Re: RT and Cascade interrupts

[Trond Myklebust <trond.myklebust@fys.uio.no>]

on den 01.06.2005 Klokka 14:05 (-0400) skreiv john cooper:
> I've done so and the second assert is generated
> when running the test.  So here we have a case
> of RPC_TASK_HAS_TIMER set but the associated
> timer->base == NULL.  It would seem this could
> easily be the scenario of executing in:
> 
> __run_timers()
>      timer->base = NULL;
>          rpc_run_timer()
>              task->tk_timeout_fn(task)
>              /* ksoftirqd preempted */
>                                               :
>                                           /* RPC client */
>                                           rpc_execute()
>                                              rpc_delete_timer()
>                                                  del_timer() returns 0
>                                                  BUG_ON(test_bit(RPC_TASK_HAS_TIMER,
>                                                    &task->tk_runstate));
>                                               :
>           /* rpc_run_timer() resumes */
>           clear_bit(RPC_TASK_HAS_TIMER,
>             &task->tk_runstate);
> 
> I don't see how this would imply a kernel/timer.c
> problem.  It also appears this wouldn't be causing
> the timer cascade corruption I've seen as the
> end result is deleting an already dequeued timer
> which is safe here.

If timer->base==NULL, then del_timer() returns 0 (as you correctly state
above). What you appear to be missing is that this will trigger a call
to del_timer_sync() inside del_singleshot_timer_sync().

In other words, your scenario above should, if the timer code is working
according to spec instead lead to

__run_timers()
     timer->base = NULL;
         rpc_run_timer()
             task->tk_timeout_fn(task)
             /* ksoftirqd preempted */
                                              :
                                       /* RPC client */
                                       rpc_execute()
                                          rpc_delete_timer()
					     del_singleshot_timer_sync()
                                                 del_timer() returns 0
						 del_timer_sync()
						/* wait until timer has
						   completed */
          /*rpc_run_timer() resumes */
          clear_bit(RPC_TASK_HAS_TIMER,
            &task->tk_runstate);
         ....
     set_running_timer(some_other_value);

				    BUG_ON(test_bit(RPC_TASK_HAS_TIMER,
                                                   &task->tk_runstate));
                                              :

Cheers,
  Trond

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