Avoding deadlocks

[Pradeep bhatt]

What are the practices to be followed for avoiding deadlocks?

[Alexandru Popescu]

Java Tutorial has a specialized tutorial chapter on this

./pope

[David Ulicny]

How can I recognize deadlock in application?

[Alexandru Popescu]

At least two threads waiting for locks.
Thread A - lock ObjA, waiting to lock ObjB
Thread B - lock ObjB, waiting to lock ObjA

the simplest example

./pope

[David Ulicny]

Yes, I know what deadlock is, but how can I detect him in the application?
How can I say: "In this method is a deadlock?"

[Vedhas Pitkar]

Java has added doug Lea's widely appreciated util.concurrent package to it library.Checkout Threads for more

[Alexandru Popescu]

Originally posted by David Ulicny:
Yes, I know what deadlock is, but how can I detect him in the application?
How can I say: "In this method is a deadlock?"

Involving a profiler as JProfiler or OptimizeIT.

./pope

[David Ulicny]

Thanks, I'm using JProfiler but I missed the option Deadlock Detection.
Do you know how Deadlock Detection is working?

[Alexandru Popescu]

Originally posted by David Ulicny:
Thanks, I'm using JProfiler but I missed the option Deadlock Detection.
Do you know how Deadlock Detection is working?

Not by heart but I think you may try to look for this into the help.

./pope

[Francis Siu]

What are the practices to be followed for avoiding deadlocks?
Deadlock can arise if four conditions hold simultaneously, so you can check these conditions with your program carefully.
Mutual exclusion: only one process can use a resource at a time

Hold and wait: a process holding at least one resource and also is waiting to acquire additional resources held by another's process.

No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.

Circular wait: there exists a set {p0,p1,p2,.....pi} of waiting processes such that pi is waiting for a resource that is held by p1, p1 is waiting for a resource that is held by p2,....pi-1 is waiting for a resource that is held by pi and pi is waiting for a resource that is held by p0.

[Stan James]

I get a newsletter from somebody who wrote an analyzer that can detect deadlock situations. Guess I'll have to read how he did it some day. As I recall it is slow enough that it's a debugging tool, not something to leave in production code.

One rule is to always lock resources in the same order. If T1 is holding A trying to get B and T2 is holding B trying to get A we're in trouble. If all threads get A first, then B they can coexist pretty happily. I saw a utility - maybe in Doug Lea's library? - that given a list of resources obtains locks in an order determined by comparators. So if T1 asked for B,A,C and T2 asked for C,B it would get the locks in A,B,C order for both threads and avoid deadlock.

[Henry Wong]

Originally posted by Pradeep Bhat:
What are the practices to be followed for avoiding deadlocks?

Deadlocks are best avoided by programming practices. Common techniques are to establish a locking order, or a higher order lock. It is a very important topic, and we spend a whole chapter to discuss it.

Another technique is to use optimistic locking techniques. After all, what better way to avoid deadlocks, than not locking at all...

Henry

[Henry Wong]

Originally posted by David Ulicny:
How can I recognize deadlock in application?

In Java Threads, we actually develop a class that can be used to detect a deadlock. Even so, it is not completely reliable. The issue is because, a "deadlock" may be caused by other causes than locks, it could even be caused by two threads waiting on variables to be in particular states.

In any case, feel free to download the source code from http://www.oreilly.com/catalog/jthreads3, if you are interested in deadlock detection. The source for the deadlock detecting lock is in the chapter 6 package.

Henry