Would you explain lock ordering?

by

In the simple case given, unlocking in the reverse order is not necessary to avoid a deadlock.

However, as the code gets more complicated, unlocking in the reverse order helps you maintain proper lock ordering.

Consider:

A.lock();
B.lock();
Foo();
A.unlock();
Bar();
B.unlock();

If Bar() attempts to reacquire A, you’ve effectively broken your lock ordering. You’re holding B and then trying to get A. Now it can deadlock.

If you unlock in the reverse order style (which is very natural if you use RAII):

A.lock();
B.lock();
Foo();
B.unlock();
Bar();
A.unlock();

then it doesn’t matter if Bar() attempts to take a lock, as lock ordering will be preserved.

More Related Contents:

  1. What is the difference between lock, mutex and semaphore?
  2. What is a deadlock?
  3. Only inserting a row if it’s not already there
  4. Is there an advantage to use a Synchronized Method instead of a Synchronized Block?
  5. SSE instructions: which CPUs can do atomic 16B memory operations?
  6. Recursive Lock (Mutex) vs Non-Recursive Lock (Mutex)
  7. Is x86 CMPXCHG atomic, if so why does it need LOCK?
  8. Re-entrant locks in C#
  9. How to solve the “Double-Checked Locking is Broken” Declaration in Java?
  10. What is the Swift equivalent to Objective-C’s “@synchronized”?
  11. To what level does MongoDB lock on writes? (or: what does it mean by “per connection”
  12. What is the difference between synchronized on lockObject and using this as the lock?
  13. What is a coroutine?
  14. Write a program that will surely go into deadlock [closed]
  15. Java Concurrency: CAS vs Locking [closed]
  16. What is a memory fence?
  17. Programmatic deadlock detection in java
  18. Java : Does wait() release lock from synchronized block
  19. How does LMAX’s disruptor pattern work?
  20. In SQL Server, how can I lock a single row in a way similar to Oracle’s “SELECT FOR UPDATE WAIT”?
  21. How to implement a lock in JavaScript
  22. How do goroutines work? (or: goroutines and OS threads relation)
  23. anonymous struct and empty struct
  24. How to configure concurrency in .NET Core Web API?
  25. what is a loader lock?
  26. Are POSIX’ read() and write() system calls atomic?
  27. EF Core takes a lot of time, sometimes, to perform SELECT query
  28. What exactly does runtime.Gosched do?
  29. Is a bool read/write atomic in C#
  30. ReentrantReadWriteLock: what’s the difference between ReadLock and WriteLock?

Leave a Comment