#Mutex vs semaphor code#
#Mutex vs semaphor series#

Neither task can continue until the mutex is unlocked and neither mutex can be unlocked until either task runs – classic deadlock.įor circular deadlock to occur the following conditions must all be true:

It is blocked as the DAC is held by the SI task. The Control task now runs and tries to lock the DAC. The SI task is now blocked as the ADC is already owned by the Control task. The SI task then locks the DAC and tries to lock the ADC. In this example the Control task has locked the ADC, but before locking the DAC has been pre-empted by the higher priory SI task. Unfortunately, under certain timing conditions, this can lead to deadlock. The SI Task waits for mutexes DAC and ADC: The Control task waits for mutexes ADC and DAC: There is also an analogue output protected by a different mutex. There is an analogue input shared by the Control and the SI tasks, which is protected by a mutex. The system is made up of three tasks, a low priority Control task, a medium priority System Identification (SI) task and a high priority Alarm task. So how can this happen? Take as an example a small control system. That other task is also block waiting on a mutex held by the first task. Simply put, one task is blocked waiting on a mutex owned by another task. These are:Ĭircular deadlock, often referred to as the “deadly embrace” problem is a condition where two or more tasks develop a circular dependency of mutual exclusion. Nevertheless, there are still a couple of problems that use of the mutex (in preference to the semaphore) will not solve. The mutex is a significantly safer mechanism to use for implementing mutual exclusion around shared resources.

#Mutex vs semaphor code#

This makes the code cleaner by not confusing the issues of mutual exclusion with synchronization.Ī specific Operating Systems mutex implementation may or may not support the following: This task is now responsible for ensuring integrity of critical region, and can unlock the mutex as normal.ĭue to ownership a mutex cannot be used for synchronization due to lock/unlock pairing. When only one task is readied, ownership of the mutex is passed from the terminated task to the readied task. The mutex is in an undefined state (and cannot be locked) and must be reinitialized. In this model no task currently has ownership of the mutex. When all tasks are readied, these tasks must then assume critical region is in an undefined state. Only one task readied this task is responsible for ensuring integrity of critical region.All tasks readied with error condition.In terms of what happens to the waiting tasks, there are various models, but two dominate: If a task terminates for any reason, the RTOS can detect if that task current owns a mutex and signal waiting tasks of this condition. The details of the Priority Inheritance Protocol and Priority Ceiling Protocol (a slight variant) are covered later in this article. The low priority task can now run and unlock the mutex – at this point it is returned back to its original priority. The Basic Priority Inheritance Protocol enables a low-priority task to inherit a higher-priorities task’s priority if this higher-priority task becomes blocked waiting on a mutex currently owned by the low-priority task. With ownership this problem can be addressed using one of the following priority inheritance protocols: The concept of ownership enables mutex implementations to address the problems discussed in part 1:Īs already stated, ownership stops accidental release of a mutex as a check is made on the release and an error is raised if current task is not owner.ĭue to ownership, a mutex can support relocking of the same mutex by the owning task as long as it is released the same number of times. If the mutual exclusion object doesn’t have ownership then, irrelevant of what it is called, it is not a mutex. If a task tries to unlock a mutex it hasn’t locked (thus doesn’t own) then an error condition is encountered and, most importantly, the mutex is not unlocked. Ownership is the simple concept that when a task locks (acquires) a mutex only it can unlock (release) it.

The mutex is similar to the principles of the binary semaphore with one significant difference: the principle of ownership. In 1990 this was formalised by the IEEE as standard IEEE Std 1003.1 commonly known as POSIX. I have struggled to find it’s first clear definition, but the major use of the term mutex (another neologism based around MUTual EXclusion) appears to have been driven through the development of the common programming specification for UNIX based systems. To address the problems associated with semaphore, a new concept was developed during the late 1980’s.

In this posting I aim to show how a different RTOS construct, the mutex, may overcome some, if not all, of these weaknesses.

#Mutex vs semaphor series#

Part 1 of this series we looked at the history of the binary and counting semaphore, and then went on to discuss some of the associated problem areas.