.. _mutexes_v2:

Mutexes
#######

A :dfn:`mutex` is a kernel object that implements a traditional
reentrant mutex. A mutex allows multiple threads to safely share
an associated hardware or software resource by ensuring mutually exclusive
access to the resource.

.. contents::
    :local:
    :depth: 2

Concepts
********

Any number of mutexes can be defined. Each mutex is referenced by its memory
address.

A mutex has the following key properties:

* A **lock count** that indicates the number of times the mutex has be locked
  by the thread that has locked it. A count of zero indicates that the mutex
  is unlocked.

* An **owning thread** that identifies the thread that has locked the mutex,
  when it is locked.

A mutex must be initialized before it can be used. This sets its lock count
to zero.

A thread that needs to use a shared resource must first gain exclusive rights
to access it by **locking** the associated mutex. If the mutex is already locked
by another thread, the requesting thread may choose to wait for the mutex
to be unlocked.

After locking a mutex, the thread may safely use the associated resource
for as long as needed; however, it is considered good practise to hold the lock
for as short a time as possible to avoid negatively impacting other threads
that want to use the resource. When the thread no longer needs the resource
it must **unlock** the mutex to allow other threads to use the resource.

Any number of threads may wait on a locked mutex simultaneously.
When the mutex becomes unlocked it is then locked by the highest-priority
thread that has waited the longest.

.. note::
    Mutex objects are *not* designed for use by ISRs.

Reentrant Locking
=================

A thread is permitted to lock a mutex it has already locked.
This allows the thread to access the associated resource at a point
in its execution when the mutex may or may not already be locked.

A mutex that is repeatedly locked by a thread must be unlocked an equal number
of times before the mutex becomes fully unlocked so it can be claimed
by another thread.

Priority Inheritance
====================

The thread that has locked a mutex is eligible for :dfn:`priority inheritance`.
This means the kernel will *temporarily* elevate the thread's priority
if a higher priority thread begins waiting on the mutex. This allows the owning
thread to complete its work and release the mutex more rapidly by executing
at the same priority as the waiting thread. Once the mutex has been unlocked,
the unlocking thread resets its priority to the level it had before locking
that mutex.

.. note::
    The :option:`CONFIG_PRIORITY_CEILING` configuration option limits
    how high the kernel can raise a thread's priority due to priority
    inheritance. The default value of 0 permits unlimited elevation.

When two or more threads wait on a mutex held by a lower priority thread, the
kernel adjusts the owning thread's priority each time a thread begins waiting
(or gives up waiting). When the mutex is eventually unlocked, the unlocking
thread's priority correctly reverts to its original non-elevated priority.

The kernel does *not* fully support priority inheritance when a thread holds
two or more mutexes simultaneously. This situation can result in the thread's
priority not reverting to its original non-elevated priority when all mutexes
have been released. It is recommended that a thread lcok only a single mutex
at a time when multiple mutexes are shared between threads of different
priorities.

Implementation
**************

Defining a Mutex
================

A mutex is defined using a variable of type :c:type:`struct k_mutex`.
It must then be initialized by calling :cpp:func:`k_mutex_init()`.

The following code defines and initializes a mutex.

.. code-block:: c

    struct k_mutex my_mutex;

    k_mutex_init(&my_mutex);

Alternatively, a mutex can be defined and initialized at compile time
by calling :c:macro:`K_MUTEX_DEFINE`.

The following code has the same effect as the code segment above.

.. code-block:: c

    K_MUTEX_DEFINE(my_mutex);

Locking a Mutex
===============

A mutex is locked by calling :cpp:func:`k_mutex_lock()`.

The following code builds on the example above, and waits indefinitely
for the mutex to become available if it is already locked by another thread.

.. code-block:: c

    k_mutex_lock(&my_mutex, K_FOREVER);

The following code waits up to 100 milliseconds for the mutex to become
available, and gives a warning if the mutex does not become availablee.

.. code-block:: c

    if (k_mutex_lock(&my_mutex, K_MSEC(100)) == 0) {
        /* mutex successfully locked */
    } else {
        printf("Cannot lock XYZ display\n");
    }

Unlocking a Mutex
=================

A mutex is unlocked by calling :cpp:func:`k_mutex_unlock()`.

The following code builds on the example above,
and unlocks the mutex that was previously locked by the thread.

.. code-block:: c

    k_mutex_unlock(&my_mutex);

Suggested Uses
**************

Use a mutex to provide exclusive access to a resource, such as a physical
device.

Configuration Options
*********************

Related configuration options:

* :option:`CONFIG_PRIORITY_CEILING`

APIs
****

The following mutex APIs are provided by :file:`kernel.h`:

* :c:macro:`K_MUTEX_DEFINE`
* :cpp:func:`k_mutex_init()`
* :cpp:func:`k_mutex_lock()`
* :cpp:func:`k_mutex_unlock()`