Priority Inversion in C++

In real-time systems, if you have threads with different priorities, it is possible to encounter a priority inversion. A priority inversion occurs when a low-priority thread prevents a higher-priority thread from running. This situation arises when a low-priority thread acquires a mutex and is pre-empted by one or more medium-priority threads that do not relinquish the CPU. If a high-priority thread then attempts to acquire the mutex locked by the low-priority thread, it will wait (potentially forever) for the medium-priority threads to complete.

One way to deal with this problem is to use a priority inheritance protocol. If a low-priority thread holds a mutex and a high-priority thread attempts to acquire the mutex, the priority of the thread holding the mutex is temporarily raised to the level of the thread waiting for the mutex. This allows the low-priority thread to keep running until it releases the mutex; as soon as it does, its priority is reduced back to its previous level and the high-priority thread acquires the mutex.

Ice supports the priority inheritance protocol on many POSIX platforms. (Windows does not provide such a protocol.)

For POSIX platforms that support the priority inheritance protocol, mutexes by default do not use it. You can use the getDefaultMutexProtocol function to retrieve the current default for your platform:

namespace IceUtil
    enum MutexProtocol { PrioInherit, PrioNone };

    MutexProtocol getDefaultMutexProtocol();

On POSIX systems that do not support priority inheritance and on Windows, this function always returns PrioNone.

The return value of getDefaultMutexProtocol determines whether a default-constructed Mutex or RecMutex uses priority inheritance. By default, this function returns PrioNone. You can override this default by explicitly specifying a different protocol when you construct a Mutex or RecMutex. On Windows, if you specify PrioInherit when you construct a mutex, the setting is ignored and the mutex is constructed as if you had specified PrioNone.

To change the value returned by getDefaultMutexProtocol, you can edit cpp/config/Make.rules, modify the value of the DEFAULT_MUTEX_PROTOCOL macro, and then rebuild the Ice library.

See Also