cpu_time_budget race condition

[Thomas Rauscher]

Hi,

I've found a race condition between 
_Thread_Dispatch (called from a task) and _Thread_Tickle_timeslice 
(called from the tick ISR).

This is a topic for 4.5 and later (checked against cvs head revision).

Problem:

A task is using the classic RTEMS API. The thread control structure
is called my_thread in advance. 

my_thread->budget_algorithm ==
THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE
my_thread->cpu_time_budget == 0 (or even uninitialized, I'm not sure)

is true at the beginning.

Now the task is started and rtems_task_create invokes _Thread_Dispatch.

threaddispatch.c

  ...
  _ISR_Disable( level );
  while ( _Context_Switch_necessary == TRUE ) {
    heir = _Thread_Heir;
    _Thread_Dispatch_disable_level = 1;
    _Context_Switch_necessary = FALSE;
    _Thread_Executing = heir;
    executing->rtems_ada_self = rtems_ada_self;
    rtems_ada_self = heir->rtems_ada_self;
    _ISR_Enable( level );
    ...

At this point, interrupts are enabled again and a tick interrupt occurs
calling _Thread_Tickle_timeslice. _Thread_Executing is my_thread now.

threadtickletimeslice.c:

    ...
    executing = _Thread_Executing;
    ...
    case THREAD_CPU_BUDGET_ALGORITHM_RESET_TIMESLICE:
    case THREAD_CPU_BUDGET_ALGORITHM_EXHAUST_TIMESLICE:
      if ( --executing->cpu_time_budget == 0 ) {
        _Thread_Reset_timeslice();
        executing->cpu_time_budget = _Thread_Ticks_per_timeslice;
      }
      break;
    ...

Now, thread->cpu_time_budget == 0xFFFFFFFF which means that
the task won't stop until it preempts itself or its timeslice
is exhausted (approx. 49 days using a 1 ms tick).

I think, there are two possible fixes:

1) Move the cpu_time_budget initialization in _Thread_Dispatch
   to the beginning where interrupts are locked.
2) Initialize cpu_time_budget to _Thread_Ticks_per_timeslice
   in _Thread_Initialize().

I've successfully tried the first patch, but the second
one seems to be a little better as the interrupt latency isn't
increased.

Any comments? If my observation is correct, I can file a PR and
provide a patch.

Best regards,
Thomas

-- 
Thomas Rauscher
LOYTEC electronics GmbH
Stolzenthalergasse 24/3
A-1080 Wien
Austria/Europe
trauscher at loytec.com
www.loytec.com
Phone: +43-1-4020805-15
FAX:   +43-1-4020805-99