Some Issues With Synchronized

[Andrew Wiley]

On Fri, Dec 23, 2011 at 8:33 PM, Andrew Wiley <wiley.andrew.j at gmail.com> wrote:
> On Fri, Dec 23, 2011 at 1:25 AM, Somedude <lovelydear at mailmetrash.com> wrote:
>> On windows XP with DMD 2.057, I get
>> Queue1: deadlock
>> Queue2: works
>> Queue3: works
>
> Yes, I posted another (much shorter) post describing the issue with
> Queue1. In short, since Queue1 is a synchronized class, the
> constructor is synchronized (which is mostly worthless). As a
> consequence, when I replace the lock in the middle of the function,
> bad things happen when the runtime tries to unlock the lock at the end
> and sees the new lock.
>
> This version of Queue1 shows a very hacky way to get around this:
> ---
> synchronized class Queue1 {private:   bool _work;   Condition
> _cond;public:   this() {       auto lock = new Mutex(this); //
> initialize the monitor for this object so we can use the same lock in
> the Condition       lock.lock(); // HACK: acquire the lock so we can
> unlock it at the end of the function       _cond = new
> Condition(lock);       _work = false;   }   void doWork() {
> while(!_work) (cast()_cond).wait();       _work = false;
> writeln("did work");       return;   }   void addWork() {       _work
> = true;       (cast()_cond).notify();       writeln("added work");
> }}---
> Queue2 looks like a bug where GDC is acquiring all locks twice in
> synchronized functions, and since the condition variable only unlocks
> the lock once, a deadlock results. I'll get a bug report up about it
> shortly.

Gah, my hacked/fixed Queue1 got garbled:
---
synchronized class Queue1 {
private:
    bool _work;
    Condition _cond;
public:
    this() {
        auto lock = new Mutex(this); // initialize the monitor for this object
        // so we can use the same lock in the Condition
        lock.lock(); // HACK: acquire the lock so we can unlock it
        // at the end of the function
        _cond = new Condition(lock);
        _work = false;
    }
    void doWork() {
        while(!_work) (cast()_cond).wait();
        _work = false;
        writeln("did work");
        return;
    }
    void addWork() {
        _work = true;
        (cast()_cond).notify();
        writeln("added work");
    }
}
---