Kernel Priority Inversion

Priority inversion occurs when a higher priority process is blocked by one or more lower priority processes for a long time. The use of synchronization primitives such as mutual-exclusion locks in the SunOS 5.0 through 5.8 kernel can lead to priority inversion.

A process is blocked when it must wait for one or more processes to relinquish resources. If blocking continues, it can lead to missed deadlines, even for low levels of utilization.

The problem of priority inversion has been addressed for mutual-exclusion locks for the SunOS 5.0 through 5.8 kernel by implementing a basic priority inheritance policy. The policy states that a lower priority process inherits the priority of a higher priority process when the lower priority process blocks the execution of the higher priority process. This places an upper bound on the amount of time a process can remain blocked. The policy is a property of the kernel's behavior, not a solution that a programmer institutes through system calls or function execution. User-level processes can still exhibit priority inversion, however.