diff options
Diffstat (limited to 'cpukit/score/src/threaddispatch.c')
-rw-r--r-- | cpukit/score/src/threaddispatch.c | 47 |
1 files changed, 43 insertions, 4 deletions
diff --git a/cpukit/score/src/threaddispatch.c b/cpukit/score/src/threaddispatch.c index 672a0999ab..8a77d1b4fe 100644 --- a/cpukit/score/src/threaddispatch.c +++ b/cpukit/score/src/threaddispatch.c @@ -55,9 +55,41 @@ void _Thread_Dispatch( void ) Thread_Control *heir; ISR_Level level; - _Thread_Disable_dispatch(); #if defined(RTEMS_SMP) /* + * WARNING: The SMP sequence has severe defects regarding the real-time + * performance. + * + * 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(). + */ + _Thread_Disable_dispatch(); + + /* * If necessary, send dispatch request to other cores. */ _SMP_Request_other_cores_to_dispatch(); @@ -69,8 +101,10 @@ void _Thread_Dispatch( void ) executing = _Thread_Executing; _ISR_Disable( level ); while ( _Thread_Dispatch_necessary == true ) { - heir = _Thread_Heir; + #ifndef RTEMS_SMP + _Thread_Dispatch_set_disable_level( 1 ); + #endif _Thread_Dispatch_necessary = false; _Thread_Executing = heir; @@ -167,10 +201,15 @@ void _Thread_Dispatch( void ) } post_switch: + #ifndef RTEMS_SMP + _Thread_Dispatch_set_disable_level( 0 ); + #endif _ISR_Enable( level ); - _Thread_Unnest_dispatch(); - + #ifdef RTEMS_SMP + _Thread_Unnest_dispatch(); + #endif + _API_extensions_Run_postswitch(); } |