change log for rtems (2011-08-22)

[Sebastian Huber]

On 08/22/2011 09:10 PM, rtems-vc at rtems.org wrote:
> 2011-08-22	Jennifer Averett <Jennifer.Averett at OARcorp.com>
>
> 	PR 1876
> 	* score/Makefile.am, score/include/rtems/score/isr.h, score/src/isr.c,
> 	score/src/smp.c, score/src/smplock.c, score/src/threaddispatch.c,
> 	score/src/threaddispatchdisablelevel.c: Add smp isr support.
> 	* score/src/isrsmp.c: New file.
>
[...]
> diff -u rtems/cpukit/score/src/threaddispatch.c:1.25 rtems/cpukit/score/src/threaddispatch.c:1.26
> --- rtems/cpukit/score/src/threaddispatch.c:1.25	Thu May 26 13:07:07 2011
> +++ rtems/cpukit/score/src/threaddispatch.c	Mon Aug 22 13:26:08 2011
> @@ -169,9 +169,10 @@
>     }
>
>   post_switch:
> -  _Thread_Dispatch_set_disable_level( 0 );
>
>     _ISR_Enable( level );
>
> +  _Thread_Unnest_dispatch();
> +
>     _API_extensions_Run_postswitch();
>   }
[...]

This change and previous changes in the file threaddispatch.c are very bad.  In 
fact they destroy the real-time properties of RTEMS completely.

Consider the following scenario.  We have three tasks L (lowest priority), M 
(middle priority), and H (highest priority).  Now let a thread dispatch from M 
to L happen.  An interrupt occurs in _Thread_Dispatch() here:

void _Thread_Dispatch( void )
{
   [...]

post_switch:

   _ISR_Enable( level );

   <-- INTERRUPT
   <-- AFTER INTERRUPT

   _Thread_Unnest_dispatch();

   _API_extensions_Run_postswitch();
}

The interrupt event makes task H ready.  The interrupt code will see 
_Thread_Dispatch_disable_level > 0 and thus doesn't perform a 
_Thread_Dispatch().  Now we return to position "AFTER INTERRUPT".  This means 
task L executes now although task H is ready!  Task H will execute once someone 
calls _Thread_Dispatch().

We should restore the previous behaviour at least for non-SMP configurations. 
I am not sure how to deal with SMP here, but the current solution turns RTEMS 
into a non-real time system.

-- 
Sebastian Huber, embedded brains GmbH

Address : Obere Lagerstr. 30, D-82178 Puchheim, Germany
Phone   : +49 89 18 90 80 79-6
Fax     : +49 89 18 90 80 79-9
E-Mail  : sebastian.huber at embedded-brains.de
PGP     : Public key available on request.

Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.


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