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--- realtime:pi [2016/06/23 07:19]
anna-maria created
+++ — (current)
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-====== Priority inversion - priority inheritance ======
-Priority inheritance is a way to solve the problem called priority inversion.
-===== Priority inversion =====
-{{ priority-inversion.png?direct&700 }}
-  * 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:
-{{ priority-inheritance.png?direct&700 }}
-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).

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