Firmware-Specific Bug #8: Priority Inversion

A wide range of nasty things can go wrong when two or more tasks coordinate their work through, or otherwise share, a singleton resource such as a global data area, heap object, or peripheral’s register set. In the first part of this column, I described two of the most common problems in task-sharing scenarios: race conditions and non-reentrant functions. But resource sharing combined with the priority-based preemption found in commercial real-time operating systems can also cause priority inversion, which is equally difficult to reproduce and debug.

The problem of priority inversion stems from the use of an operating system with fixed relative task priorities. In such a system, the programmer must assign each task it’s priority. The scheduler inside the RTOS provides a guarantee that the highest-priority task that’s ready to run gets the CPU—at all times. To meet this goal, the scheduler may preempt a lower-priority task in mid-execution. But when tasks share resources, events outside the scheduler’s control can sometimes prevent the highest-priority ready task from running when it should. When this happens, a critical deadline could be missed, causing the system to fail.

At least three tasks are required for a priority inversion to actually occur: the pair of highest and lowest relative priority must share a resource, say by a mutex, and the third must have a priority between the other two. The scenario is always as shown in the figure below. First, the low-priority task acquires the shared resource (time t1). After the high priority task preempts low, it next tries but fails to acquire their shared resource (time t2); control of the CPU returns back to low as high blocks. Finally, the medium priority task—which has no interest at all in the resource shared by low and high—preempts low (time t3). At this point the priorities are inverted: medium is allowed to use the CPU for as long as it wants, while high waits for low. There could even be multiple medium priority tasks.

FiveMoreBugs_fig2-300x243

The risk with priority inversion is that it can prevent the high-priority task in the set from meeting a real-time deadline. The need to meet deadlines often goes hand-in-hand with the choice of a preemptive RTOS. Depending on the end product, this missed deadline outcome might even be deadly for its user!

One of the major challenges with priority inversion is that it’s generally not a reproducible problem. First, the three steps need to happen—and in that order. And then the high priority task needs to actually miss a deadline. One or both of these may be rare or hard to reproduce events. Unfortunately, no amount of testing can assure they won’t ever happen in the field.[5]

Best Practice: The good news is that an easy three-step fix will eliminate all priority inversions from your system.
Choose an RTOS that includes a priority-inversion work-around in its mutex API. These work-arounds come by various names, such as priority inheritance protocol and priority ceiling emulation. Ask your sales rep for details.
Only use the mutex API (never the semaphore API, which lacks this work-around) to protect shared resources within real-time software.

Take the additional execution time cost of the work-around into account when performing the analysis to prove that all deadlines will always be met. Note that the method for doing this varies by the specific work-around.
Note that it’s safe to ignore the possibility of priority inversions if you don’t have any tasks with consequences for missing deadlines.

Footnotes

[5] Barr, Michael and Dave Stewart. “Introduction to Rate Monotonic Scheduling,” Beginner’s Corner, Embedded Systems Programming, February 2002. Available online at www.embedded.com/showArticle.jhtml?articleID=9900522.

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Firmware-Specific Bug #9

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