On Aug 21, 2007, at 3:22 PM, Yuval Ronen wrote:
Howard Hinnant wrote:
On Aug 21, 2007, at 8:46 AM, Yuval Ronen wrote:
Howard Hinnant wrote:
Here is a link to a reference implementation and a FAQ for mutexes, locks and condition variables I am currently anticipating proposing for C++ standardization (or subsequent TR).
http://home.twcny.rr.com/hinnant/cpp_extensions/concurrency_rationale.html After some not-so-thorough reading of this, a few comments:
1. I couldn't understand what defer_lock is good for, even after reading Q.9 of the FAQ. I believe the use-case shown in Q.9 should actually use accept_ownership instead. Can you elaborate please?
See if this is any clearer:
http://home.twcny.rr.com/hinnant/cpp_extensions/concurrency_rationale.html#u...
I'm afraid not...
This example has 3 lines, the first 2 create unique_lock with defer_lock, and the 3rd calls std::lock. Those unique_lock don't lock, because std::lock to lock. OK. But who unlocks? The unique_locks don't own the mutexes, and therefore don't unlock them. But someone needs to unlock, and it sounds logical that the unique_locks would... Had we used accept_ownership, the unique_locks would have owned the mutexes, and unlock them. That's the difference between defer_lock and accept_ownership, the ownership, isn't it?
Ok, the lightbulb went off in my head and I think I understand your question now. Thanks for not giving up on me. I've tried again here: http://home.twcny.rr.com/hinnant/cpp_extensions/concurrency_rationale.html#u... and see the next question (#10) as well. If that doesn't do it, see: http://home.twcny.rr.com/hinnant/cpp_extensions/mutex_base and search for "defer_lock_type" and "accept_ownership_type" for the unique_lock implementation of these constructors. Neither constructor does anything to the mutex, and simply sets the owns() flag to false or true respectively. The unique_lock destructor will unlock the mutex iff it owns()'s the mutex.
Either way, I believe this design wouldn't meet the use case which I didn't effectively communicate in #14:
Given a read-write mutex and its associated condition variable:
my::shared_mutex rw_mut; std::condition<my::shared_mutex> cv(rw_mut);
client code wants to wait on that cv in two different ways:
1. With rw_mut read-locked. 2. With rw_mut write-locked.
If we initialized the condition variable with cv(rw_mut.exclusive()), then cv.wait() would wait with rw_mut write-locked, but we wouldn't be able to wait on cv with rw_mut read-locked.
If we initialized the condition variable with cv(rw_mut.shared()), then cv.wait() would wait with rw_mut read-locked, but we wouldn't be able to wait on cv with rw_mut write-locked.
This use case desires *both* types of waits on the *same* mutex/cv pair.
The last sentence starts with "This use case", but I see no use case. Do we really have such a use case? I haven't seen one yet. But even if we had, then maybe the solution is the same solution to the requirement you phrased as "The freedom to dynamically associate mutexes with condition variables" or "The ability to wait on general mutex / lock types" (what's the difference between those two sentences anyway?) in your response to Peter. Add a 'set_mutex(mutex_type &)', or maybe even 'set_mutex(mutex_type *)' to std::condition. I think it will solve this rare case.
Ok, perhaps I'll clean the following use case up, and include it in the faq. First I'll try it out here. :-) I've got a "communication node" class. It serves as a node in a network. It gets data from somewhere and temporarily stores it while it uses several threads to forward (or relay) the information to other nodes in the network. For simplicity I'm using vector<int> for the data, and only two relay threads. The example is incomplete (not even compiled), just illustrative right now: class communication_node { std::vector<int>* from_; std::vector<int> data_; std::vector<int> to_[2]; typedef std::tr2::shared_mutex Mutex; Mutex mut_; std::condition<Mutex> cv_; bool get_data_; bool fresh_data_; bool data_relayed_[2]; public: void supplier() { while (true) { std::unique_lock<Mutex> write_lock(mut_); while (!get_data_ || !data_relayed_[0] || ! data_relayed_[1]) cv_.wait(write_lock); std::copy(from_->begin(), from_->end(), data_.begin()); get_data_ = false; fresh_data_ = true; data_relayed_[0] = false; data_relayed_[1] = false; cv_.notify_all(); } } void relayer(int id) { while (true) { std::tr2::shared_lock<Mutex> read_lock(mut_); while (data_relayed_[id]) cv_.wait(read_lock); std::copy(data_.begin(), data_.end(), to[id].begin()); data_relayed_[id] = true; cv_.notify_all(); } } }; One thread will be running "supplier", while two other threads will run "relayer". Additionally some fourth thread (not shown) will tell "supplier" when there is new data that it needs to go get. In this design, which may not be the best way to do things, but it looks like reasonable client-written code to me, there is one shared_mutex and one condition to control the data flow. The supplier waits for there to be new data to get, and also waits until the relayers have done their jobs, before getting new data. It needs write access to the data. So it holds mut_ write-locked, and waits on the cv_ with it. The relayer threads only need read access to the data. So they each have mut_ read-locked, and wait on the cv until they get the instruction that it is time to relay the data. Once they relay the data, they notify everyone else that they're done. This example is meant to demonstrate a reasonable use case where one thread wants to wait on a cv with a read-lock while another thread wants to wait on the same cv/mutex with a write lock. Both readers and the writer may all be waiting at the same time for there to be data available from an upstream node (and a fourth thread would have to notify them when said data is available). Because of this, it is not possible (in the above use case) for there to be a set_mutex on the condition to change the facade, since both facades are simultaneously in use. Did I make more sense this time? I often complain when people use too much English and not enough C++ in their arguments, and then I find myself being guilty of the same thing. :-) -Howard