Question

I was asked this question in a .net/C# interview:

If we have two threads T1 and T2. T1
acquires a lock on obj1 and then does
some processing and acquires a lock on
obj2. T2 acquires a lock on obj2 and
then does some processing and acquires
a lock on obj1. So, we can have a
deadlock. What is common technique
that we use in multithreading to avoid
this situation?

I answered saying that T1 and T2 should have some mechanism to communicate and we should do the coding in such a way that T2 starts doing its job only after T1 has signalled that it is done with its job. The interviewer asked me if I knew about transactions and how we can use it to encounter this deadlock situation.
I have some amount of multithreading experience on the UI side in winforms. But, I have never used transactions. Can some one tell me more about this or point me to a url/book,

Answer 1

One general approach to avoid deadlocks is to ensure your threads/processes acquire locks on resources in the same order. eg, T2 should lock obj1 first and then obj2 (the same as T1).

That way you can’t have both threads holding a resource that the other thread wants, ie a deadlock.

If the wording in your quote was the exact question, it is badly written. It should be:

T1 acquires a lock on obj1, does
something and then tries to also lock
obj2 without unlocking obj1. At the same time T2 acquires
a lock on obj2, does something and
then tries to also lock obj1 without unlocking obj2. A
deadlock will occur.

I highly recommend reading Concurrent Programming on Windows by Joe Duffy. It’s probably the most comprehensive book on threading theory and practice for Windows.