Details
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Bug
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Status: Resolved
-
Major
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Resolution: Fixed
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None
Description
An extremely simplified version of the task scheduler's ScheduleMore method it looks something like:
void ScheduleMore(int num_to_schedule) {
tasks_that_need_running_.fetch_add(num_to_schedule);
if (!weak_lock.lock()) {
// If someone else is scheduling then return early
return;
}
auto tasks = PickTasks();
weak_lock.unlock();
}
It is possible for one thread to have the lock, and find 0 tasks. But then, before it gives up the lock, another thread adds tasks and fails to acquire the lock. Neither thread will schedule anything even though there are tasks to run. This can lead to deadlock.
The proposed PR changes the logic to (still extremely simplified):
void ScheduleMore(int num_to_schedule) {
tasks_that_need_running_.fetch_add(num_to_schedule);
tasks_added_recently.store(true);
if (!weak_lock.lock()) {
// If someone else is scheduling then return early
return;
}
auto tasks = PickTasks();
if (tasks_added_recently.compare_exchange_strong(true, false)) {
if (tasks.empty()) {
ScheduleMore();
}
}
weak_lock.unlock();
}
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