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370CT_21-22_Open/lectures/deadlock.org

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Deadlock setup Deadlock Deadlock Preventative measures Curative Measures Obligatory XKCD
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Deadlock

setup

Deadlock

Deadlock

  • What Deadlock is and how to avoid it.

  • This occurs when two or more threads need more than one shared resource in order to complete a task.

img/deadlock.png

  • Unless access to these resources is strictly controlled by means of careful program design, deadlock can easily happen.

Preventative measures

  • Condition Variables
    • By introducing an extra layer of control, being imposing order of access rather than simply granting of access, Condition variables can go some way in prevention of deadlock.
  • They can also, if used correctly, be used to release resources if held for too long without a complete set of requirements (all required resources) being met. This is complex to code, and not something we can cover.

Curative Measures

  • Here we have two options
  • Deadlock Breaking Algorithms
    • These act after the system in question has been in a state of deadlock for a set period of time, typically numbered in milliseconds or less. It works by forcing all resource gathering threads to let go of all of their resources and start again. Essentially a soft reset.
  • Redundant System
    • The second option, cheaper and becoming more common as hardware gets cheaper, is to emplace a duplicate system, running in parallel and ready to take over if the first system locks up.

Obligatory XKCD

file:img/travelling_salesman_problem.png