From 72a62ad88f82fe1ffee50024db4dd0f3fa5806f7 Mon Sep 17 00:00:00 2001 From: Chris Johns Date: Thu, 3 Nov 2016 16:58:08 +1100 Subject: Rename all manuals with an _ to have a -. It helps released naming of files. --- c-user/barrier_manager.rst | 429 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 429 insertions(+) create mode 100644 c-user/barrier_manager.rst (limited to 'c-user/barrier_manager.rst') diff --git a/c-user/barrier_manager.rst b/c-user/barrier_manager.rst new file mode 100644 index 0000000..7e80879 --- /dev/null +++ b/c-user/barrier_manager.rst @@ -0,0 +1,429 @@ +.. comment SPDX-License-Identifier: CC-BY-SA-4.0 + +.. COMMENT: COPYRIGHT (c) 1988-2008. +.. COMMENT: On-Line Applications Research Corporation (OAR). +.. COMMENT: All rights reserved. + +Barrier Manager +############### + +.. index:: barrier + +Introduction +============ + +The barrier manager provides a unique synchronization capability which can be +used to have a set of tasks block and be unblocked as a set. The directives +provided by the barrier manager are: + +- rtems_barrier_create_ - Create a barrier + +- rtems_barrier_ident_ - Get ID of a barrier + +- rtems_barrier_delete_ - Delete a barrier + +- rtems_barrier_wait_ - Wait at a barrier + +- rtems_barrier_release_ - Release a barrier + +Background +========== + +A barrier can be viewed as a gate at which tasks wait until the gate is opened. +This has many analogies in the real world. Horses and other farm animals may +approach a closed gate and gather in front of it, waiting for someone to open +the gate so they may proceed. Similarly, cticket holders gather at the gates +of arenas before concerts or sporting events waiting for the arena personnel to +open the gates so they may enter. + +Barriers are useful during application initialization. Each application task +can perform its local initialization before waiting for the application as a +whole to be initialized. Once all tasks have completed their independent +initializations, the "application ready" barrier can be released. + +Automatic Versus Manual Barriers +-------------------------------- + +Just as with a real-world gate, barriers may be configured to be manually +opened or automatically opened. All tasks calling the ``rtems_barrier_wait`` +directive will block until a controlling task invokes +the ``rtems_barrier_release`` directive. + +Automatic barriers are created with a limit to the number of tasks which may +simultaneously block at the barrier. Once this limit is reached, all of the +tasks are released. For example, if the automatic limit is ten tasks, then the +first nine tasks calling the ``rtems_barrier_wait`` directive will block. When +the tenth task calls the ``rtems_barrier_wait`` directive, the nine blocked +tasks will be released and the tenth task returns to the caller without +blocking. + +Building a Barrier Attribute Set +-------------------------------- + +In general, an attribute set is built by a bitwise OR of the desired attribute +components. The following table lists the set of valid barrier attributes: + +``RTEMS_BARRIER_AUTOMATIC_RELEASE`` + automatically release the barrier when the configured number of tasks are + blocked + +``RTEMS_BARRIER_MANUAL_RELEASE`` + only release the barrier when the application invokes the + ``rtems_barrier_release`` directive. (default) + +.. note:: + + Barriers only support FIFO blocking order because all waiting tasks are + released as a set. Thus the released tasks will all become ready to execute + at the same time and compete for the processor based upon their priority. + +Attribute values are specifically designed to be mutually exclusive, therefore +bitwise OR and addition operations are equivalent as long as each attribute +appears exactly once in the component list. An attribute listed as a default +is not required to appear in the attribute list, although it is a good +programming practice to specify default attributes. If all defaults are +desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this +call. + +This example demonstrates the attribute_set parameter needed to create a +barrier with the automatic release policy. The ``attribute_set`` parameter +passed to the ``rtems_barrier_create`` directive will be +``RTEMS_BARRIER_AUTOMATIC_RELEASE``. In this case, the user must also specify +the ``maximum_waiters`` parameter. + +Operations +========== + +Creating a Barrier +------------------ + +The ``rtems_barrier_create`` directive creates a barrier with a user-specified +name and the desired attributes. RTEMS allocates a Barrier Control Block (BCB) +from the BCB free list. This data structure is used by RTEMS to manage the +newly created barrier. Also, a unique barrier ID is generated and returned to +the calling task. + +Obtaining Barrier IDs +--------------------- + +When a barrier is created, RTEMS generates a unique barrier ID and assigns it +to the created barrier until it is deleted. The barrier ID may be obtained by +either of two methods. First, as the result of an invocation of the +``rtems_barrier_create`` directive, the barrier ID is stored in a user provided +location. Second, the barrier ID may be obtained later using the +``rtems_barrier_ident`` directive. The barrier ID is used by other barrier +manager directives to access this barrier. + +Waiting at a Barrier +-------------------- + +The ``rtems_barrier_wait`` directive is used to wait at +the specified barrier. Since a barrier is, by definition, never immediately, +the task may wait forever for the barrier to be released or it may +specify a timeout. Specifying a timeout limits the interval the task will +wait before returning with an error status code. + +If the barrier is configured as automatic and there are already one less then +the maximum number of waiters, then the call will unblock all tasks waiting at +the barrier and the caller will return immediately. + +When the task does wait to acquire the barrier, then it is placed in the +barrier's task wait queue in FIFO order. All tasks waiting on a barrier are +returned an error code when the barrier is deleted. + +Releasing a Barrier +------------------- + +The ``rtems_barrier_release`` directive is used to release the specified +barrier. When the ``rtems_barrier_release`` is invoked, all tasks waiting at +the barrier are immediately made ready to execute and begin to compete for the +processor to execute. + +Deleting a Barrier +------------------ + +The ``rtems_barrier_delete`` directive removes a barrier from the system and +frees its control block. A barrier can be deleted by any local task that knows +the barrier's ID. As a result of this directive, all tasks blocked waiting for +the barrier to be released, will be readied and returned a status code which +indicates that the barrier was deleted. Any subsequent references to the +barrier's name and ID are invalid. + +Directives +========== + +This section details the barrier manager's directives. A subsection is +dedicated to each of this manager's directives and describes the calling +sequence, related constants, usage, and status codes. + +.. _rtems_barrier_create: + +BARRIER_CREATE - Create a barrier +--------------------------------- +.. index:: create a barrier + +**CALLING SEQUENCE:** + +.. index:: rtems_barrier_create + +.. code-block:: c + + rtems_status_code rtems_barrier_create( + rtems_name name, + rtems_attribute attribute_set, + uint32_t maximum_waiters, + rtems_id *id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - barrier created successfully + * - ``RTEMS_INVALID_NAME`` + - invalid barrier name + * - ``RTEMS_INVALID_ADDRESS`` + - ``id`` is NULL + * - ``RTEMS_TOO_MANY`` + - too many barriers created + +**DESCRIPTION:** + +This directive creates a barrier which resides on the local node. The created +barrier has the user-defined name specified in ``name`` and the initial count +specified in ``count``. For control and maintenance of the barrier, RTEMS +allocates and initializes a BCB. The RTEMS-assigned barrier id is returned in +``id``. This barrier id is used with other barrier related directives to +access the barrier. + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_BARRIER_MANUAL_RELEASE`` + - only release + +Specifying ``RTEMS_BARRIER_AUTOMATIC_RELEASE`` in ``attribute_set`` causes +tasks calling the ``rtems_barrier_wait`` directive to block until there are +``maximum_waiters - 1`` tasks waiting at the barrier. When the +``maximum_waiters`` task invokes the ``rtems_barrier_wait`` directive, the +previous ``maximum_waiters - 1`` tasks are automatically released and the +caller returns. + +In contrast, when the ``RTEMS_BARRIER_MANUAL_RELEASE`` attribute is specified, +there is no limit on the number of tasks that will block at the barrier. Only +when the ``rtems_barrier_release`` directive is invoked, are the tasks waiting +at the barrier unblocked. + +**NOTES:** + +This directive will not cause the calling task to be preempted. + +The following barrier attribute constants are defined by RTEMS: + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_BARRIER_AUTOMATIC_RELEASE`` + - automatically release the barrier when the configured number of tasks are + blocked + * - ``RTEMS_BARRIER_MANUAL_RELEASE`` + - only release the barrier when the application invokes + the ``rtems_barrier_release`` directive. (default) + +.. _rtems_barrier_ident: + +BARRIER_IDENT - Get ID of a barrier +----------------------------------- +.. index:: get ID of a barrier +.. index:: obtain ID of a barrier + +**CALLING SEQUENCE:** + +.. index:: rtems_barrier_ident + +.. code-block:: c + + rtems_status_code rtems_barrier_ident( + rtems_name name, + rtems_id *id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - barrier identified successfully + * - ``RTEMS_INVALID_NAME`` + - barrier name not found + * - ``RTEMS_INVALID_NODE`` + - invalid node id + +**DESCRIPTION:** + +This directive obtains the barrier id associated with the barrier name. If the +barrier name is not unique, then the barrier id will match one of the barriers +with that name. However, this barrier id is not guaranteed to correspond to +the desired barrier. The barrier id is used by other barrier related +directives to access the barrier. + +**NOTES:** + +This directive will not cause the running task to be preempted. + +.. _rtems_barrier_delete: + +BARRIER_DELETE - Delete a barrier +--------------------------------- +.. index:: delete a barrier + +**CALLING SEQUENCE:** + +.. index:: rtems_barrier_delete + +.. code-block:: c + + rtems_status_code rtems_barrier_delete( + rtems_id id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - barrier deleted successfully + * - ``RTEMS_INVALID_ID`` + - invalid barrier id + +**DESCRIPTION:** + +This directive deletes the barrier specified by ``id``. All tasks blocked +waiting for the barrier to be released will be readied and returned a status +code which indicates that the barrier was deleted. The BCB for this barrier is +reclaimed by RTEMS. + +**NOTES:** + +The calling task will be preempted if it is enabled by the task's execution +mode and a higher priority local task is waiting on the deleted barrier. The +calling task will NOT be preempted if all of the tasks that are waiting on the +barrier are remote tasks. + +The calling task does not have to be the task that created the barrier. Any +local task that knows the barrier id can delete the barrier. + +.. _rtems_barrier_wait: + +BARRIER_OBTAIN - Acquire a barrier +---------------------------------- +.. index:: obtain a barrier +.. index:: lock a barrier + +**CALLING SEQUENCE:** + +.. index:: rtems_barrier_wait + +.. code-block:: c + + rtems_status_code rtems_barrier_wait( + rtems_id id, + rtems_interval timeout + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - barrier released and task unblocked + * - ``RTEMS_UNSATISFIED`` + - barrier not available + * - ``RTEMS_TIMEOUT`` + - timed out waiting for barrier + * - ``RTEMS_OBJECT_WAS_DELETED`` + - barrier deleted while waiting + * - ``RTEMS_INVALID_ID`` + - invalid barrier id + +**DESCRIPTION:** + +This directive acquires the barrier specified by ``id``. The ``RTEMS_WAIT`` +and ``RTEMS_NO_WAIT`` components of the options parameter indicate whether the +calling task wants to wait for the barrier to become available or return +immediately if the barrier is not currently available. With either +``RTEMS_WAIT`` or ``RTEMS_NO_WAIT``, if the current barrier count is positive, +then it is decremented by one and the barrier is successfully acquired by +returning immediately with a successful return code. + +Conceptually, the calling task should always be thought of as blocking when it +makes this call and being unblocked when the barrier is released. If the +barrier is configured for manual release, this rule of thumb will always be +valid. If the barrier is configured for automatic release, all callers will +block except for the one which is the Nth task which trips the automatic +release condition. + +The timeout parameter specifies the maximum interval the calling task is +willing to be blocked waiting for the barrier. If it is set to +``RTEMS_NO_TIMEOUT``, then the calling task will wait forever. If the barrier +is available or the ``RTEMS_NO_WAIT`` option component is set, then timeout is +ignored. + +**NOTES:** + +The following barrier acquisition option constants are defined by RTEMS: + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_WAIT`` + - task will wait for barrier (default) + * - ``RTEMS_NO_WAIT`` + - task should not wait + +A clock tick is required to support the timeout functionality of this +directive. + +.. _rtems_barrier_release: + +BARRIER_RELEASE - Release a barrier +----------------------------------- +.. index:: wait at a barrier +.. index:: release a barrier + +**CALLING SEQUENCE:** + +.. index:: rtems_barrier_release + +.. code-block:: c + + rtems_status_code rtems_barrier_release( + rtems_id id, + uint32_t *released + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``RTEMS_SUCCESSFUL`` + - barrier released successfully + * - ``RTEMS_INVALID_ID`` + - invalid barrier id + +**DESCRIPTION:** + +This directive releases the barrier specified by id. All tasks waiting at the +barrier will be unblocked. If the running task's preemption mode is enabled +and one of the unblocked tasks has a higher priority than the running task. + +**NOTES:** + +The calling task may be preempted if it causes a higher priority task to be +made ready for execution. -- cgit v1.2.3