tn_mutex.h File Reference

Detailed Description

A mutex is an object used to protect shared resource.

There is a lot of confusion about differences between semaphores and mutexes, so, it's quite recommended to read small article by Michael Barr: Mutexes and Semaphores Demystified.

Very short:

While mutex is seemingly similar to a semaphore with maximum count of 1 (the so-called binary semaphore), their usage is very different: the purpose of mutex is to protect shared resource. A locked mutex is "owned" by the task that locked it, and only the same task may unlock it. This ownership allows to implement algorithms to prevent priority inversion. So, mutex is a locking mechanism.

Semaphore, on the other hand, is signaling mechanism. It's quite legal and encouraged for semaphore to be acquired in the task A, and then signaled from task B or even from ISR. It may be used in situations like "producer and consumer", etc.

In addition to the article mentioned above, you may want to look at the related question on stackoverflow.com.

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Mutex features in TNeoKernel:

• Recursive locking is supported (if option TN_MUTEX_REC is non-zero);
• Deadlock detection (if option TN_MUTEX_DEADLOCK_DETECT is non-zero);
• Two protocols available to avoid unbounded priority inversion: priority inheritance and priority ceiling.

A discussion about strengths and weaknesses of each protocol as well as priority inversions problem is beyond the scope of this document.

The priority inheritance protocol solves the priority inversions problem but doesn't prevents deadlocks, although the kernel can notify you if a deadlock has occured (see TN_MUTEX_DEADLOCK_DETECT).

The priority ceiling protocol prevents deadlocks and chained blocking but it is slower than the priority inheritance protocol.

See also

TN_USE_MUTEXES

Definition in file tn_mutex.h.

Go to the source code of this file.

Data Structures
struct	TN_Mutex
	Mutex. More...

Enumerations
enum	TN_MutexProtocol { TN_MUTEX_PROT_CEILING = 1, TN_MUTEX_PROT_INHERIT = 2 }
	Mutex protocol for avoid priority inversion. More...

Functions
enum TN_RCode	tn_mutex_create (struct TN_Mutex *mutex, enum TN_MutexProtocol protocol, int ceil_priority)
	Construct the mutex. More...
enum TN_RCode	tn_mutex_delete (struct TN_Mutex *mutex)
	Destruct mutex. More...
enum TN_RCode	tn_mutex_lock (struct TN_Mutex *mutex, TN_Timeout timeout)
	Lock mutex. More...
enum TN_RCode	tn_mutex_lock_polling (struct TN_Mutex *mutex)
	The same as tn_mutex_lock() with zero timeout. More...
enum TN_RCode	tn_mutex_unlock (struct TN_Mutex *mutex)
	Unlock mutex. More...

Enumeration Type Documentation

enum TN_MutexProtocol

Mutex protocol for avoid priority inversion.

Enumerator
TN_MUTEX_PROT_CEILING	Mutex uses priority ceiling protocol.
TN_MUTEX_PROT_INHERIT	Mutex uses priority inheritance protocol.

Definition at line 109 of file tn_mutex.h.

Function Documentation

enum TN_RCode tn_mutex_create	(	struct TN_Mutex *	mutex,
		enum TN_MutexProtocol	protocol,
		int	ceil_priority
	)

Construct the mutex.

The field id_mutex should not contain TN_ID_MUTEX, otherwise, TN_RC_WPARAM is returned.

(refer to Legend for details)

Parameters

mutex	Pointer to already allocated struct TN_Mutex
protocol	Mutex protocol: priority ceiling or priority inheritance. See enum TN_MutexProtocol.
ceil_priority	Used if only protocol is TN_MUTEX_PROT_CEILING: maximum priority of the task that may lock the mutex.

Returns

• TN_RC_OK if mutex was successfully created;
• If TN_CHECK_PARAM is non-zero, additional return code is available: TN_RC_WPARAM.

enum TN_RCode tn_mutex_delete

(

struct TN_Mutex *

mutex

)

Destruct mutex.

All tasks that wait for lock the mutex become runnable with TN_RC_DELETED code returned.

(refer to Legend for details)

Parameters

mutex

mutex to destruct

Returns

• TN_RC_OK if mutex was successfully destroyed;
• TN_RC_WCONTEXT if called from wrong context;
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.

enum TN_RCode tn_mutex_lock	(	struct TN_Mutex *	mutex,
		TN_Timeout	timeout
	)

Lock mutex.

• If the mutex is not locked, function immediately locks the mutex and returns TN_RC_OK.
• If the mutex is already locked by the same task, lock count is merely incremented and TN_RC_OK is returned immediately.
• If the mutex is locked by different task, behavior depends on timeout value: refer to TN_Timeout.

(refer to Legend for details)

Parameters

mutex	mutex to lock
timeout	refer to TN_Timeout

Returns

• TN_RC_OK if mutex is successfully locked or if lock count was merely incremented (this is possible if recursive locking is enabled, see TN_MUTEX_REC)
• TN_RC_WCONTEXT if called from wrong context;
• TN_RC_ILLEGAL_USE
  • if mutex protocol is TN_MUTEX_PROT_CEILING and calling task's priority is higher than ceil_priority given to tn_mutex_create()
  • if recursive locking is disabled (see TN_MUTEX_REC) and the mutex is already locked by calling task
• Other possible return codes depend on timeout value, refer to TN_Timeout
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.

See also

TN_MutexProtocol

enum TN_RCode tn_mutex_lock_polling

(

struct TN_Mutex *

mutex

)

The same as tn_mutex_lock() with zero timeout.

(refer to Legend for details)

enum TN_RCode tn_mutex_unlock

(

struct TN_Mutex *

mutex

)

Unlock mutex.

• If mutex is not locked or locked by different task, TN_RC_ILLEGAL_USE is returned.
• If mutex is already locked by calling task, lock count is decremented. Now, if lock count is zero, mutex gets unlocked (and if there are task(s) waiting for mutex, the first one from the wait queue locks the mutex). Otherwise, mutex remains locked with lock count decremented and function returns TN_RC_OK.

(refer to Legend for details)

Returns

• TN_RC_OK if mutex is unlocked of if lock count was merely decremented (this is possible if recursive locking is enabled, see TN_MUTEX_REC)
• TN_RC_WCONTEXT if called from wrong context;
• TN_RC_ILLEGAL_USE if mutex is either not locked or locked by different task
• If TN_CHECK_PARAM is non-zero, additional return codes are available: TN_RC_WPARAM and TN_RC_INVALID_OBJ.