------------------------------------------------------------------------------ -- -- -- GNAT RUN-TIME COMPONENTS -- -- -- -- A D A . T A S K _ A T T R I B U T E S -- -- -- -- B o d y -- -- -- -- Copyright (C) 1991-1994, Florida State University -- -- Copyright (C) 1995-2004, Ada Core Technologies -- -- -- -- GNARL is free software; you can redistribute it and/or modify it under -- -- terms of the GNU General Public License as published by the Free Soft- -- -- ware Foundation; either version 2, or (at your option) any later ver- -- -- sion. GNARL is distributed in the hope that it will be useful, but WITH- -- -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- -- for more details. You should have received a copy of the GNU General -- -- Public License distributed with GNARL; see file COPYING. If not, write -- -- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, -- -- MA 02111-1307, USA. -- -- -- -- As a special exception, if other files instantiate generics from this -- -- unit, or you link this unit with other files to produce an executable, -- -- this unit does not by itself cause the resulting executable to be -- -- covered by the GNU General Public License. This exception does not -- -- however invalidate any other reasons why the executable file might be -- -- covered by the GNU Public License. -- -- -- -- GNARL was developed by the GNARL team at Florida State University. -- -- Extensive contributions were provided by Ada Core Technologies, Inc. -- -- -- ------------------------------------------------------------------------------ -- The following notes are provided in case someone decides the -- implementation of this package is too complicated, or too slow. -- Please read this before making any "simplifications". -- Correct implementation of this package is more difficult than one -- might expect. After considering (and coding) several alternatives, -- we settled on the present compromise. Things we do not like about -- this implementation include: -- - It is vulnerable to bad Task_Id values, to the extent of -- possibly trashing memory and crashing the runtime system. -- - It requires dynamic storage allocation for each new attribute value, -- except for types that happen to be the same size as System.Address, -- or shorter. -- - Instantiations at other than the library level rely on being able to -- do down-level calls to a procedure declared in the generic package body. -- This makes it potentially vulnerable to compiler changes. -- The main implementation issue here is that the connection from -- task to attribute is a potential source of dangling references. -- When a task goes away, we want to be able to recover all the storage -- associated with its attributes. The Ada mechanism for this is -- finalization, via controlled attribute types. For this reason, -- the ARM requires finalization of attribute values when the -- associated task terminates. -- This finalization must be triggered by the tasking runtime system, -- during termination of the task. Given the active set of instantiations -- of Ada.Task_Attributes is dynamic, the number and types of attributes -- belonging to a task will not be known until the task actually terminates. -- Some of these types may be controlled and some may not. The RTS must find -- some way to determine which of these attributes need finalization, and -- invoke the appropriate finalization on them. -- One way this might be done is to create a special finalization chain -- for each task, similar to the finalization chain that is used for -- controlled objects within the task. This would differ from the usual -- finalization chain in that it would not have a LIFO structure, since -- attributes may be added to a task at any time during its lifetime. -- This might be the right way to go for the longer term, but at present -- this approach is not open, since GNAT does not provide such special -- finalization support. -- Lacking special compiler support, the RTS is limited to the -- normal ways an application invokes finalization, i.e. -- a) Explicit call to the procedure Finalize, if we know the type -- has this operation defined on it. This is not sufficient, since -- we have no way of determining whether a given generic formal -- Attribute type is controlled, and no visibility of the associated -- Finalize procedure, in the generic body. -- b) Leaving the scope of a local object of a controlled type. -- This does not help, since the lifetime of an instantiation of -- Ada.Task_Attributes does not correspond to the lifetimes of the -- various tasks which may have that attribute. -- c) Assignment of another value to the object. This would not help, -- since we then have to finalize the new value of the object. -- d) Unchecked deallocation of an object of a controlled type. -- This seems to be the only mechanism available to the runtime -- system for finalization of task attributes. -- We considered two ways of using unchecked deallocation, both based -- on a linked list of that would hang from the task control block. -- In the first approach the objects on the attribute list are all derived -- from one controlled type, say T, and are linked using an access type to -- T'Class. The runtime system has an Unchecked_Deallocation for T'Class -- with access type T'Class, and uses this to deallocate and finalize all -- the items in the list. The limitation of this approach is that each -- instantiation of the package Ada.Task_Attributes derives a new record -- extension of T, and since T is controlled (RM 3.9.1 (3)), instantiation -- is only allowed at the library level. -- In the second approach the objects on the attribute list are of -- unrelated but structurally similar types. Unchecked conversion is -- used to circument Ada type checking. Each attribute-storage node -- contains not only the attribute value and a link for chaining, but -- also a pointer to a descriptor for the corresponding instantiation -- of Task_Attributes. The instantiation-descriptor contains a -- pointer to a procedure that can do the correct deallocation and -- finalization for that type of attribute. On task termination, the -- runtime system uses the pointer to call the appropriate deallocator. -- While this gets around the limitation that instantations be at -- the library level, it relies on an implementation feature that -- may not always be safe, i.e. that it is safe to call the -- Deallocate procedure for an instantiation of Ada.Task_Attributes -- that no longer exists. In general, it seems this might result in -- dangling references. -- Another problem with instantiations deeper than the library level -- is that there is risk of storage leakage, or dangling references -- to reused storage. That is, if an instantiation of Ada.Task_Attributes -- is made within a procedure, what happens to the storage allocated for -- attributes, when the procedure call returns? Apparently (RM 7.6.1 (4)) -- any such objects must be finalized, since they will no longer be -- accessible, and in general one would expect that the storage they occupy -- would be recovered for later reuse. (If not, we would have a case of -- storage leakage.) Assuming the storage is recovered and later reused, -- we have potentially dangerous dangling references. When the procedure -- containing the instantiation of Ada.Task_Attributes returns, there -- may still be unterminated tasks with associated attribute values for -- that instantiation. When such tasks eventually terminate, the RTS -- will attempt to call the Deallocate procedure on them. If the -- corresponding storage has already been deallocated, when the master -- of the access type was left, we have a potential disaster. This -- disaster is compounded since the pointer to Deallocate is probably -- through a "trampoline" which will also have been destroyed. -- For this reason, we arrange to remove all dangling references -- before leaving the scope of an instantiation. This is ugly, since -- it requires traversing the list of all tasks, but it is no more ugly -- than a similar traversal that we must do at the point of instantiation -- in order to initialize the attributes of all tasks. At least we only -- need to do these traversals if the type is controlled. -- We chose to defer allocation of storage for attributes until the -- Reference function is called or the attribute is first set to a value -- different from the default initial one. This allows a potential -- savings in allocation, for attributes that are not used by all tasks. -- For efficiency, we reserve space in the TCB for a fixed number of -- direct-access attributes. These are required to be of a size that -- fits in the space of an object of type System.Address. Because -- we must use unchecked bitwise copy operations on these values, they -- cannot be of a controlled type, but that is covered automatically -- since controlled objects are too large to fit in the spaces. -- We originally deferred the initialization of these direct-access -- attributes, just as we do for the indirect-access attributes, and -- used a per-task bit vector to keep track of which attributes were -- currently defined for that task. We found that the overhead of -- maintaining this bit-vector seriously slowed down access to the -- attributes, and made the fetch operation non-atomic, so that even -- to read an attribute value required locking the TCB. Therefore, -- we now initialize such attributes for all existing tasks at the time -- of the attribute instantiation, and initialize existing attributes -- for each new task at the time it is created. -- The latter initialization requires a list of all the instantiation -- descriptors. Updates to this list, as well as the bit-vector that -- is used to reserve slots for attributes in the TCB, require mutual -- exclusion. That is provided by the Lock/Unlock_RTS. -- One special problem that added complexity to the design is that -- the per-task list of indirect attributes contains objects of -- different types. We use unchecked pointer conversion to link -- these nodes together and access them, but the records may not have -- identical internal structure. Initially, we thought it would be -- enough to allocate all the common components of the records at the -- front of each record, so that their positions would correspond. -- Unfortunately, GNAT adds "dope" information at the front of a record, -- if the record contains any controlled-type components. -- -- This means that the offset of the fields we use to link the nodes is -- at different positions on nodes of different types. To get around this, -- each attribute storage record consists of a core node and wrapper. -- The core nodes are all of the same type, and it is these that are -- linked together and generally "seen" by the RTS. Each core node -- contains a pointer to its own wrapper, which is a record that contains -- the core node along with an attribute value, approximately -- as follows: -- type Node; -- type Node_Access is access all Node; -- type Node_Access; -- type Access_Wrapper is access all Wrapper; -- type Node is record -- Next : Node_Access; -- ... -- Wrapper : Access_Wrapper; -- end record; -- type Wrapper is record -- Noed : aliased Node; -- Value : aliased Attribute; -- the generic formal type -- end record; -- Another interesting problem is with the initialization of -- the instantiation descriptors. Originally, we did this all via -- the Initialize procedure of the descriptor type and code in the -- package body. It turned out that the Initialize procedure needed -- quite a bit of information, including the size of the attribute -- type, the initial value of the attribute (if it fits in the TCB), -- and a pointer to the deallocator procedure. These needed to be -- "passed" in via access discriminants. GNAT was having trouble -- with access discriminants, so all this work was moved to the -- package body. with Ada.Task_Identification; -- used for Task_Id -- Null_Task_Id -- Current_Task with System.Error_Reporting; -- used for Shutdown; with System.Storage_Elements; -- used for Integer_Address with System.Task_Primitives.Operations; -- used for Write_Lock -- Unlock -- Lock/Unlock_RTS with System.Tasking; -- used for Access_Address -- Task_Id -- Direct_Index_Vector -- Direct_Index with System.Tasking.Initialization; -- used for Defer_Abortion -- Undefer_Abortion -- Initialize_Attributes_Link -- Finalize_Attributes_Link with System.Tasking.Task_Attributes; -- used for Access_Node -- Access_Dummy_Wrapper -- Deallocator -- Instance -- Node -- Access_Instance with Ada.Exceptions; -- used for Raise_Exception with Unchecked_Conversion; with Unchecked_Deallocation; pragma Elaborate_All (System.Tasking.Task_Attributes); -- to ensure the initialization of object Local (below) will work package body Ada.Task_Attributes is use System.Error_Reporting, System.Tasking.Initialization, System.Tasking, System.Tasking.Task_Attributes, Ada.Exceptions; use type System.Tasking.Access_Address; package POP renames System.Task_Primitives.Operations; --------------------------- -- Unchecked Conversions -- --------------------------- -- The following type corresponds to Dummy_Wrapper, -- declared in System.Tasking.Task_Attributes. type Wrapper; type Access_Wrapper is access all Wrapper; pragma Warnings (Off); -- We turn warnings off for the following declarations of the -- To_Attribute_Handle conversions, since these are used only -- for small attributes where we know that there are no problems -- with alignment, but the compiler will generate warnings for -- the occurrences in the large attribute case, even though -- they will not actually be used. function To_Attribute_Handle is new Unchecked_Conversion (System.Address, Attribute_Handle); function To_Direct_Attribute_Element is new Unchecked_Conversion (System.Address, Direct_Attribute_Element); -- For reference to directly addressed task attributes type Access_Integer_Address is access all System.Storage_Elements.Integer_Address; function To_Attribute_Handle is new Unchecked_Conversion (Access_Integer_Address, Attribute_Handle); -- For reference to directly addressed task attributes pragma Warnings (On); -- End of warnings off region for directly addressed -- attribute conversion functions. function To_Access_Address is new Unchecked_Conversion (Access_Node, Access_Address); -- To store pointer to list of indirect attributes pragma Warnings (Off); function To_Access_Wrapper is new Unchecked_Conversion (Access_Dummy_Wrapper, Access_Wrapper); pragma Warnings (On); -- To fetch pointer to actual wrapper of attribute node. We turn off -- warnings since this may generate an alignment warning. The warning -- can be ignored since Dummy_Wrapper is only a non-generic standin -- for the real wrapper type (we never actually allocate objects of -- type Dummy_Wrapper). function To_Access_Dummy_Wrapper is new Unchecked_Conversion (Access_Wrapper, Access_Dummy_Wrapper); -- To store pointer to actual wrapper of attribute node function To_Task_Id is new Unchecked_Conversion (Task_Identification.Task_Id, Task_Id); -- To access TCB of identified task type Local_Deallocator is access procedure (P : in out Access_Node); function To_Lib_Level_Deallocator is new Unchecked_Conversion (Local_Deallocator, Deallocator); -- To defeat accessibility check pragma Warnings (On); ------------------------ -- Storage Management -- ------------------------ procedure Deallocate (P : in out Access_Node); -- Passed to the RTS via unchecked conversion of a pointer to -- permit finalization and deallocation of attribute storage nodes -------------------------- -- Instantiation Record -- -------------------------- Local : aliased Instance; -- Initialized in package body type Wrapper is record Noed : aliased Node; Value : aliased Attribute := Initial_Value; -- The generic formal type, may be controlled end record; -- A number of unchecked conversions involving Wrapper_Access sources -- are performed in this unit. We have to ensure that the designated -- object is always strictly enough aligned. for Wrapper'Alignment use Standard'Maximum_Alignment; procedure Free is new Unchecked_Deallocation (Wrapper, Access_Wrapper); procedure Deallocate (P : in out Access_Node) is T : Access_Wrapper := To_Access_Wrapper (P.Wrapper); begin Free (T); end Deallocate; --------------- -- Reference -- --------------- function Reference (T : Task_Identification.Task_Id := Task_Identification.Current_Task) return Attribute_Handle is TT : constant Task_Id := To_Task_Id (T); Error_Message : constant String := "Trying to get the reference of a "; begin if TT = null then Raise_Exception (Program_Error'Identity, Error_Message & "null task"); end if; if TT.Common.State = Terminated then Raise_Exception (Tasking_Error'Identity, Error_Message & "terminated task"); end if; -- Directly addressed case if Local.Index /= 0 then -- Return the attribute handle. Warnings off because this return -- statement generates alignment warnings for large attributes -- (but will never be executed in this case anyway). pragma Warnings (Off); return To_Attribute_Handle (TT.Direct_Attributes (Local.Index)'Address); pragma Warnings (On); -- Not directly addressed else declare P : Access_Node := To_Access_Node (TT.Indirect_Attributes); W : Access_Wrapper; begin Defer_Abortion; POP.Lock_RTS; while P /= null loop if P.Instance = Access_Instance'(Local'Unchecked_Access) then POP.Unlock_RTS; Undefer_Abortion; return To_Access_Wrapper (P.Wrapper).Value'Access; end if; P := P.Next; end loop; -- Unlock the RTS here to follow the lock ordering rule -- that prevent us from using new (i.e the Global_Lock) while -- holding any other lock. POP.Unlock_RTS; W := new Wrapper' ((null, Local'Unchecked_Access, null), Initial_Value); POP.Lock_RTS; P := W.Noed'Unchecked_Access; P.Wrapper := To_Access_Dummy_Wrapper (W); P.Next := To_Access_Node (TT.Indirect_Attributes); TT.Indirect_Attributes := To_Access_Address (P); POP.Unlock_RTS; Undefer_Abortion; return W.Value'Access; exception when others => POP.Unlock_RTS; Undefer_Abortion; raise; end; end if; pragma Assert (Shutdown ("Should never get here in Reference")); return null; exception when Tasking_Error | Program_Error => raise; when others => raise Program_Error; end Reference; ------------------ -- Reinitialize -- ------------------ procedure Reinitialize (T : Task_Identification.Task_Id := Task_Identification.Current_Task) is TT : constant Task_Id := To_Task_Id (T); Error_Message : constant String := "Trying to Reinitialize a "; begin if TT = null then Raise_Exception (Program_Error'Identity, Error_Message & "null task"); end if; if TT.Common.State = Terminated then Raise_Exception (Tasking_Error'Identity, Error_Message & "terminated task"); end if; if Local.Index /= 0 then Set_Value (Initial_Value, T); else declare P, Q : Access_Node; W : Access_Wrapper; begin Defer_Abortion; POP.Lock_RTS; Q := To_Access_Node (TT.Indirect_Attributes); while Q /= null loop if Q.Instance = Access_Instance'(Local'Unchecked_Access) then if P = null then TT.Indirect_Attributes := To_Access_Address (Q.Next); else P.Next := Q.Next; end if; W := To_Access_Wrapper (Q.Wrapper); Free (W); POP.Unlock_RTS; Undefer_Abortion; return; end if; P := Q; Q := Q.Next; end loop; POP.Unlock_RTS; Undefer_Abortion; exception when others => POP.Unlock_RTS; Undefer_Abortion; raise; end; end if; exception when Tasking_Error | Program_Error => raise; when others => raise Program_Error; end Reinitialize; --------------- -- Set_Value -- --------------- procedure Set_Value (Val : Attribute; T : Task_Identification.Task_Id := Task_Identification.Current_Task) is TT : constant Task_Id := To_Task_Id (T); Error_Message : constant String := "Trying to Set the Value of a "; begin if TT = null then Raise_Exception (Program_Error'Identity, Error_Message & "null task"); end if; if TT.Common.State = Terminated then Raise_Exception (Tasking_Error'Identity, Error_Message & "terminated task"); end if; -- Directly addressed case if Local.Index /= 0 then -- Set attribute handle, warnings off, because this code can generate -- alignment warnings with large attributes (but of course will not -- be executed in this case, since we never have direct addressing in -- such cases). pragma Warnings (Off); To_Attribute_Handle (TT.Direct_Attributes (Local.Index)'Address).all := Val; pragma Warnings (On); return; end if; -- Not directly addressed declare P : Access_Node := To_Access_Node (TT.Indirect_Attributes); W : Access_Wrapper; begin Defer_Abortion; POP.Lock_RTS; while P /= null loop if P.Instance = Access_Instance'(Local'Unchecked_Access) then To_Access_Wrapper (P.Wrapper).Value := Val; POP.Unlock_RTS; Undefer_Abortion; return; end if; P := P.Next; end loop; -- Unlock RTS here to follow the lock ordering rule that -- prevent us from using new (i.e the Global_Lock) while -- holding any other lock. POP.Unlock_RTS; W := new Wrapper'((null, Local'Unchecked_Access, null), Val); POP.Lock_RTS; P := W.Noed'Unchecked_Access; P.Wrapper := To_Access_Dummy_Wrapper (W); P.Next := To_Access_Node (TT.Indirect_Attributes); TT.Indirect_Attributes := To_Access_Address (P); POP.Unlock_RTS; Undefer_Abortion; exception when others => POP.Unlock_RTS; Undefer_Abortion; raise; end; exception when Tasking_Error | Program_Error => raise; when others => raise Program_Error; end Set_Value; ----------- -- Value -- ----------- function Value (T : Task_Identification.Task_Id := Task_Identification.Current_Task) return Attribute is TT : constant Task_Id := To_Task_Id (T); Error_Message : constant String := "Trying to get the Value of a "; begin if TT = null then Raise_Exception (Program_Error'Identity, Error_Message & "null task"); end if; if TT.Common.State = Terminated then Raise_Exception (Program_Error'Identity, Error_Message & "terminated task"); end if; -- Directly addressed case if Local.Index /= 0 then -- Get value of attribute. Warnings off, because for large -- attributes, this code can generate alignment warnings. -- But of course large attributes are never directly addressed -- so in fact we will never execute the code in this case. pragma Warnings (Off); return To_Attribute_Handle (TT.Direct_Attributes (Local.Index)'Address).all; pragma Warnings (On); end if; -- Not directly addressed declare P : Access_Node; Result : Attribute; begin Defer_Abortion; POP.Lock_RTS; P := To_Access_Node (TT.Indirect_Attributes); while P /= null loop if P.Instance = Access_Instance'(Local'Unchecked_Access) then Result := To_Access_Wrapper (P.Wrapper).Value; POP.Unlock_RTS; Undefer_Abortion; return Result; end if; P := P.Next; end loop; POP.Unlock_RTS; Undefer_Abortion; return Initial_Value; exception when others => POP.Unlock_RTS; Undefer_Abortion; raise; end; exception when Tasking_Error | Program_Error => raise; when others => raise Program_Error; end Value; -- Start of elaboration code for package Ada.Task_Attributes begin -- This unchecked conversion can give warnings when alignments -- are incorrect, but they will not be used in such cases anyway, -- so the warnings can be safely ignored. pragma Warnings (Off); Local.Deallocate := To_Lib_Level_Deallocator (Deallocate'Access); pragma Warnings (On); declare Two_To_J : Direct_Index_Vector; begin Defer_Abortion; -- Need protection for updating links to per-task initialization and -- finalization routines, in case some task is being created or -- terminated concurrently. POP.Lock_RTS; -- Add this instantiation to the list of all instantiations. Local.Next := System.Tasking.Task_Attributes.All_Attributes; System.Tasking.Task_Attributes.All_Attributes := Local'Unchecked_Access; -- Try to find space for the attribute in the TCB. Local.Index := 0; Two_To_J := 1; if Attribute'Size <= System.Address'Size then for J in Direct_Index_Range loop if (Two_To_J and In_Use) = 0 then -- Reserve location J for this attribute In_Use := In_Use or Two_To_J; Local.Index := J; -- This unchecked conversions can give a warning when the -- the alignment is incorrect, but it will not be used in -- such a case anyway, so the warning can be safely ignored. pragma Warnings (Off); To_Attribute_Handle (Local.Initial_Value'Access).all := Initial_Value; pragma Warnings (On); exit; end if; Two_To_J := Two_To_J * 2; end loop; end if; -- Attribute goes directly in the TCB if Local.Index /= 0 then -- Replace stub for initialization routine -- that is called at task creation. Initialization.Initialize_Attributes_Link := System.Tasking.Task_Attributes.Initialize_Attributes'Access; -- Initialize the attribute, for all tasks. declare C : System.Tasking.Task_Id := System.Tasking.All_Tasks_List; begin while C /= null loop C.Direct_Attributes (Local.Index) := To_Direct_Attribute_Element (System.Storage_Elements.To_Address (Local.Initial_Value)); C := C.Common.All_Tasks_Link; end loop; end; -- Attribute goes into a node onto a linked list else -- Replace stub for finalization routine -- that is called at task termination. Initialization.Finalize_Attributes_Link := System.Tasking.Task_Attributes.Finalize_Attributes'Access; end if; POP.Unlock_RTS; Undefer_Abortion; end; end Ada.Task_Attributes;