score: Critical fix for thread dispatching

The changes in _Thread_Dispatch() of commits
dad36c52b8be5d7b46bc7af85655055db7208652 and
d4dc7c8196355f08044e67a3f5c1e19485f17ff1 introduced a severe bug which
destroys 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().

0 files changed, 0 insertions, 0 deletions