Recently I've been working on projects that heavily use threading. I think that I'm OK at designing them; use stateless design as much as possible, lock access to all resources that more than one thread needs, etc. My experience in functional programming has helped that immensely.
However, when reading other people's thread code, I get confused. I am debugging a deadlock right now, and since the coding style and design are different from my personal style, I am having a difficult time seeing potential deadlock conditions.
What do you look for when debugging deadlocks?
Best Answer
If the situation is a real deadlock (i.e. two threads hold two different locks, but at least one thread wants a lock the other thread holds) then you need to first abandon all pre-conceptions of how the threads order locking. Assume nothing. You may want to remove all comments from the code you're looking at, as those comments may cause you to believe something that doesn't hold true. It's hard to emphasize this enough: assume nothing.
After that, determine what locks get held while a thread attempts to lock something else. If you can, ensure that a thread unlocks in reverse order from locking. Even better, ensure that a thread holds only one lock at a time.
Painstakingly work through a thread's execution, and examine all locking events. At each lock, determine whether a thread holds other locks, and if so, under what circumstances another thread, doing a similar execution path, can get to the locking event under consideration.
It's certainly possible you will not find the problem before you run out of time or money.