Commit 5c9ae868 authored by Peter Bell's avatar Peter Bell
Browse files

Complete fix for race condition

parent f077b1fb
......@@ -89,8 +89,8 @@ template <typename T> class concurrent_queue
public:
void push(T val)
{
++size_;
lock_t lock(mut_);
++size_;
q_.push(std::move(val));
}
......@@ -101,28 +101,13 @@ template <typename T> class concurrent_queue
// Queue might have been emptied while we acquired the lock
if (q_.empty()) return false;
--size_;
val = std::move(q_.front());
--size_;
q_.pop();
return true;
}
// bool try_pop(T &val)
// {
// while (size_ > 0)
// {
// lock_t lock(mut_);
// // Item might not have been added yet
// if (q_.empty()) continue;
// --size_;
// val = std::move(q_.front());
// q_.pop();
// return true;
// }
// return false;
// }
bool empty() const { return size_==0; }
};
class thread_pool
......@@ -138,27 +123,49 @@ class thread_pool
std::atomic_flag busy_flag = ATOMIC_FLAG_INIT;
std::function<void()> work;
void worker_main(std::atomic<bool> & shutdown_flag,
void worker_main(
std::atomic<bool> &shutdown_flag,
std::atomic<size_t> &unscheduled_tasks,
concurrent_queue<std::function<void()>> &overflow_work)
{
using lock_t = std::unique_lock<std::mutex>;
while (!shutdown_flag)
bool expect_work = true;
while (!shutdown_flag || expect_work)
{
std::function<void()> local_work;
{
lock_t lock(mut);
// Wait to be woken by the thread pool with a piece of work
work_ready.wait(lock, [&]{ return (work || shutdown_flag); });
local_work.swap(work);
}
if (expect_work || unscheduled_tasks == 0)
{
lock_t lock(mut);
// Wait until there is work to be executed
work_ready.wait(lock, [&]{ return (work || shutdown_flag); });
local_work.swap(work);
expect_work = false;
}
if (local_work) local_work();
bool marked_busy = false;
if (local_work)
{
marked_busy = true;
local_work();
}
// Execute any work which queued up while we were busy
while (overflow_work.try_pop(local_work)) local_work();
if (!overflow_work.empty())
{
if (!marked_busy && busy_flag.test_and_set())
{
expect_work = true;
continue;
}
marked_busy = true;
while (overflow_work.try_pop(local_work))
{
--unscheduled_tasks;
local_work();
}
}
// Mark ourself as available before going back to sleep
busy_flag.clear();
if (marked_busy) busy_flag.clear();
}
}
};
......@@ -167,6 +174,7 @@ class thread_pool
std::mutex mut_;
std::vector<worker> workers_;
std::atomic<bool> shutdown_;
std::atomic<size_t> unscheduled_tasks_;
using lock_t = std::lock_guard<std::mutex>;
void create_threads()
......@@ -181,7 +189,7 @@ class thread_pool
worker->busy_flag.clear();
worker->work = nullptr;
worker->thread = std::thread(
[worker, this]{ worker->worker_main(shutdown_, overflow_work_); });
[worker, this]{ worker->worker_main(shutdown_, unscheduled_tasks_, overflow_work_); });
}
catch (...)
{
......@@ -217,32 +225,23 @@ class thread_pool
if (shutdown_)
throw std::runtime_error("Work item submitted after shutdown");
auto submit_to_idle = [&](std::function<void()> &work) -> bool
{
for (auto &worker : workers_)
if (!worker.busy_flag.test_and_set())
{
{
lock_t lock(worker.mut);
worker.work = std::move(work);
}
worker.work_ready.notify_one();
return true;
}
return false;
};
++unscheduled_tasks_;
// First check for any idle workers and wake those
if (submit_to_idle(work)) return;
for (auto &worker : workers_)
if (!worker.busy_flag.test_and_set())
{
--unscheduled_tasks_;
{
lock_t lock(worker.mut);
worker.work = std::move(work);
}
worker.work_ready.notify_one();
return;
}
// If no workers were idle, push onto the overflow queue for later
overflow_work_.push(std::move(work));
// Possible race: All workers might have gone idle between the first
// submit attempt and pushing the work item into the queue. So, there
// could be no active workers to check the queue.
// Resolve with another check for idle workers.
std::function<void()> dummy_work = []{};
submit_to_idle(dummy_work);
}
void shutdown()
......
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