ChibiOS/RT, like other RTOSs, implements multiple synchronization primitives. Some primitives may appear similar and this can generate confusion when deciding which primitive is best suited for a given scenario.
We will consider three synchronization mechanisms implemented in ChibiOS/RT:
Note that other RTOSs may have implementation differences or even name them in a different way, there is not a standard about both terminology and exact behavior. Often the binary semaphore term is used as a synonym of mutex, this is not the case in ChibiOS/RT, there are fundamental differences as we will see.
A binary semaphore is a synchronization object that can have only two states:
Two operations are defined:
chBSemWait()in ChibiOS/RT). Taking a binary semaphore brings it in the “taken” state, trying to take a semaphore that is already taken enters the invoking thread into a waiting queue.
chBSemSignal()in ChibiOS/RT). Releasing a binary semaphore brings it in the “not taken” state if there are not queued threads. If there are queued threads then a thread is removed from the queue and resumed, the binary semaphore remains in the “taken” state. Releasing a semaphore that is already in its “not taken” state has no effect.
Binary semaphores have no ownership attribute and can be released by any thread or interrupt handler regardless of who performed the last take operation. Because of this binary semaphores are often used to synchronize threads with external events implemented as ISRs, for example waiting for a packet from a network or waiting that a button is pressed.
Because there is no ownership concept a binary semaphore object can be created to be either in the “taken” or “not taken” state initially.
A counting semaphore is a synchronization object that can have an arbitrarily large number of states. The internal state is defined by a signed integer variable, the counter. The counter value (N) has a precise meaning:
Two operations are defined for counting semaphores:
chSemWait()in ChibiOS/RT). This operation decreases the semaphore counter, if the result is negative then the invoking thread is queued.
chSemSignal()in ChibiOS/RT). This operation increases the semaphore counter, if the result is non-negative then a waiting thread is removed from the queue and resumed.
Counting semaphores have no ownership attribute and can be signaled by any thread or interrupt handler regardless of who performed the last wait operation.
Because there is no ownership concept a counting semaphore object can be created with any initial counter value as long it is non-negative.
The counting semaphores are usually used as guards of resources available in a discrete quantity. For example the counter may represent the number of used slots into a circular queue, producer threads would “signal” the semaphores when inserting items in the queue, consumer threads would “wait” for an item to appear in queue, this would ensure that no consumer would be able to fetch an item from the queue if there are no items available. Note that this is exactly how I/O queues are implemented in ChibiOS/RT, very convenient.
A mutex is a synchronization object that can have only two states:
Two operations are defined for mutexes:
chMtxLock()in ChibiOS/RT). This operation attempts to take ownership of a mutex, if the mutex is already owned by another thread then the invoking thread is queued.
chMtxUnlock()in ChibiOS/RT). This operation relinquishes ownership of a mutex. If there are queued threads then a thread is removed from the queue and resumed, ownership is implicitly assigned to the thread.
Note that, unlike semaphores, mutexes do have owners. A mutex can be unlocked only by the thread that owns it, this precludes the use of mutexes from interrupt handles but enables the implementation of the Priority Inheritance protocol, most RTOSs implement this protocol in order to address the Priority Inversion problem. It must be said that few RTOSs implement this protocol fully (any number of threads and mutexes involved) and even less do that efficiently.
Mutexes have one single use, Mutual Exclusion, and are optimized for that. Semaphores can also handle mutual exclusion scenarios but are best used as a communication mechanism between threads or between ISRs and threads. Also see the following articles:
|Feature||Binary Semaphores||Counter Semaphores||Mutexes|
|Use from ISRs||yes||yes||no|
|Initialization||either taken or not taken||counter >= 0||always not owned|
|Queue organization*3||FIFO or priority||FIFO or priority||Priority|
CH_CFG_USE_SEMAPHORES_PRIORITYis enabled then the semaphores queues are subject to reordering because priority inheritance triggered by a mutex operation.
ChibiOS/RT extends the above mechanisms in several ways:
MSG_OK, normal exit from a wait.
MSG_RESET, the semaphore has been reset while waiting.
MSG_TIMEOUT, timeout occurred while waiting.