Re: RT and Cascade interrupts

[john cooper]

Trond Myklebust wrote:
> on den 01.06.2005 Klokka 14:05 (-0400) skreiv john cooper:
> > 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().

You might have missed in my earlier mail as
this is a not an MP kernel ie: !CONFIG_SMP
The synchronous timer delete primitives don't
exist in this configuration:

include/linux/timer.h:

#ifdef CONFIG_SMP
  extern int del_timer_sync(struct timer_list *timer);
  extern int del_singleshot_timer_sync(struct timer_list *timer);
#else
# define del_timer_sync(t) del_timer(t)
# define del_singleshot_timer_sync(t) del_timer(t)
#endif

BTW, I don't know if you happened on the mail I sent
yesterday.  It details rpc_run_timer() waking up an
application task blocked in call_transmit().  The
app task preempts ksoftirqd and eventually does a
__rpc_sleep_on()/__mod_timer() which requeues the
timer in the cascade.  When ksoftirqd/rpc_run_timer()
resumes RPC_TASK_HAS_TIMER is unconditionally cleared
however the timer is queued in the cascade.  This
appears to be at least one cause of the timer cascade
corruption I've seen.

-john


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