Mutexes are a critical tool for ensuring thread safety in multi-threaded applications on ARM Cortex M3 microcontrollers. A mutex provides mutual exclusion, allowing only one thread to access a shared resource or section of code at a time. This prevents data corruption and race conditions when multiple threads read and write the same data concurrently.
Introduction to Thread Safety
In single threaded applications, the code executes sequentially in one flow. There are no concerns about the ordering of operations or sharing data between threads. However, in multi-threaded applications where the processor switches between threads rapidly, the order of execution can be non-deterministic.
If two threads access the same resource without synchronization, unpredictable errors can occur. For example, one thread may be part way through updating a data structure when another thread reads the same data. This results in a race condition – the outcome depends on the thread timing.
To avoid such issues, critical sections of code must be protected by synchronization primitives like mutexes. This ensures only one thread can execute the protected code at a time. When implemented correctly, thread synchronization guarantees predictable, reliable execution.
Mutex Overview
A mutex object provides mutual exclusion to a section of code referred to as a critical section. To use a mutex, a thread must first acquire or lock the mutex. This will succeed only if no other thread holds that lock.
If another thread has already locked the mutex, the requesting thread will block, suspending execution until the lock becomes available. After the critical section has completed, the thread must unlock the mutex. This allows other threads waiting on the same mutex to acquire the lock and proceed.
The general usage pattern is: mutex_lock(mutex); // critical section mutex_unlock(mutex);
The key properties of a mutex are:
- A thread must lock the mutex before entering the critical section.
- Only one thread can lock a mutex at a time.
- Threads attempting to lock an already locked mutex will block until it is unlocked.
- A thread must unlock a mutex it has locked when exiting the critical section.
Enforcing these constraints guarantees that only one thread can be inside the critical section at any given time. This mutual exclusion prevents race conditions and inconsistent data access.
Mutex APIs on ARM Cortex M3
The ARM Cortex M3 core includes SVCall and NVIC hardware to support mutexes and thread synchronization. However, the processor itself does not provide mutex APIs. These are implemented in software libraries.
Here are some common mutex APIs available on ARM Cortex M3:
- CMSIS RTOS – The Cortex Microcontroller Software Interface Standard (CMSIS) provides RTOS aware mutex APIs like osMutexCreate, osMutexWait and osMutexRelease.
- FreeRTOS – The FreeRTOS real time operating system implements mutexes through APIs like xSemaphoreCreateMutex, xSemaphoreTake and xSemaphoreGive.
- ChibiOS – The ChibiOS RTOS provides mutex functions such as chMtxObjectInit, chMtxLock and chMtxUnlock.
- embOS – The embOS real time kernel includes mutex management APIs like OS_CreateMutex, OS_MutexWait and OS_MutexRelease.
These libraries provide thread safe mutex implementations for the Cortex M3. Developers should choose the one that best aligns with their RTOS or framework requirements.
Using Mutexes to Protect Shared Resources
Let’s look at a simple example of using a mutex to protect shared data between threads in a Cortex M3 application.
We have two threads – Thread 1 and Thread 2. Both threads access a shared global counter variable named g_counter. int g_counter = 0; // shared global counter
Each thread increments this counter in a loop. Without synchronization, the operations will interleave non-deterministically and the final counter value will be unpredictable. // Thread 1 void thread1_func() { for(int i = 0; i < 1000; i++) { g_counter++; } } // Thread 2 void thread2_func() { for(int i = 0; i < 1000; i++) { g_counter++; } }
To fix this, we protect the critical section (incrementing g_counter) using a mutex as follows: // Create mutex osMutexId g_counter_mutex; osMutexCreate(&g_counter_mutex); // Thread 1 void thread1_func() { for(int i = 0; i < 1000; i++) { osMutexWait(g_counter_mutex, osWaitForever); // lock mutex g_counter++; osMutexRelease(g_counter_mutex); // unlock mutex } } // Thread 2 void thread2_func() { for(int i = 0; i < 1000; i++) { osMutexWait(g_counter_mutex, osWaitForever); g_counter++; osMutexRelease(g_counter_mutex); } }
Now only one thread can increment g_counter at a time, ensuring consistent, predictable results. The mutex protects the integrity of the shared data from concurrent access.
Avoiding Deadlock
While mutexes provide thread safety, they can also introduce deadlock scenarios if not used carefully. Deadlock occurs when two or more threads are blocked forever, each waiting on locks held by the other.
For example, consider two mutexes, mutex1 and mutex2. Thread 1 locks mutex1 then tries to lock mutex2. Meanwhile, Thread 2 locks mutex2 first, then tries to lock mutex1. This results in an unavoidable deadlock.
To prevent deadlocks:
- Avoid nested locking of mutexes within critical sections.
- Always lock mutexes in the same consistent order.
- Use timeout periods when waiting on mutexes and handle failure appropriately.
- Carefully analyze dependencies between threads and shared resources.
Proper mutex design up front and good application architecture helps mitigate deadlock risks in multi-threaded Cortex M3 development.
Choosing the Right Mutex Type
Mutex implementations generally fall into two categories:
- Normal mutexes – Basic mutual exclusion locks.
- Recursive mutexes – Allow a thread to lock a mutex it has already locked.
Recursive mutexes prevent self-deadlock when making nested calls within the same thread context. However they are slower than normal mutexes and have some limitations.
Factors to consider when selecting mutex types:
- Number of threads contending for the mutex.
- Frequency of lock/unlock operations.
- Need for priority inheritance or priority ceiling emulation.
- Reentrancy requirements – is nested locking needed?
- Memory and performance constraints.
Profiling threaded application behavior helps choose the optimal mutex implementation.
Conclusion
Mutexes are an essential synchronization tool for building reliable multi-threaded applications using ARM Cortex M3 MCUs. They allow safe access to shared data by ensuring critical code runs atomically.
Proper mutex usage prevents race conditions, inconsistent reads and writes, and undeterministic behavior caused by thread concurrency. But care must be taken to avoid deadlocks through good design.
ARM Cortex M3 supports mutex APIs through software libraries like CMSIS-RTOS, FreeRTOS and embOS. Selecting the right mutex type and properly protecting shared resources with minimal overhead allows building high performance, deadlock-free concurrent systems on Cortex M3 processors.
