Lecture Notes for CS 140
Spring 2019
John Ousterhout

  • 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
      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.
    • 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.