[adfdb1c] | 1 | @c |
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[1b03eed] | 2 | @c COPYRIGHT (c) 1988-2007. |
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[adfdb1c] | 3 | @c On-Line Applications Research Corporation (OAR). |
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| 4 | @c All rights reserved. |
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| 5 | @c |
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| 6 | @c $Id$ |
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| 7 | @c |
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| 8 | |
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| 9 | @chapter Barrier Manager |
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| 10 | |
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| 11 | @cindex barrier |
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| 12 | |
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| 13 | @section Introduction |
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| 14 | |
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| 15 | The barrier manager provides a unique synchronization |
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| 16 | capability which can be used to have a set of tasks block |
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| 17 | and be unblocked as a set. The directives provided by the |
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| 18 | barrier manager are: |
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| 19 | |
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| 20 | @itemize @bullet |
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| 21 | @item @code{@value{DIRPREFIX}barrier_create} - Create a barrier |
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| 22 | @item @code{@value{DIRPREFIX}barrier_ident} - Get ID of a barrier |
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| 23 | @item @code{@value{DIRPREFIX}barrier_delete} - Delete a barrier |
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| 24 | @item @code{@value{DIRPREFIX}barrier_wait} - Wait at a barrier |
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| 25 | @item @code{@value{DIRPREFIX}barrier_release} - Release a barrier |
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| 26 | @end itemize |
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| 27 | |
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| 28 | @section Background |
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| 29 | |
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| 30 | A barrier can be viewed as a gate at which tasks wait until |
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| 31 | the gate is opened. This has many analogies in the real world. |
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| 32 | Horses and other farm animals may approach a closed gate and |
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| 33 | gather in front of it, waiting for someone to open the gate so |
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| 34 | they may proceed. Similarly, cticket holders gather at the gates |
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| 35 | of arenas before concerts or sporting events waiting for the |
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| 36 | arena personnel to open the gates so they may enter. |
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| 37 | |
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| 38 | Barriers are useful during application initialization. Each |
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| 39 | application task can perform its local initialization before |
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| 40 | waiting for the application as a whole to be initialized. Once |
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| 41 | all tasks have completed their independent initializations, |
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| 42 | the "application ready" barrier can be released. |
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| 43 | |
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| 44 | @subsection Automatic Versus Manual Barriers |
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| 45 | |
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| 46 | Just as with a real-world gate, barriers may be configured to |
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| 47 | be manually opened or automatically opened. All tasks |
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| 48 | calling the @code{@value{DIRPREFIX}barrier_wait} directive |
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| 49 | will block until a controlling task invokes the |
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| 50 | @code{@value{DIRPREFIX}barrier_release} directive. |
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| 51 | |
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| 52 | Automatic barriers are created with a limit to the number of |
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| 53 | tasks which may simultaneously block at the barrier. Once |
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| 54 | this limit is reached, all of the tasks are released. For |
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| 55 | example, if the automatic limit is ten tasks, then the first |
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| 56 | nine tasks calling the @code{@value{DIRPREFIX}barrier_wait} directive |
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| 57 | will block. When the tenth task calls the |
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| 58 | @code{@value{DIRPREFIX}barrier_wait} directive, the nine |
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| 59 | blocked tasks will be released and the tenth task returns |
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| 60 | to the caller without blocking. |
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| 61 | |
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| 62 | @subsection Building a Barrier Attribute Set |
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| 63 | |
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| 64 | In general, an attribute set is built by a bitwise OR |
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| 65 | of the desired attribute components. The following table lists |
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| 66 | the set of valid barrier attributes: |
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| 67 | |
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| 68 | @itemize @bullet |
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| 69 | |
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| 70 | @item @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} - automatically |
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| 71 | release the barrier when the configured number of tasks are blocked |
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| 72 | |
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| 73 | @item @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release |
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| 74 | the barrier when the application invokes the |
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| 75 | @code{@value{DIRPREFIX}barrier_release} directive. (default) |
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| 76 | |
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| 77 | @end itemize |
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| 78 | |
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| 79 | @b{NOTE}: Barriers only support FIFO blocking order because all |
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| 80 | waiting tasks are released as a set. Thus the released tasks |
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| 81 | will all become ready to execute at the same time and compete |
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| 82 | for the processor based upon their priority. |
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| 83 | |
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| 84 | Attribute values are specifically designed to be |
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| 85 | mutually exclusive, therefore bitwise OR and addition operations |
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| 86 | are equivalent as long as each attribute appears exactly once in |
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| 87 | the component list. An attribute listed as a default is not |
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| 88 | required to appear in the attribute list, although it is a good |
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| 89 | programming practice to specify default attributes. If all |
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| 90 | defaults are desired, the attribute |
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| 91 | @code{@value{RPREFIX}DEFAULT_ATTRIBUTES} should be |
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| 92 | specified on this call. |
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| 93 | |
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| 94 | This example demonstrates the attribute_set parameter needed to create a |
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| 95 | barrier with the automatic release policy. The |
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| 96 | @code{attribute_set} parameter passed to the |
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| 97 | @code{@value{DIRPREFIX}barrier_create} directive will be |
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| 98 | @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE}. In this case, the |
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| 99 | user must also specify the @i{maximum_waiters} parameter. |
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| 100 | |
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| 101 | @section Operations |
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| 102 | |
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| 103 | @subsection Creating a Barrier |
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| 104 | |
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| 105 | The @code{@value{DIRPREFIX}barrier_create} directive creates |
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| 106 | a barrier with a user-specified name and the desired attributes. |
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| 107 | RTEMS allocates a Barrier Control Block (BCB) from the BCB free list. |
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| 108 | This data structure is used by RTEMS to manage the newly created |
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| 109 | barrier. Also, a unique barrier ID is generated and returned to |
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| 110 | the calling task. |
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| 111 | |
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| 112 | @subsection Obtaining Barrier IDs |
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| 113 | |
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| 114 | When a barrier is created, RTEMS generates a unique |
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| 115 | barrier ID and assigns it to the created barrier until it is |
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| 116 | deleted. The barrier ID may be obtained by either of two |
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| 117 | methods. First, as the result of an invocation of the |
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| 118 | @code{@value{DIRPREFIX}barrier_create} directive, the |
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| 119 | barrier ID is stored in a user provided location. Second, |
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| 120 | the barrier ID may be obtained later using the |
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| 121 | @code{@value{DIRPREFIX}barrier_ident} directive. The barrier ID is |
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| 122 | used by other barrier manager directives to access this |
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| 123 | barrier. |
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| 124 | |
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| 125 | @subsection Waiting at a Barrier |
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| 126 | |
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| 127 | The @code{@value{DIRPREFIX}barrier_wait} directive is used to wait at |
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| 128 | the specified barrier. Since a barrier is, by definition, never immediately, |
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| 129 | the task may wait forever for the barrier to be released or it may |
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| 130 | specify a timeout. Specifying a timeout limits the interval the task will |
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| 131 | wait before returning with an error status code. |
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| 132 | |
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| 133 | If the barrier is configured as automatic and there are already |
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| 134 | one less then the maximum number of waiters, then the call will |
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| 135 | unblock all tasks waiting at the barrier and the caller will |
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| 136 | return immediately. |
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| 137 | |
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| 138 | When the task does wait to acquire the barrier, then it |
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| 139 | is placed in the barrier's task wait queue in FIFO order. |
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| 140 | All tasks waiting on a barrier are returned an error |
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| 141 | code when the barrier is deleted. |
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| 142 | |
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| 143 | @subsection Releasing a Barrier |
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| 144 | |
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| 145 | The @code{@value{DIRPREFIX}barrier_release} directive is used to release |
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| 146 | the specified barrier. When the @code{@value{DIRPREFIX}barrier_release} |
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| 147 | is invoked, all tasks waiting at the barrier are immediately made ready |
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| 148 | to execute and begin to compete for the processor to execute. |
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| 149 | |
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| 150 | @subsection Deleting a Barrier |
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| 151 | |
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| 152 | The @code{@value{DIRPREFIX}barrier_delete} directive removes a barrier |
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| 153 | from the system and frees its control block. A barrier can be |
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| 154 | deleted by any local task that knows the barrier's ID. As a |
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| 155 | result of this directive, all tasks blocked waiting for the |
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| 156 | barrier to be released, will be readied and returned a status code which |
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| 157 | indicates that the barrier was deleted. Any subsequent |
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| 158 | references to the barrier's name and ID are invalid. |
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| 159 | |
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| 160 | @section Directives |
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| 161 | |
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| 162 | This section details the barrier manager's |
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| 163 | directives. A subsection is dedicated to each of this manager's |
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| 164 | directives and describes the calling sequence, related |
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| 165 | constants, usage, and status codes. |
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| 166 | |
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| 167 | @c |
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| 168 | @c |
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| 169 | @c |
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| 170 | @page |
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| 171 | @subsection BARRIER_CREATE - Create a barrier |
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| 172 | |
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| 173 | @cindex create a barrier |
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| 174 | |
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| 175 | @subheading CALLING SEQUENCE: |
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| 176 | |
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| 177 | @ifset is-C |
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| 178 | @findex rtems_barrier_create |
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| 179 | @example |
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| 180 | rtems_status_code rtems_barrier_create( |
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| 181 | rtems_name name, |
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| 182 | rtems_attribute attribute_set, |
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| 183 | uint32_t maximum_waiters, |
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| 184 | rtems_id *id |
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| 185 | ); |
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| 186 | @end example |
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| 187 | @end ifset |
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| 188 | |
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| 189 | @ifset is-Ada |
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| 190 | @example |
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| 191 | procedure Barrier_Create ( |
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| 192 | Name : in RTEMS.Name; |
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| 193 | Attribute_Set : in RTEMS.Attribute; |
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| 194 | Maximum_Waiters : in RTEMS.Unsigned32; |
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| 195 | ID : out RTEMS.ID; |
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| 196 | Result : out RTEMS.Status_Codes |
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| 197 | ); |
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| 198 | @end example |
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| 199 | @end ifset |
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| 200 | |
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| 201 | @subheading DIRECTIVE STATUS CODES: |
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| 202 | @code{@value{RPREFIX}SUCCESSFUL} - barrier created successfully@* |
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| 203 | @code{@value{RPREFIX}INVALID_NAME} - invalid task name@* |
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| 204 | @code{@value{RPREFIX}INVALID_ADDRESS} - @code{id} is NULL@* |
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| 205 | @code{@value{RPREFIX}TOO_MANY} - too many barriers created@ |
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| 206 | |
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| 207 | @subheading DESCRIPTION: |
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| 208 | |
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| 209 | This directive creates a barrier which resides on |
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| 210 | the local node. The created barrier has the user-defined name |
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| 211 | specified in @code{name} and the initial count specified in @code{count}. For |
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| 212 | control and maintenance of the barrier, RTEMS allocates and |
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| 213 | initializes a BCB. The RTEMS-assigned barrier id is returned |
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| 214 | in @code{id}. This barrier id is used with other barrier related |
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| 215 | directives to access the barrier. |
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| 216 | |
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| 217 | @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release |
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| 218 | |
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| 219 | Specifying @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} in |
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| 220 | @code{attribute_set} causes tasks calling the |
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| 221 | @code{@value{DIRPREFIX}barrier_wait} directive to block until |
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| 222 | there are @code{maximum_waiters - 1} tasks waiting at the barrier. |
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| 223 | When the @code{maximum_waiters} task invokes the |
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| 224 | @code{@value{DIRPREFIX}barrier_wait} directive, the previous |
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| 225 | @code{maximum_waiters - 1} tasks are automatically released |
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| 226 | and the caller returns. |
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| 227 | |
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| 228 | In contrast, when the @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} |
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| 229 | attribute is specified, there is no limit on the number of |
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| 230 | tasks that will block at the barrier. Only when the |
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| 231 | @code{@value{DIRPREFIX}barrier_release} directive is invoked, |
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| 232 | are the tasks waiting at the barrier unblocked. |
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| 233 | |
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| 234 | @subheading NOTES: |
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| 235 | |
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| 236 | This directive will not cause the calling task to be preempted. |
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| 237 | |
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| 238 | The following barrier attribute constants are defined by RTEMS: |
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| 239 | |
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| 240 | @itemize @bullet |
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| 241 | |
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| 242 | @item @code{@value{RPREFIX}BARRIER_AUTOMATIC_RELEASE} - automatically |
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| 243 | release the barrier when the configured number of tasks are blocked |
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| 244 | |
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| 245 | @item @code{@value{RPREFIX}BARRIER_MANUAL_RELEASE} - only release |
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| 246 | the barrier when the application invokes the |
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| 247 | @code{@value{DIRPREFIX}barrier_release} directive. (default) |
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| 248 | |
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| 249 | @end itemize |
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| 250 | |
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| 251 | @c |
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| 252 | @c |
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| 253 | @c |
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| 254 | @page |
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| 255 | @subsection BARRIER_IDENT - Get ID of a barrier |
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| 256 | |
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| 257 | @cindex get ID of a barrier |
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| 258 | @cindex obtain ID of a barrier |
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| 259 | |
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| 260 | @subheading CALLING SEQUENCE: |
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| 261 | |
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| 262 | @ifset is-C |
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| 263 | @findex rtems_barrier_ident |
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| 264 | @example |
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| 265 | rtems_status_code rtems_barrier_ident( |
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| 266 | rtems_name name, |
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| 267 | rtems_id *id |
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| 268 | ); |
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| 269 | @end example |
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| 270 | @end ifset |
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| 271 | |
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| 272 | @ifset is-Ada |
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| 273 | @example |
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| 274 | procedure Barrier_Ident ( |
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| 275 | Name : in RTEMS.Name; |
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| 276 | ID : out RTEMS.ID; |
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| 277 | Result : out RTEMS.Status_Codes |
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| 278 | ); |
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| 279 | @end example |
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| 280 | @end ifset |
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| 281 | |
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| 282 | @subheading DIRECTIVE STATUS CODES: |
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| 283 | @code{@value{RPREFIX}SUCCESSFUL} - barrier identified successfully@* |
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| 284 | @code{@value{RPREFIX}INVALID_NAME} - barrier name not found@* |
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| 285 | @code{@value{RPREFIX}INVALID_NODE} - invalid node id |
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| 286 | |
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| 287 | @subheading DESCRIPTION: |
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| 288 | |
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| 289 | This directive obtains the barrier id associated |
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| 290 | with the barrier name. If the barrier name is not unique, |
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| 291 | then the barrier id will match one of the barriers with that |
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| 292 | name. However, this barrier id is not guaranteed to |
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| 293 | correspond to the desired barrier. The barrier id is used |
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| 294 | by other barrier related directives to access the barrier. |
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| 295 | |
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| 296 | @subheading NOTES: |
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| 297 | |
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| 298 | This directive will not cause the running task to be |
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| 299 | preempted. |
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| 300 | |
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| 301 | @c |
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| 302 | @c |
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| 303 | @c |
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| 304 | @page |
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| 305 | @subsection BARRIER_DELETE - Delete a barrier |
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| 306 | |
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| 307 | @cindex delete a barrier |
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| 308 | |
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| 309 | @subheading CALLING SEQUENCE: |
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| 310 | |
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| 311 | @ifset is-C |
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| 312 | @findex rtems_barrier_delete |
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| 313 | @example |
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| 314 | rtems_status_code rtems_barrier_delete( |
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| 315 | rtems_id id |
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| 316 | ); |
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| 317 | @end example |
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| 318 | @end ifset |
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| 319 | |
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| 320 | @ifset is-Ada |
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| 321 | @example |
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| 322 | procedure Barrier_Delete ( |
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| 323 | ID : in RTEMS.ID; |
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| 324 | Result : out RTEMS.Status_Codes |
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| 325 | ); |
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| 326 | @end example |
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| 327 | @end ifset |
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| 328 | |
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| 329 | @subheading DIRECTIVE STATUS CODES: |
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| 330 | @code{@value{RPREFIX}SUCCESSFUL} - barrier deleted successfully@* |
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| 331 | @code{@value{RPREFIX}INVALID_ID} - invalid barrier id@ |
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| 332 | |
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| 333 | @subheading DESCRIPTION: |
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| 334 | |
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| 335 | This directive deletes the barrier specified by @code{id}. |
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| 336 | All tasks blocked waiting for the barrier to be released will be |
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| 337 | readied and returned a status code which indicates that the |
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| 338 | barrier was deleted. The BCB for this barrier is reclaimed |
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| 339 | by RTEMS. |
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| 340 | |
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| 341 | @subheading NOTES: |
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| 342 | |
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| 343 | The calling task will be preempted if it is enabled |
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| 344 | by the task's execution mode and a higher priority local task is |
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| 345 | waiting on the deleted barrier. The calling task will NOT be |
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| 346 | preempted if all of the tasks that are waiting on the barrier |
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| 347 | are remote tasks. |
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| 348 | |
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| 349 | The calling task does not have to be the task that |
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| 350 | created the barrier. Any local task that knows the barrier |
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| 351 | id can delete the barrier. |
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| 352 | |
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| 353 | @c |
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[e8e2693c] | 354 | @c Barrier Obtain |
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[adfdb1c] | 355 | @c |
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| 356 | @page |
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| 357 | @subsection BARRIER_OBTAIN - Acquire a barrier |
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| 358 | |
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| 359 | @cindex obtain a barrier |
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| 360 | @cindex lock a barrier |
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| 361 | |
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| 362 | @subheading CALLING SEQUENCE: |
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| 363 | |
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| 364 | @ifset is-C |
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[1b03eed] | 365 | @findex rtems_barrier_wait |
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[adfdb1c] | 366 | @example |
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[1b03eed] | 367 | rtems_status_code rtems_barrier_wait( |
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[adfdb1c] | 368 | rtems_id id, |
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| 369 | rtems_unsigned32 option_set, |
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| 370 | rtems_interval timeout |
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| 371 | ); |
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| 372 | @end example |
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| 373 | @end ifset |
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| 374 | |
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| 375 | @ifset is-Ada |
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| 376 | @example |
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[1b03eed] | 377 | procedure Barrier_Wait ( |
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[adfdb1c] | 378 | ID : in RTEMS.ID; |
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| 379 | Option_Set : in RTEMS.Option; |
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| 380 | Timeout : in RTEMS.Interval; |
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| 381 | Result : out RTEMS.Status_Codes |
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| 382 | ); |
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| 383 | @end example |
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| 384 | @end ifset |
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| 385 | |
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| 386 | @subheading DIRECTIVE STATUS CODES: |
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| 387 | @code{@value{RPREFIX}SUCCESSFUL} - barrier obtained successfully@* |
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| 388 | @code{@value{RPREFIX}UNSATISFIED} - barrier not available@* |
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| 389 | @code{@value{RPREFIX}TIMEOUT} - timed out waiting for barrier@* |
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| 390 | @code{@value{RPREFIX}OBJECT_WAS_DELETED} - barrier deleted while waiting@* |
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| 391 | @code{@value{RPREFIX}INVALID_ID} - invalid barrier id |
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| 392 | |
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| 393 | @subheading DESCRIPTION: |
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| 394 | |
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| 395 | This directive acquires the barrier specified by |
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[e8e2693c] | 396 | id. The @code{@value{RPREFIX}WAIT} and @code{@value{RPREFIX}NO_WAIT} |
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| 397 | components of the options parameter indicate whether the calling task |
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| 398 | wants to wait for the barrier to become available or return immediately |
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| 399 | if the barrier is not currently available. With either |
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| 400 | @code{@value{RPREFIX}WAIT} or @code{@value{RPREFIX}NO_WAIT}, |
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| 401 | if the current barrier count is positive, then it is |
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[adfdb1c] | 402 | decremented by one and the barrier is successfully acquired by |
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| 403 | returning immediately with a successful return code. |
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| 404 | |
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[e8e2693c] | 405 | Conceptually, the calling task should always be thought |
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| 406 | of as blocking when it makes this call and being unblocked when |
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| 407 | the barrier is released. If the barrier is configured for |
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| 408 | manual release, this rule of thumb will always be valid. |
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| 409 | If the barrier is configured for automatic release, all callers |
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| 410 | will block except for the one which is the Nth task which trips |
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| 411 | the automatic release condition. |
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[adfdb1c] | 412 | |
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| 413 | The timeout parameter specifies the maximum interval the calling task is |
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| 414 | willing to be blocked waiting for the barrier. If it is set to |
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| 415 | @code{@value{RPREFIX}NO_TIMEOUT}, then the calling task will wait forever. |
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| 416 | If the barrier is available or the @code{@value{RPREFIX}NO_WAIT} option |
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| 417 | component is set, then timeout is ignored. |
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| 418 | |
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| 419 | @subheading NOTES: |
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[e8e2693c] | 420 | The following barrier acquisition option constants are defined by RTEMS: |
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[adfdb1c] | 421 | |
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| 422 | @itemize @bullet |
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| 423 | @item @code{@value{RPREFIX}WAIT} - task will wait for barrier (default) |
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| 424 | @item @code{@value{RPREFIX}NO_WAIT} - task should not wait |
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| 425 | @end itemize |
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| 426 | |
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| 427 | A clock tick is required to support the timeout functionality of |
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| 428 | this directive. |
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| 429 | |
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| 430 | @c |
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[e8e2693c] | 431 | @c Release Barrier |
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[adfdb1c] | 432 | @c |
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| 433 | @page |
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| 434 | @subsection BARRIER_RELEASE - Release a barrier |
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| 435 | |
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| 436 | @cindex wait at a barrier |
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| 437 | @cindex release a barrier |
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| 438 | |
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| 439 | @subheading CALLING SEQUENCE: |
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| 440 | |
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| 441 | @ifset is-C |
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| 442 | @findex rtems_barrier_release |
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| 443 | @example |
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| 444 | rtems_status_code rtems_barrier_release( |
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| 445 | rtems_id id |
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| 446 | ); |
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| 447 | @end example |
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| 448 | @end ifset |
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| 449 | |
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| 450 | @ifset is-Ada |
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| 451 | @example |
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| 452 | procedure Barrier_Release ( |
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| 453 | ID : in RTEMS.ID; |
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| 454 | Result : out RTEMS.Status_Codes |
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| 455 | ); |
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| 456 | @end example |
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| 457 | @end ifset |
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| 458 | |
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| 459 | @subheading DIRECTIVE STATUS CODES: |
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| 460 | @code{@value{RPREFIX}SUCCESSFUL} - barrier released successfully@* |
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| 461 | @code{@value{RPREFIX}INVALID_ID} - invalid barrier id@* |
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| 462 | |
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| 463 | @subheading DESCRIPTION: |
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| 464 | |
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[e8e2693c] | 465 | This directive releases the barrier specified by id. |
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| 466 | All tasks waiting at the barrier will be unblocked. |
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| 467 | If the running task's preemption mode is enabled and one of |
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| 468 | the unblocked tasks has a higher priority than the running task. |
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[adfdb1c] | 469 | |
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| 470 | @subheading NOTES: |
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| 471 | |
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| 472 | The calling task may be preempted if it causes a |
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| 473 | higher priority task to be made ready for execution. |
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