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Priority inversion - priority inheritance

Priority inheritance is a way to solve the problem called priority inversion.

Priority inversion

  • A task with low priority L becomes runnable and starts executing (1).
  • It acquires a mutually exclusive resource (2).
  • Now a task with a higher priority H becomes runnable and preempts L while L is holding the resource (3).
  • A third task M with priority between Hs and Ls priority (and with no need for the resource) becomes runnable, but it has to wait because H with higher priority is running (4).
  • H needs the resource still held by L (5), and so H stops running to wait until the resource is released.
  • The runnable task M prevents L from executing because of its higher priority. This leads to the priority inversion because H has to wait until M has finished (6) so that L can release the resource (7).

Priority inheritance

The priority inversion problem is solved by priority inheritance:

This means than when H needs the resource held by L, L inherits H's priority (5) in order to release the resource sooner (6). When M gets ready to run, M has to wait until the currently higher prioritized task L releases the resource and H finishes running (7).

realtime/documentation/technical_basics/pi.txt · Last modified: 2016/06/23 08:16 by anna-maria