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Priority inheritance
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<h2 id="example">Example</h2> <p>Consider three jobs: </p> <table><tbody><tr><th>Job Name</th><th>Priority</th></tr><tr><td>H</td><td>High</td></tr><tr><td>M</td><td>Medium</td></tr><tr><td>L</td><td>Low</td></tr></tbody></table> <p>Suppose that both H and L require some shared resource. If L acquires this shared resource (entering a critical section), and H subsequently requires it, H will block until L releases it (leaving its critical section). Without priority inheritance, process M could <a href="/facts/Preemption_(computing)/wjgIbWel">preempt</a> process L during the critical section and delay its completion, in effect causing the lower-priority process M to indirectly preempt the high-priority process H. This is a <a href="/facts/Priority_inversion/9gSnYRRf">priority inversion</a> bug. </p><p>With priority inheritance, L will execute its critical section at H's high priority whenever H is blocked on the shared resource. As a result, M will be unable to preempt L and will be blocked. That is, the higher-priority job M must wait for the critical section of the lower priority job L to be executed, because L has inherited H's priority. When L exits its critical section, it regains its original (low) priority and awakens H (which was blocked by L). H, having high priority, preempts L and runs to completion. This enables M and L to resume in succession and run to completion without priority inversion. </p> <h2 id="operating-systems-supporting-priority-inheritance">Operating systems supporting priority inheritance</h2> <ul><li><a href="/facts/ERIKA_Enterprise/HRAfok0e">ERIKA Enterprise</a><a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>[<i>circular reference</i>]</li> <li><a href="/facts/FreeRTOS/v9how6mE">FreeRTOS</a></li> <li><a href="/facts/ThreadX/WrRGsNVz">Microsoft Azure RTOS, formerly Express Logic's ThreadX</a></li> <li><a href="/facts/Linux/9FvCKSTq">Linux</a><a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a></li> <li><a href="/facts/VxWorks/DCajjp0d">VxWorks</a></li> <li><a href="/facts/IRMX/wXBPoFsh">iRMX</a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a></li></ul> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Priority_ceiling_protocol/MTnF3vnk">Priority ceiling protocol</a></li></ul> <ul><li>Lui Sha; Ragunathan Rajkumar & John P. Lehoczky (September 1990). <a href="https://web.archive.org/web/20220419175141/https://www.csie.ntu.edu.tw/~r95093/papers/Priority%20Inheritance%20Protocols%20An%20Approach%20to%20Real-Time%20Synchronization.pdf">"Priority Inheritance Protocols: An Approach to Real-Time Synchronization"</a> (PDF). <i><a href="/facts/IEEE_Transactions_on_Computers/jjc4KLle">IEEE Transactions on Computers</a></i>. 39 (9): 1175–1185. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1109%2F12.57058">10.1109/12.57058</a>. Archived from <a href="https://www.csie.ntu.edu.tw/~r95093/papers/Priority%20Inheritance%20Protocols%20An%20Approach%20to%20Real-Time%20Synchronization.pdf">the original</a> (PDF) on 2022-04-19.</li></ul> <h2 id="external-links">External links</h2> <ul><li>"<a href="https://web.archive.org/web/20070706071045/http://www.linuxdevices.com/articles/AT5698775833.html">Priority Inheritance: The Real Story</a>" by <a href="/facts/Doug_Locke/7V33zAHk">Doug Locke</a></li> <li>"<a href="https://web.archive.org/web/20070706071207/http://www.linuxdevices.com/articles/AT7168794919.html">Against Priority Inheritance</a>" by Victor Yodaiken</li> <li>"<a href="http://rtdoc.cs.uri.edu/downloads/wohlever_thesis.pdf">Implementing Concurrency Control With Priority Inheritance in Real-Time CORBA</a>" by Steven Wohlever, Victor Fay Wolfe and Russell Johnston</li> <li>"<a href="http://citeseer.ist.psu.edu/108383.html">Priority Inheritance Spin Locks for Multiprocessor Real-Time Systems</a>" by Cai-Dong Wang, Hiroaki Takada and <a href="/facts/Ken_Sakamura/2ys29Nw8">Ken Sakamura</a></li> <li>"<a href="http://doi.ieeecomputersociety.org/10.1109/REAL.2003.1253271">Hardware Support for Priority Inheritance</a>" by Bilge E. S. Akgul, Vincent J. Mooney, Henrik Thane and Pramote Kuacharoen</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"ERIKA Enterprise". Retrieved 2024-01-03. <a href="https://en.wikipedia.org/wiki/ERIKA_Enterprise" target="_blank">https://en.wikipedia.org/wiki/ERIKA_Enterprise</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"RT-mutex subsystem with PI support". Retrieved 2021-11-27. <a href="https://www.kernel.org/doc/html/v5.15/locking/rt-mutex.html" target="_blank">https://www.kernel.org/doc/html/v5.15/locking/rt-mutex.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Resource-centric real-time kernel and middleware services" (PDF). p. 3. Retrieved 2024-01-01. <a href="https://apps.dtic.mil/sti/tr/pdf/ADA408745.pdf#page=7" target="_blank">https://apps.dtic.mil/sti/tr/pdf/ADA408745.pdf#page=7</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> </ol>