32. Common concurrency problems

Question 1

What are two most common concurrency problems?

Answer 1

Atomicity violation and order violation.

Question 1

What four conditions must be met for a deadlock?

Answer 1

1. Mutual exclusion: Threads claim exclusive control of resources that they require (e.g., a thread grabs a lock).
2. Hold-and-wait: Threads hold resources allocated to them (e.g.,locks that they have already acquired) while waiting for additional resources (e.g., locks that they wish to acquire).
3. No preemption: Resources (e.g., locks) cannot be forcibly removed from threads that are holding them.
4. Circular wait:There exists a circular chain of threads such that each thread holds one or more resources (e.g., locks) that are being requested by the next thread in the chain.

Question 2

What can we do to prevent curicular wait?

Answer 2

Total or partial ordering

Question 3

What can we do to avoid hold-and-wait?

Answer 3

Acquire all locks at once, atomically (through additional lock).

Question 4

How can we avoid no preemtpion?

Answer 4

By using more advanced interfaces that don’t get stuck waiting for lock such as pthread_mutex_trylock() .

Question 5

How can we avoid mutual exclusion?

Answer 5

By not using mutual exclusion. This is accomplished by using wait-free and lock-free data structures, such as those that use atomic machine instructions (test-and-set, compare-and-swap and so on).