Deadlock

Optional readings for this topic from Operating Systems: Principles and Practice: Section 6.5.

The deadlock problem:

• Systems usually have multiple locks
• Threads often need to hold multiple locks at the same time.
• Simple example:

  Thread A               Thread B
  mutex1.lock();         mutex2.lock();
  mutex2.lock();         mutex1.lock();
  ...                    ...
  mutex2.unlock();       mutex1.unlock();
  mutex1.unlock();       mutex2.unlock();

• Deadlock definition:
  • A collection of threads are all blocked.
  • Each thread is waiting for a resource owned by one of the other threads.
  • Since all threads are blocked, none can release their resources.

Four conditions for deadlock:

• Limited access: resources cannot be shared.
• No preemption. Once given, a resource cannot be taken away.
• Multiple independent requests: threads don't ask for resources all at once (hold resources while waiting).
• A circularity in the graph of requests and ownership.

Complexities:

• Deadlock can occur over anything that causes waiting:
  • Locks
  • Network messages
  • Disk drive
  • Memory space exhausted
• Deadlock can occur over discrete resources (e.g. locks) or quantities of a more continuous resource (pages of memory).
• In general, don't know in advance which resources a thread will need.

Solution #1: deadlock detection

• Determine when system is deadlocked
• Break the deadlock by terminating one of the threads
• Usually not practical in operating systems, but often used in database systems where transactions can be aborted and retried

Solution #2: deadlock prevention: eliminate one of the necessary conditions for deadlock

• Don't allow exclusive access? Not reasonable for most applications.
• Create enough resources so that they never run out? May work for things like disk space, but locks for synchronization are intentionally limited in number.
• Allow preemption? Works for some resources but not others (e.g., can't preempt a lock).
• Require threads to request all resources at the same time; either get them all or wait for them all.
  • Tricky to implement: must wait for several things without locking any of them.
  • Inconvenient for thread: hard to predict needs in advance. May require thread to over-allocate just to be safe.
• Prevent circularities: all threads request resources in the same order (e.g., always lock l1 before l2). This is the most common approach used in operating systems.