Deadlock

Lecture Notes for CS 140
Spring 2018
John Ousterhout

• Readings for this topic from Operating Systems: Principles and Practice: Section 6.5.
• The deadlock problem:
  • Threads often need to hold multiple locks at the same time.
  • Simple example:

    Thread A               Thread B
    lock_acquire(l1);      lock_acquire(l2);
    lock_acquire(l2);      lock_acquire(l1);
    ...                    ...
    lock_release(l2);      lock_release(l1);
    lock_release(l1);      lock_release(l2);
    

  • 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 a transaction can be 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.
  • Break the circularity: 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.