OCEOS/oceos inter-task communication/mutex
Mutex
Introduction
In OCEOS all mutexes are defined at compile time when the tasks that use them are known. A job obtains and returns a mutex by ‘wait’ and ‘signal’ calls to OCEOS.
Each mutex has a priority ceiling, the priority of the highest priority job that can ‘wait’ for that mutex. This is identified by the application developer and used in defining the mutex.
At compile time it is possible to determine the maximum time for which a mutex can be held by examining the source code of each task that uses the mutex.
Note
If a job simultaneously holds more than one mutex, this must be done in a LIFO nested way, e.g. Wait1, Wait2, Wait3, Signal3, Signal2, Signal1.
The maximum time across all mutexes for which a mutex can be held gives the maximum time for which a higher priority task can be blocked waiting for a mutex (assuming mutex overlaps are nested).
In OCEOS when a mutex ‘wait’ call succeeds, the current system priority ceiling value is replaced by a value derived from the mutex and the current system priority ceiling.
When a mutex ‘signal’ call succeeds, the previous system priority ceiling is restored and OCEOS automatically reschedules and pre-empts if appropriate.
In OCEOS a pending job only becomes active if its priority is higher than the system priority ceiling. As a result when a job becomes active no mutex that could block it is currently held.
In OCEOS if a job does a mutex ‘wait’ and then a ‘wait’ on the same mutex without an intervening ‘signal’ the second ‘wait’ is treated as a no-operation but is logged and the system state updated.
In OCEOS a ‘signal’ by a job on a mutex not currently held by the job is treated as a no-operation but is logged and the system state updated.
In OCEOS if a job terminates while holding a mutex the ‘signal’ operation is performed automatically for all mutexes held by the job and this is logged and the system state updated.
Mutexes can be read by interrupt handling code but do not provide mutual exclusion for such code. Mutual exclusion in interrupt handling code is achieved by enabling and disabling traps/interrupts.
API Functions
| Directive | Description | main | task | IRQ handler |
|---|---|---|---|---|
| oceos_mutex_create() | Create Mutex | * | ||
| oceos_mutex_wait() | Wait on mutex with ID | * | ||
| oceos_mutex_signal() | Signal a mutex with ID | * | ||
| oceos_mutex_get_value() | Get current value of mutex with ID | * | * |