Breaking a condition variable deadlock

I have a situation where thread 1 is waiting on a condition variable A, whi开发者_JAVA百科ch should be woken up by thread 2. Now thread 2 is waiting on a condition variable B , which should be woken up by thread 1. In the scenario I am using the condition variable, I cannot avoid such a deadlock situation. I detect cycle(deadlock) and terminate one of the threads which is a participant in the deadlock.

Now, what I am not sure is how to simply terminate a thread say thread 1 which is waiting on a condition variable.

Would be grateful for some pointers. Thanks

Condition variables aren't like mutexes. By that I mean they aren't only usable by a single thread controlling them. The mutex that protects the condition variable is treated that way but that's only locked for short periods of time, unlocked manually by a thread after kicking (signalling) the condition variable, and automatically by a thread waiting for such a kick.

You can have a totally separate thread (like your deadlock detector, let's call it thread 3) simply kick one of the condition variables and it will wake up the thread waiting for it.

The usual use case for condition variables is for threads to wait for the kick then check to ensure you have work anyway (don't assume there is work simply because the variable was kicked). That's to take care of spurious wake-ups.

One possibility is to have a "global" deadlock_occurred flag which thread 3 sets when it detects deadlock, then also have thread 3 kick all the condition variables.

The first thing that threads 1 and 2 should do after being woken, should be to check that flag and take appropriate action (probably exit the thread).

You'll find you get into a lot less deadlock-type trouble if you architect your applications so that threads are responsible for their own lifetime. It's too easy to externally kill threads when they're not in a state amenable to being terminated. Don't get me wrong, there are other ways to handle it (such as with cancel points), but my tried and tested solution is by far the easiest I've ever found.