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9.10 Shared Variables
Static Semantics
1
{shared variable (protection
of)} {independently
addressable} If two different objects,
including nonoverlapping parts of the same object, are
independently
addressable, they can be manipulated concurrently by two different
tasks without synchronization. Normally, any two nonoverlapping objects
are independently addressable. However, if packing, record layout, or
Component_Size is specified for a given composite object, then it is
implementation defined whether or not two nonoverlapping parts of that
composite object are independently addressable.
1.a
Implementation defined: Whether
or not two nonoverlapping parts of a composite object are independently
addressable, in the case where packing, record layout, or Component_Size
is specified for the object.
1.b
Implementation Note: Independent
addressability is the only high level semantic effect of a pragma
Pack. If two objects are independently addressable, the implementation
should allocate them in such a way that each can be written by the hardware
without writing the other. For example, unless the user asks for it,
it is generally not feasible to choose a bit-packed representation on
a machine without an atomic bit field insertion instruction, because
there might be tasks that update neighboring subcomponents concurrently,
and locking operations on all subcomponents is generally not a good idea.
1.c
Even if packing or one of the
other above-mentioned aspects is specified, subcomponents should still
be updated independently if the hardware efficiently supports it.
Dynamic Semantics
2
[Separate tasks
normally proceed independently and concurrently with one another. However,
task interactions can be used to synchronize the actions of two or more
tasks to allow, for example, meaningful communication by the direct updating
and reading of variables shared between the tasks.] The actions of two
different tasks are synchronized in this sense when an action of one
task
signals an action of the other task;
{signal
(as defined between actions)} an action
A1 is defined to signal an action A2 under the following circumstances:
3
- If A1 and A2 are part of the execution
of the same task, and the language rules require A1 to be performed before
A2;
4
- If A1 is the action of an activator
that initiates the activation of a task, and A2 is part of the execution
of the task that is activated;
5
- If A1 is part of the activation of
a task, and A2 is the action of waiting for completion of the activation;
6
- If A1 is part of the execution of
a task, and A2 is the action of waiting for the termination of the task;
6.1/1
- {8652/0031}
If A1 is the termination of a task T, and A2 is either the evaluation
of the expression T'Terminated or a call to Ada.Task_Identification.Is_Terminated
with an actual parameter that identifies T (see C.7.1);
7
- If A1 is the action of issuing an
entry call, and A2 is part of the corresponding execution of the appropriate
entry_body or accept_statement.
7.a
Ramification: Evaluating
the entry_index of an accept_statement
is not synchronized with a corresponding entry call, nor is evaluating
the entry barrier of an entry_body.
8
- If A1 is part of the execution of
an accept_statement or entry_body,
and A2 is the action of returning from the corresponding entry call;
9
- If A1 is part of the execution of
a protected procedure body or entry_body
for a given protected object, and A2 is part of a later execution of
an entry_body for the same protected
object;
9.a
Reason: The underlying
principle here is that for one action to ``signal'' a second, the second
action has to follow a potentially blocking operation, whose blocking
is dependent on the first action in some way. Protected procedures are
not potentially blocking, so they can only be "signalers,"
they cannot be signaled.
9.b
Ramification: Protected
subprogram calls are not defined to signal one another, which means that
such calls alone cannot be used to synchronize access to shared data
outside of a protected object.
9.c
Reason: The point of
this distinction is so that on multiprocessors with inconsistent caches,
the caches only need to be refreshed at the beginning of an entry body,
and forced out at the end of an entry body or protected procedure that
leaves an entry open. Protected function calls, and protected subprogram
calls for entryless protected objects do not require full cache consistency.
Entryless protected objects are intended to be treated roughly like atomic
objects -- each operation is indivisible with respect to other operations
(unless both are reads), but such operations cannot be used to synchronize
access to other nonvolatile shared variables.
10
- If A1
signals some action that in turn signals A2.
Erroneous Execution
11
{erroneous
execution (cause) [partial]} Given an
action of assigning to an object, and an action of reading or updating
a part of the same object (or of a neighboring object if the two are
not independently addressable), then the execution of the actions is
erroneous unless the actions are
sequential.
{sequential
(actions)} Two actions are sequential
if one of the following is true:
12
- One action signals the other;
13
- Both actions occur as part of the
execution of the same task;
13.a
Reason: Any two actions
of the same task are sequential, even if one does not signal the other
because they can be executed in an ``arbitrary'' (but necessarily equivalent
to some ``sequential'') order.
14
- Both actions occur as part of protected
actions on the same protected object, and at most one of the actions
is part of a call on a protected function of the protected object.
14.a
Reason: Because actions
within protected actions do not always imply signaling, we have to mention
them here explicitly to make sure that actions occurring within different
protected actions of the same protected object are sequential with respect
to one another (unless both are part of calls on protected functions).
14.b
Ramification: It doesn't
matter whether or not the variable being assigned is actually a subcomponent
of the protected object; globals can be safely updated from within the
bodies of protected procedures or entries.
15
A
pragma
Atomic or Atomic_Components may also be used to ensure that certain reads
and updates are sequential -- see
C.6.
15.a
Ramification: If two
actions are ``sequential'' it is known that their executions don't overlap
in time, but it is not necessarily specified which occurs first. For
example, all actions of a single task are sequential, even though the
exact order of execution is not fully specified for all constructs.
15.b
Discussion: Note that
if two assignments to the same variable are sequential, but neither signals
the other, then the program is not erroneous, but it is not specified
which assignment ultimately prevails. Such a situation usually corresponds
to a programming mistake, but in some (rare) cases, the order makes no
difference, and for this reason this situation is not considered erroneous
nor even a bounded error. In Ada 83, this was considered an ``incorrect
order dependence'' if the ``effect'' of the program was affected, but
``effect'' was never fully defined. In Ada 95, this situation represents
a potential nonportability, and a friendly compiler might want to warn
the programmer about the situation, but it is not considered an error.
An example where this would come up would be in gathering statistics
as part of referencing some information, where the assignments associated
with statistics gathering don't need to be ordered since they are just
accumulating aggregate counts, sums, products, etc.
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