
Thanks
You don't just want to lock the container, you also want to lock accesses into the container i.e. any iterators or pointers into it. You need to move those accesses into the locked region of the code.
std::string Read(std::string key)
{
std::string value = "DNE";
pthread_mutex_lock(&map_mutex);
std::map<std::string, std::string>::iterator it = my_map.find(key);
if(it != my_map.end())
{
value = it->second;
}
pthread_mutex_unlock(&map_mutex);
return value;
}
There's really no practical way to do this from inside the object itself.
Warning: I have not compiled or tested any of this, but I've done similar things in the past.
Step one would be to control the mutex with class, as such:
class Lock {
public:
Lock(Mutex& mutex) {
pthread_mutex_lock(mutex);
}
~Lock(Mutex& mutex) {
pthread_mutex_unlock(mutex);
}
};
This saves you from all sorts of issues, for instance, if your map throws an exception.
Then your modify becomes:
void Modify(std::string value)
{
Lock(map_mutex);
my_map[value] = value;
}
Create a reference counted lock class:
class RefCntLock {
private:
static int count;
static Lock* lock;
public:
RefCountLock(Mutex& mutex) {
// probably want to check that the mutex matches prior instances.
if( !lock ) {
lock = new Lock(mutex);
count++;
}
}
~RefCountLock() {
--count;
if( count == 0 ) {
delete lock;
lock = NULL;
}
}
};
(Note: it'd be easy to generalize this to deal with multiple mutexes.)
In your read, use the RefCntLock class:
std::string Read(std::string key)
{
{
RefCntLock(&map_mutex);
std::map<std::string, std::string>::iterator it = my_map.find(key);
}
if(it != my_map.end())
{
return it->second;
}
else
{
return "DNE";
}
}
This means that each write gets a lock but all reads share a lock.
In the upcoming C++17 standard, you can use std::shared_mutex and std::shared_lock to allow multiple readers exclusive read access, and std::unique_lock to implement exclusive write access.
std::shared_mutex map_lock;
void Modify(std::string value)
{
auto write_lock = std::unique_lock(map_lock);
my_map[value] = value;
}
std::string Read(std::string key)
{
auto read_lock = std::shared_lock(map_lock);
auto it = my_map.find(key);
return (it != my_map.end()) ? it->second : "DNE";
}