Re: Suspension_Object problem

[Jeffrey Carter <jrcarter@acm.org>]

Philippe Laval wrote:
> The following program compiles without error in gnat 3.14p.
> However the line "From H: P is now released" never shows up. What is
> wrong?
> 
> -- File Susp_O.adb
> with P_Pkg;
> with H_Pkg;
> procedure Susp_O is
> begin
>    null;
> end Susp_O;
> ----------------------------------------------------------------------------------------------
> 
> -- File P_Pkg.ads
> with Ada.Synchronous_Task_Control; use Ada.Synchronous_Task_Control;
> package P_Pkg is
>    protected type Protected_Lock is
>       procedure Set_Lock;
>       procedure Release;
>    private
>       Lock : Suspension_Object;
>    end Protected_Lock;
> 
>    type Lock_Access is access Protected_Lock;
>    Attack_Lock : Lock_Access;
> 
>    task P;
> 
> end P_Pkg;
> 
> ------------------------------------------------------------------------------------------------
> 
> -- File P_Pkg.ads
> package H_Pkg is
>    task H;
> end H_Pkg;
> ------------------------------------------------------------------------------------------------
> 
> -- File P_Pkg.adb
> with Ada.Text_IO; use Ada.Text_IO;
> package body P_Pkg is
>    protected body Protected_Lock is
>       procedure Set_Lock is
>       begin
>          Suspend_Until_True (Lock);
>       end Set_Lock;
> 
>       procedure Release is
>       begin
>          Set_True (Lock);
>       end Release;
>    end Protected_Lock;
> 
>    task body P is
>    begin
>       Attack_Lock := new Protected_Lock;
>       Put_Line ("P is being suspended.");
>       Put_Line ("Waiting to be released by H...");
>       Attack_Lock.Set_Lock;
> 
>       Put_Line ("P is now released.");
>    end P;
> end P_Pkg;
> ------------------------------------------------------------------------------------------------
> 
> -- File H_Pkg.adb
> with Ada.Text_IO; use Ada.Text_IO;
> with P_Pkg;
> package body H_Pkg is
>    task body H is
>    begin
>       delay 1.0;
>       P_Pkg.Attack_Lock.Release;
>       Put_Line ("From H : P is released now!");
>    end H;
> end H_Pkg;

This seems entirely inappropriate. Either your 2 tasks should operate on 
a shared Suspension_Object or they should operate on a shared protected 
object. Mixing the 2 seems inappropriate, especially as some operations 
on Suspension_Objects are intended to be blocking operations, and 
therefore illegal to call from a protected object.

I suspect that your problem comes because the call to Suspend_Until_True 
blocks P before it completes executing Set_Lock, so it never frees the 
protected object to allow H to execute Release.

There is also no need to use access types in this example.

-- 
Jeff Carter
"Pray that there's intelligent life somewhere up in
space, 'cause there's bugger all down here on earth."
Monty Python's Meaning of Life