Editing Priority inversion (section)

== Formulation ==
Consider two tasks ''H'' and ''L'', of high and low priority respectively, either of which can acquire exclusive use of a shared resource ''R''. If ''H'' attempts to acquire ''R'' after ''L'' has acquired it, then ''H'' becomes blocked until ''L'' relinquishes the resource. Sharing an exclusive-use resource (''R'' in this case) in a well-designed system typically involves ''L'' relinquishing ''R'' promptly so that ''H'' (a higher-priority task) does not stay blocked for excessive periods of time. Despite good design, however, it is possible that a third task ''M'' of medium priority becomes runnable during ''L''<nowiki>'</nowiki>s use of ''R''. At this point, ''M'' being higher in priority than ''L'', preempts ''L'' (since ''M'' does not depend on ''R''), causing ''L'' to not be able to relinquish ''R'' promptly, in turn causing ''H''—the highest-priority process—to be unable to run (that is, ''H'' suffers unexpected blockage indirectly caused by lower-priority tasks like ''M'').