source: rtems/doc/user/barrier.t @ 880e2f37

@c
@c  COPYRIGHT (c) 1988-2007.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.

@chapter Barrier Manager

@cindex barrier

@section 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:

@itemize @bullet
@item @code{@value{DIRPREFIX}barrier_create} - Create a barrier
@item @code{@value{DIRPREFIX}barrier_ident} - Get ID of a barrier
@item @code{@value{DIRPREFIX}barrier_delete} - Delete a barrier
@item @code{@value{DIRPREFIX}barrier_wait} - Wait at a barrier
@item @code{@value{DIRPREFIX}barrier_release} - Release a barrier
@end itemize

@section 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.

@subsection 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 @code{@value{DIRPREFIX}barrier_wait} directive
will block until a controlling task invokes the 
@code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}barrier_wait} directive
will block.  When the tenth task calls the
@code{@value{DIRPREFIX}barrier_wait} directive, the nine 
blocked tasks will be released and the tenth task returns
to the caller without blocking.

@subsection 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:

@itemize @bullet

@item @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} - automatically
release the barrier when the configured number of tasks are blocked

@item @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release
the barrier when the application invokes the
@code{@value{DIRPREFIX}barrier_release} directive.  (default)

@end itemize

@b{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
@code{@value{RPREFIX}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
@code{attribute_set} parameter passed to the
@code{@value{DIRPREFIX}barrier_create} directive will be
@code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE}.  In this case, the
user must also specify the @i{maximum_waiters} parameter.

@section Operations

@subsection Creating a Barrier

The @code{@value{DIRPREFIX}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.

@subsection 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
@code{@value{DIRPREFIX}barrier_create} directive, the
barrier ID is stored in a user provided location.  Second,
the barrier ID may be obtained later using the
@code{@value{DIRPREFIX}barrier_ident} directive.  The barrier ID is
used by other barrier manager directives to access this
barrier.

@subsection Waiting at a Barrier

The @code{@value{DIRPREFIX}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.

@subsection Releasing a Barrier

The @code{@value{DIRPREFIX}barrier_release} directive is used to release
the specified barrier.  When the @code{@value{DIRPREFIX}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.

@subsection Deleting a Barrier

The @code{@value{DIRPREFIX}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.

@section 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.

@c
@c
@c
@page
@subsection BARRIER_CREATE - Create a barrier

@cindex create a barrier

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_barrier_create
@example
rtems_status_code rtems_barrier_create(
  rtems_name           name,
  rtems_attribute      attribute_set,
  uint32_t             maximum_waiters,
  rtems_id            *id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Barrier_Create (
   Name            : in     RTEMS.Name;
   Attribute_Set   : in     RTEMS.Attribute;
   Maximum_Waiters : in     RTEMS.Unsigned32;
   ID              :    out RTEMS.ID;
   Result          :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - barrier created successfully@*
@code{@value{RPREFIX}INVALID_NAME} - invalid barrier name@*
@code{@value{RPREFIX}INVALID_ADDRESS} - @code{id} is NULL@*
@code{@value{RPREFIX}TOO_MANY} - too many barriers created@

@subheading DESCRIPTION:

This directive creates a barrier which resides on
the local node. The created barrier has the user-defined name
specified in @code{name} and the initial count specified in @code{count}.  For
control and maintenance of the barrier, RTEMS allocates and
initializes a BCB.  The RTEMS-assigned barrier id is returned
in @code{id}.  This barrier id is used with other barrier related
directives to access the barrier.

@code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release

Specifying @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} in
@code{attribute_set} causes tasks calling the 
@code{@value{DIRPREFIX}barrier_wait} directive to block until
there are @code{maximum_waiters - 1} tasks waiting at the barrier.
When the @code{maximum_waiters} task invokes the 
@code{@value{DIRPREFIX}barrier_wait} directive, the previous
@code{maximum_waiters - 1} tasks are automatically released
and the caller returns.

In contrast, when the @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE}
attribute is specified, there is no limit on the number of
tasks that will block at the barrier. Only when the
@code{@value{DIRPREFIX}barrier_release} directive is invoked,
are the tasks waiting at the barrier unblocked.

@subheading NOTES:

This directive will not cause the calling task to be preempted.

The following barrier attribute constants are defined by RTEMS:

@itemize @bullet

@item @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} - automatically
release the barrier when the configured number of tasks are blocked

@item @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release
the barrier when the application invokes the
@code{@value{DIRPREFIX}barrier_release} directive.  (default)

@end itemize

@c
@c
@c
@page
@subsection BARRIER_IDENT - Get ID of a barrier

@cindex get ID of a barrier
@cindex obtain ID of a barrier

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_barrier_ident
@example
rtems_status_code rtems_barrier_ident(
  rtems_name        name,
  rtems_id         *id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Barrier_Ident (
   Name   : in     RTEMS.Name;
   ID     :    out RTEMS.ID;
   Result :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - barrier identified successfully@*
@code{@value{RPREFIX}INVALID_NAME} - barrier name not found@*
@code{@value{RPREFIX}INVALID_NODE} - invalid node id

@subheading 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.

@subheading NOTES:

This directive will not cause the running task to be
preempted.

@c
@c
@c
@page
@subsection BARRIER_DELETE - Delete a barrier

@cindex delete a barrier

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_barrier_delete
@example
rtems_status_code rtems_barrier_delete(
  rtems_id id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Barrier_Delete (
   ID     : in     RTEMS.ID;
   Result :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - barrier deleted successfully@*
@code{@value{RPREFIX}INVALID_ID} - invalid barrier id@

@subheading DESCRIPTION:

This directive deletes the barrier specified by @code{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.

@subheading 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.

@c
@c Barrier Obtain
@c
@page
@subsection BARRIER_OBTAIN - Acquire a barrier

@cindex obtain a barrier
@cindex lock a barrier

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_barrier_wait
@example
rtems_status_code rtems_barrier_wait(
  rtems_id         id,
  rtems_interval   timeout
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Barrier_Wait (
   ID         : in     RTEMS.ID;
   Timeout    : in     RTEMS.Interval;
   Result     :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - barrier released and task unblocked@*
@code{@value{RPREFIX}UNSATISFIED} - barrier not available@*
@code{@value{RPREFIX}TIMEOUT} - timed out waiting for barrier@*
@code{@value{RPREFIX}OBJECT_WAS_DELETED} - barrier deleted while waiting@*
@code{@value{RPREFIX}INVALID_ID} - invalid barrier id

@subheading DESCRIPTION:

This directive acquires the barrier specified by
id.  The @code{@value{RPREFIX}WAIT} and @code{@value{RPREFIX}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
@code{@value{RPREFIX}WAIT} or @code{@value{RPREFIX}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
@code{@value{RPREFIX}NO_TIMEOUT}, then the calling task will wait forever.  
If the barrier is available or the @code{@value{RPREFIX}NO_WAIT} option
component is set, then timeout is ignored.

@subheading NOTES:
The following barrier acquisition option constants are defined by RTEMS:

@itemize @bullet
@item @code{@value{RPREFIX}WAIT} - task will wait for barrier (default)
@item @code{@value{RPREFIX}NO_WAIT} - task should not wait
@end itemize

A clock tick is required to support the timeout functionality of
this directive.

@c
@c Release Barrier
@c
@page
@subsection BARRIER_RELEASE - Release a barrier

@cindex wait at a barrier
@cindex release a barrier

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_barrier_release
@example
rtems_status_code rtems_barrier_release(
  rtems_id  id,
  uint32_t *released
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Barrier_Release (
   ID       : in     RTEMS.ID;
   Released :    out RTEMS.Unsigned32;
   Result   :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - barrier released successfully@*
@code{@value{RPREFIX}INVALID_ID} - invalid barrier id@*

@subheading 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.

@subheading NOTES:

The calling task may be preempted if it causes a
higher priority task to be made ready for execution.