1 ------------------------------------------------------------------------------
3 -- GNAT RUN-TIME COMPONENTS --
5 -- A D A . T A S K _ A T T R I B U T E S --
9 -- Copyright (C) 1991-1994, Florida State University --
10 -- Copyright (C) 1995-2007, AdaCore --
12 -- GNARL is free software; you can redistribute it and/or modify it under --
13 -- terms of the GNU General Public License as published by the Free Soft- --
14 -- ware Foundation; either version 2, or (at your option) any later ver- --
15 -- sion. GNARL is distributed in the hope that it will be useful, but WITH- --
16 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
17 -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
18 -- for more details. You should have received a copy of the GNU General --
19 -- Public License distributed with GNARL; see file COPYING. If not, write --
20 -- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, --
21 -- Boston, MA 02110-1301, USA. --
23 -- As a special exception, if other files instantiate generics from this --
24 -- unit, or you link this unit with other files to produce an executable, --
25 -- this unit does not by itself cause the resulting executable to be --
26 -- covered by the GNU General Public License. This exception does not --
27 -- however invalidate any other reasons why the executable file might be --
28 -- covered by the GNU Public License. --
30 -- GNARL was developed by the GNARL team at Florida State University. --
31 -- Extensive contributions were provided by Ada Core Technologies, Inc. --
33 ------------------------------------------------------------------------------
35 -- The following notes are provided in case someone decides the implementation
36 -- of this package is too complicated, or too slow. Please read this before
37 -- making any "simplifications".
39 -- Correct implementation of this package is more difficult than one might
40 -- expect. After considering (and coding) several alternatives, we settled on
41 -- the present compromise. Things we do not like about this implementation
44 -- - It is vulnerable to bad Task_Id values, to the extent of possibly
45 -- trashing memory and crashing the runtime system.
47 -- - It requires dynamic storage allocation for each new attribute value,
48 -- except for types that happen to be the same size as System.Address, or
51 -- - Instantiations at other than the library level rely on being able to
52 -- do down-level calls to a procedure declared in the generic package body.
53 -- This makes it potentially vulnerable to compiler changes.
55 -- The main implementation issue here is that the connection from task to
56 -- attribute is a potential source of dangling references.
58 -- When a task goes away, we want to be able to recover all the storage
59 -- associated with its attributes. The Ada mechanism for this is finalization,
60 -- via controlled attribute types. For this reason, the ARM requires
61 -- finalization of attribute values when the associated task terminates.
63 -- This finalization must be triggered by the tasking runtime system, during
64 -- termination of the task. Given the active set of instantiations of
65 -- Ada.Task_Attributes is dynamic, the number and types of attributes
66 -- belonging to a task will not be known until the task actually terminates.
67 -- Some of these types may be controlled and some may not. The RTS must find
68 -- some way to determine which of these attributes need finalization, and
69 -- invoke the appropriate finalization on them.
71 -- One way this might be done is to create a special finalization chain for
72 -- each task, similar to the finalization chain that is used for controlled
73 -- objects within the task. This would differ from the usual finalization
74 -- chain in that it would not have a LIFO structure, since attributes may be
75 -- added to a task at any time during its lifetime. This might be the right
76 -- way to go for the longer term, but at present this approach is not open,
77 -- since GNAT does not provide such special finalization support.
79 -- Lacking special compiler support, the RTS is limited to the normal ways an
80 -- application invokes finalization, i.e.
82 -- a) Explicit call to the procedure Finalize, if we know the type has this
83 -- operation defined on it. This is not sufficient, since we have no way
84 -- of determining whether a given generic formal Attribute type is
85 -- controlled, and no visibility of the associated Finalize procedure, in
88 -- b) Leaving the scope of a local object of a controlled type. This does not
89 -- help, since the lifetime of an instantiation of Ada.Task_Attributes
90 -- does not correspond to the lifetimes of the various tasks which may
91 -- have that attribute.
93 -- c) Assignment of another value to the object. This would not help, since
94 -- we then have to finalize the new value of the object.
96 -- d) Unchecked deallocation of an object of a controlled type. This seems to
97 -- be the only mechanism available to the runtime system for finalization
98 -- of task attributes.
100 -- We considered two ways of using unchecked deallocation, both based on a
101 -- linked list of that would hang from the task control block.
103 -- In the first approach the objects on the attribute list are all derived
104 -- from one controlled type, say T, and are linked using an access type to
105 -- T'Class. The runtime system has an Ada.Unchecked_Deallocation for T'Class
106 -- with access type T'Class, and uses this to deallocate and finalize all the
107 -- items in the list. The limitation of this approach is that each
108 -- instantiation of the package Ada.Task_Attributes derives a new record
109 -- extension of T, and since T is controlled (RM 3.9.1 (3)), instantiation is
110 -- only allowed at the library level.
112 -- In the second approach the objects on the attribute list are of unrelated
113 -- but structurally similar types. Unchecked conversion is used to circument
114 -- Ada type checking. Each attribute-storage node contains not only the
115 -- attribute value and a link for chaining, but also a pointer to descriptor
116 -- for the corresponding instantiation of Task_Attributes. The instantiation
117 -- descriptor contains pointer to a procedure that can do the correct
118 -- deallocation and finalization for that type of attribute. On task
119 -- termination, the runtime system uses the pointer to call the appropriate
122 -- While this gets around the limitation that instantations be at the library
123 -- level, it relies on an implementation feature that may not always be safe,
124 -- i.e. that it is safe to call the Deallocate procedure for an instantiation
125 -- of Ada.Task_Attributes that no longer exists. In general, it seems this
126 -- might result in dangling references.
128 -- Another problem with instantiations deeper than the library level is that
129 -- there is risk of storage leakage, or dangling references to reused storage.
130 -- That is, if an instantiation of Ada.Task_Attributes is made within a
131 -- procedure, what happens to the storage allocated for attributes, when the
132 -- procedure call returns? Apparently (RM 7.6.1 (4)) any such objects must be
133 -- finalized, since they will no longer be accessible, and in general one
134 -- would expect that the storage they occupy would be recovered for later
135 -- reuse. (If not, we would have a case of storage leakage.) Assuming the
136 -- storage is recovered and later reused, we have potentially dangerous
137 -- dangling references. When the procedure containing the instantiation of
138 -- Ada.Task_Attributes returns, there may still be unterminated tasks with
139 -- associated attribute values for that instantiation. When such tasks
140 -- eventually terminate, the RTS will attempt to call the Deallocate procedure
141 -- on them. If the corresponding storage has already been deallocated, when
142 -- the master of the access type was left, we have a potential disaster. This
143 -- disaster is compounded since the pointer to Deallocate is probably through
144 -- a "trampoline" which will also have been destroyed.
146 -- For this reason, we arrange to remove all dangling references before
147 -- leaving the scope of an instantiation. This is ugly, since it requires
148 -- traversing the list of all tasks, but it is no more ugly than a similar
149 -- traversal that we must do at the point of instantiation in order to
150 -- initialize the attributes of all tasks. At least we only need to do these
151 -- traversals if the type is controlled.
153 -- We chose to defer allocation of storage for attributes until the Reference
154 -- function is called or the attribute is first set to a value different from
155 -- the default initial one. This allows a potential savings in allocation,
156 -- for attributes that are not used by all tasks.
158 -- For efficiency, we reserve space in the TCB for a fixed number of direct-
159 -- access attributes. These are required to be of a size that fits in the
160 -- space of an object of type System.Address. Because we must use unchecked
161 -- bitwise copy operations on these values, they cannot be of a controlled
162 -- type, but that is covered automatically since controlled objects are too
163 -- large to fit in the spaces.
165 -- We originally deferred initialization of these direct-access attributes,
166 -- just as we do for the indirect-access attributes, and used a per-task bit
167 -- vector to keep track of which attributes were currently defined for that
168 -- task. We found that the overhead of maintaining this bit-vector seriously
169 -- slowed down access to the attributes, and made the fetch operation non-
170 -- atomic, so that even to read an attribute value required locking the TCB.
171 -- Therefore, we now initialize such attributes for all existing tasks at the
172 -- time of the attribute instantiation, and initialize existing attributes for
173 -- each new task at the time it is created.
175 -- The latter initialization requires a list of all the instantiation
176 -- descriptors. Updates to this list, as well as the bit-vector that is used
177 -- to reserve slots for attributes in the TCB, require mutual exclusion. That
178 -- is provided by the Lock/Unlock_RTS.
180 -- One special problem that added complexity to the design is that the per-
181 -- task list of indirect attributes contains objects of different types. We
182 -- use unchecked pointer conversion to link these nodes together and access
183 -- them, but the records may not have identical internal structure. Initially,
184 -- we thought it would be enough to allocate all the common components of
185 -- the records at the front of each record, so that their positions would
186 -- correspond. Unfortunately, GNAT adds "dope" information at the front
187 -- of a record, if the record contains any controlled-type components.
189 -- This means that the offset of the fields we use to link the nodes is at
190 -- different positions on nodes of different types. To get around this, each
191 -- attribute storage record consists of a core node and wrapper. The core
192 -- nodes are all of the same type, and it is these that are linked together
193 -- and generally "seen" by the RTS. Each core node contains a pointer to its
194 -- own wrapper, which is a record that contains the core node along with an
195 -- attribute value, approximately as follows:
198 -- type Node_Access is access all Node;
200 -- type Access_Wrapper is access all Wrapper;
201 -- type Node is record
202 -- Next : Node_Access;
204 -- Wrapper : Access_Wrapper;
206 -- type Wrapper is record
207 -- Dummy_Node : aliased Node;
208 -- Value : aliased Attribute; -- the generic formal type
211 -- Another interesting problem is with the initialization of the instantiation
212 -- descriptors. Originally, we did this all via the Initialize procedure of
213 -- the descriptor type and code in the package body. It turned out that the
214 -- Initialize procedure needed quite a bit of information, including the size
215 -- of the attribute type, the initial value of the attribute (if it fits in
216 -- the TCB), and a pointer to the deallocator procedure. These needed to be
217 -- "passed" in via access discriminants. GNAT was having trouble with access
218 -- discriminants, so all this work was moved to the package body.
220 -- Note that references to objects declared in this package body must in
221 -- general use 'Unchecked_Access instead of 'Access as the package can be
222 -- instantiated from within a local context.
224 with System
.Error_Reporting
;
225 -- Used for Shutdown;
227 with System
.Storage_Elements
;
228 -- Used for Integer_Address
230 with System
.Task_Primitives
.Operations
;
231 -- Used for Write_Lock
236 -- Used for Access_Address
238 -- Direct_Index_Vector
241 with System
.Tasking
.Initialization
;
242 -- Used for Defer_Abort
244 -- Initialize_Attributes_Link
245 -- Finalize_Attributes_Link
247 with System
.Tasking
.Task_Attributes
;
248 -- Used for Access_Node
249 -- Access_Dummy_Wrapper
256 -- Used for Raise_Exception
258 with Ada
.Unchecked_Conversion
;
259 with Ada
.Unchecked_Deallocation
;
261 pragma Elaborate_All
(System
.Tasking
.Task_Attributes
);
262 -- To ensure the initialization of object Local (below) will work
264 package body Ada
.Task_Attributes
is
266 use System
.Error_Reporting
,
267 System
.Tasking
.Initialization
,
269 System
.Tasking
.Task_Attributes
,
272 package POP
renames System
.Task_Primitives
.Operations
;
274 ---------------------------
275 -- Unchecked Conversions --
276 ---------------------------
278 -- The following type corresponds to Dummy_Wrapper,
279 -- declared in System.Tasking.Task_Attributes.
282 type Access_Wrapper
is access all Wrapper
;
284 pragma Warnings
(Off
);
285 -- We turn warnings off for the following To_Attribute_Handle conversions,
286 -- since these are used only for small attributes where we know that there
287 -- are no problems with alignment, but the compiler will generate warnings
288 -- for the occurrences in the large attribute case, even though they will
289 -- not actually be used.
291 function To_Attribute_Handle
is new Ada
.Unchecked_Conversion
292 (System
.Address
, Attribute_Handle
);
293 function To_Direct_Attribute_Element
is new Ada
.Unchecked_Conversion
294 (System
.Address
, Direct_Attribute_Element
);
295 -- For reference to directly addressed task attributes
297 type Access_Integer_Address
is access all
298 System
.Storage_Elements
.Integer_Address
;
300 function To_Attribute_Handle
is new Ada
.Unchecked_Conversion
301 (Access_Integer_Address
, Attribute_Handle
);
302 -- For reference to directly addressed task attributes
304 pragma Warnings
(On
);
305 -- End of warnings off region for directly addressed
306 -- attribute conversion functions.
308 function To_Access_Address
is new Ada
.Unchecked_Conversion
309 (Access_Node
, Access_Address
);
310 -- To store pointer to list of indirect attributes
312 pragma Warnings
(Off
);
313 function To_Access_Wrapper
is new Ada
.Unchecked_Conversion
314 (Access_Dummy_Wrapper
, Access_Wrapper
);
315 pragma Warnings
(On
);
316 -- To fetch pointer to actual wrapper of attribute node. We turn off
317 -- warnings since this may generate an alignment warning. The warning can
318 -- be ignored since Dummy_Wrapper is only a non-generic standin for the
319 -- real wrapper type (we never actually allocate objects of type
322 function To_Access_Dummy_Wrapper
is new Ada
.Unchecked_Conversion
323 (Access_Wrapper
, Access_Dummy_Wrapper
);
324 -- To store pointer to actual wrapper of attribute node
326 function To_Task_Id
is new Ada
.Unchecked_Conversion
327 (Task_Identification
.Task_Id
, Task_Id
);
328 -- To access TCB of identified task
330 type Local_Deallocator
is access procedure (P
: in out Access_Node
);
332 function To_Lib_Level_Deallocator
is new Ada
.Unchecked_Conversion
333 (Local_Deallocator
, Deallocator
);
334 -- To defeat accessibility check
336 pragma Warnings
(On
);
338 ------------------------
339 -- Storage Management --
340 ------------------------
342 procedure Deallocate
(P
: in out Access_Node
);
343 -- Passed to the RTS via unchecked conversion of a pointer to permit
344 -- finalization and deallocation of attribute storage nodes.
346 --------------------------
347 -- Instantiation Record --
348 --------------------------
350 Local
: aliased Instance
;
351 -- Initialized in package body
353 type Wrapper
is record
354 Dummy_Node
: aliased Node
;
356 Value
: aliased Attribute
:= Initial_Value
;
357 -- The generic formal type, may be controlled
360 -- A number of unchecked conversions involving Wrapper_Access sources are
361 -- performed in this unit. We have to ensure that the designated object is
362 -- always strictly enough aligned.
364 for Wrapper
'Alignment use Standard
'Maximum_Alignment;
367 new Ada
.Unchecked_Deallocation
(Wrapper
, Access_Wrapper
);
369 procedure Deallocate
(P
: in out Access_Node
) is
370 T
: Access_Wrapper
:= To_Access_Wrapper
(P
.Wrapper
);
380 (T
: Task_Identification
.Task_Id
:= Task_Identification
.Current_Task
)
381 return Attribute_Handle
383 TT
: constant Task_Id
:= To_Task_Id
(T
);
384 Error_Message
: constant String := "Trying to get the reference of a ";
388 Raise_Exception
(Program_Error
'Identity, Error_Message
& "null task");
391 if TT
.Common
.State
= Terminated
then
392 Raise_Exception
(Tasking_Error
'Identity,
393 Error_Message
& "terminated task");
396 -- Directly addressed case
398 if Local
.Index
/= 0 then
400 -- Return the attribute handle. Warnings off because this return
401 -- statement generates alignment warnings for large attributes
402 -- (but will never be executed in this case anyway).
404 pragma Warnings
(Off
);
406 To_Attribute_Handle
(TT
.Direct_Attributes
(Local
.Index
)'Address);
407 pragma Warnings
(On
);
409 -- Not directly addressed
413 P
: Access_Node
:= To_Access_Node
(TT
.Indirect_Attributes
);
415 Self_Id
: constant Task_Id
:= POP
.Self
;
418 Defer_Abort
(Self_Id
);
422 if P
.Instance
= Access_Instance
'(Local'Unchecked_Access) then
424 Undefer_Abort (Self_Id);
425 return To_Access_Wrapper (P.Wrapper).Value'Access;
431 -- Unlock the RTS here to follow the lock ordering rule
432 -- that prevent us from using new (i.e the Global_Lock) while
433 -- holding any other lock.
437 ((null, Local
'Unchecked_Access, null), Initial_Value
);
440 P
:= W
.Dummy_Node
'Unchecked_Access;
441 P
.Wrapper
:= To_Access_Dummy_Wrapper
(W
);
442 P
.Next
:= To_Access_Node
(TT
.Indirect_Attributes
);
443 TT
.Indirect_Attributes
:= To_Access_Address
(P
);
445 Undefer_Abort
(Self_Id
);
446 return W
.Value
'Access;
451 Undefer_Abort
(Self_Id
);
456 pragma Assert
(Shutdown
("Should never get here in Reference"));
460 when Tasking_Error | Program_Error
=>
471 procedure Reinitialize
472 (T
: Task_Identification
.Task_Id
:= Task_Identification
.Current_Task
)
474 TT
: constant Task_Id
:= To_Task_Id
(T
);
475 Error_Message
: constant String := "Trying to Reinitialize a ";
479 Raise_Exception
(Program_Error
'Identity, Error_Message
& "null task");
482 if TT
.Common
.State
= Terminated
then
483 Raise_Exception
(Tasking_Error
'Identity,
484 Error_Message
& "terminated task");
487 if Local
.Index
/= 0 then
488 Set_Value
(Initial_Value
, T
);
493 Self_Id
: constant Task_Id
:= POP
.Self
;
496 Defer_Abort
(Self_Id
);
498 Q
:= To_Access_Node
(TT
.Indirect_Attributes
);
501 if Q
.Instance
= Access_Instance
'(Local'Unchecked_Access) then
503 TT.Indirect_Attributes := To_Access_Address (Q.Next);
508 W := To_Access_Wrapper (Q.Wrapper);
511 Undefer_Abort (Self_Id);
520 Undefer_Abort (Self_Id);
525 Undefer_Abort (Self_Id);
531 when Tasking_Error | Program_Error =>
544 T : Task_Identification.Task_Id := Task_Identification.Current_Task)
546 TT : constant Task_Id := To_Task_Id (T);
547 Error_Message : constant String := "Trying to Set the Value of a ";
551 Raise_Exception (Program_Error'Identity, Error_Message & "null task");
554 if TT.Common.State = Terminated then
555 Raise_Exception (Tasking_Error'Identity,
556 Error_Message & "terminated task");
559 -- Directly addressed case
561 if Local.Index /= 0 then
563 -- Set attribute handle, warnings off, because this code can generate
564 -- alignment warnings with large attributes (but of course will not
565 -- be executed in this case, since we never have direct addressing in
568 pragma Warnings (Off);
570 (TT.Direct_Attributes (Local.Index)'Address).all := Val;
571 pragma Warnings (On);
575 -- Not directly addressed
578 P : Access_Node := To_Access_Node (TT.Indirect_Attributes);
580 Self_Id : constant Task_Id := POP.Self;
583 Defer_Abort (Self_Id);
588 if P.Instance = Access_Instance'(Local
'Unchecked_Access) then
589 To_Access_Wrapper
(P
.Wrapper
).Value
:= Val
;
591 Undefer_Abort
(Self_Id
);
598 -- Unlock RTS here to follow the lock ordering rule that prevent us
599 -- from using new (i.e the Global_Lock) while holding any other lock.
602 W
:= new Wrapper
'((null, Local'Unchecked_Access, null), Val);
604 P := W.Dummy_Node'Unchecked_Access;
605 P.Wrapper := To_Access_Dummy_Wrapper (W);
606 P.Next := To_Access_Node (TT.Indirect_Attributes);
607 TT.Indirect_Attributes := To_Access_Address (P);
610 Undefer_Abort (Self_Id);
615 Undefer_Abort (Self_Id);
620 when Tasking_Error | Program_Error =>
632 (T : Task_Identification.Task_Id := Task_Identification.Current_Task)
635 TT : constant Task_Id := To_Task_Id (T);
636 Error_Message : constant String := "Trying to get the Value of a ";
640 Raise_Exception (Program_Error'Identity, Error_Message & "null task");
643 if TT.Common.State = Terminated then
645 (Program_Error'Identity, Error_Message & "terminated task");
648 -- Directly addressed case
650 if Local.Index /= 0 then
652 -- Get value of attribute. We turn Warnings off, because for large
653 -- attributes, this code can generate alignment warnings. But of
654 -- course large attributes are never directly addressed so in fact
655 -- we will never execute the code in this case.
657 pragma Warnings (Off);
658 return To_Attribute_Handle
659 (TT.Direct_Attributes (Local.Index)'Address).all;
660 pragma Warnings (On);
663 -- Not directly addressed
668 Self_Id : constant Task_Id := POP.Self;
671 Defer_Abort (Self_Id);
673 P := To_Access_Node (TT.Indirect_Attributes);
676 if P.Instance = Access_Instance'(Local
'Unchecked_Access) then
677 Result
:= To_Access_Wrapper
(P
.Wrapper
).Value
;
679 Undefer_Abort
(Self_Id
);
687 Undefer_Abort
(Self_Id
);
688 return Initial_Value
;
693 Undefer_Abort
(Self_Id
);
698 when Tasking_Error | Program_Error
=>
705 -- Start of elaboration code for package Ada.Task_Attributes
708 -- This unchecked conversion can give warnings when alignments are
709 -- incorrect, but they will not be used in such cases anyway, so the
710 -- warnings can be safely ignored.
712 pragma Warnings
(Off
);
713 Local
.Deallocate
:= To_Lib_Level_Deallocator
(Deallocate
'Access);
714 pragma Warnings
(On
);
717 Two_To_J
: Direct_Index_Vector
;
718 Self_Id
: constant Task_Id
:= POP
.Self
;
720 Defer_Abort
(Self_Id
);
722 -- Need protection for updating links to per-task initialization and
723 -- finalization routines, in case some task is being created or
724 -- terminated concurrently.
728 -- Add this instantiation to the list of all instantiations
730 Local
.Next
:= System
.Tasking
.Task_Attributes
.All_Attributes
;
731 System
.Tasking
.Task_Attributes
.All_Attributes
:=
732 Local
'Unchecked_Access;
734 -- Try to find space for the attribute in the TCB
739 if Attribute
'Size <= System
.Address
'Size then
740 for J
in Direct_Index_Range
loop
741 if (Two_To_J
and In_Use
) = 0 then
743 -- Reserve location J for this attribute
745 In_Use
:= In_Use
or Two_To_J
;
748 -- This unchecked conversions can give a warning when the the
749 -- alignment is incorrect, but it will not be used in such a
750 -- case anyway, so the warning can be safely ignored.
752 pragma Warnings
(Off
);
753 To_Attribute_Handle
(Local
.Initial_Value
'Access).all :=
755 pragma Warnings
(On
);
760 Two_To_J
:= Two_To_J
* 2;
764 -- Attribute goes directly in the TCB
766 if Local
.Index
/= 0 then
767 -- Replace stub for initialization routine that is called at task
770 Initialization
.Initialize_Attributes_Link
:=
771 System
.Tasking
.Task_Attributes
.Initialize_Attributes
'Access;
773 -- Initialize the attribute, for all tasks
776 C
: System
.Tasking
.Task_Id
:= System
.Tasking
.All_Tasks_List
;
779 C
.Direct_Attributes
(Local
.Index
) :=
780 To_Direct_Attribute_Element
781 (System
.Storage_Elements
.To_Address
(Local
.Initial_Value
));
782 C
:= C
.Common
.All_Tasks_Link
;
786 -- Attribute goes into a node onto a linked list
789 -- Replace stub for finalization routine called at task termination
791 Initialization
.Finalize_Attributes_Link
:=
792 System
.Tasking
.Task_Attributes
.Finalize_Attributes
'Access;
796 Undefer_Abort
(Self_Id
);
798 end Ada
.Task_Attributes
;