Re: [Xenomai-core] Policy switching and XNOTHER maintenance

[Philippe Gerum <rpm@xenomai.org>]

On Fri, 2011-09-16 at 22:39 +0200, Gilles Chanteperdrix wrote:
> On 09/16/2011 10:13 PM, Gilles Chanteperdrix wrote:
> > On 09/11/2011 04:29 PM, Jan Kiszka wrote:
> >> On 2011-09-11 16:24, Gilles Chanteperdrix wrote:
> >>> On 09/11/2011 12:50 PM, Jan Kiszka wrote:
> >>>> Hi all,
> >>>>
> >>>> just looked into the hrescnt issue again, specifically the corner case
> >>>> of a shadow thread switching from real-time policy to SCHED_OTHER.
> >>>
> >>> Doing this while holding a mutex looks invalid.
> >>
> >> Looking at POSIX e.g., is there anything in the spec that makes this
> >> invalid? If the kernel preserves or established proper priority
> >> boosting, I do not see what could break in principle.
> >>
> >> It is nothing I would design into some app, but we should somehow handle
> >> it (doc update or code adjustments).
> >>
> >>> If we do not do it, the current code is valid.
> >>
> >> Except for its dependency on XNOTHER which is not updated on RT->NORMAL
> >> transitions.
> > 
> > The fact that this update did not take place made the code work. No 
> > negative rescnt could happen with that code.
> > 
> > Anyway, here is a patch to allow switching back from RT to NORMAL, but 
> > send a SIGDEBUG to a thread attempting to release a mutex while its 
> > counter is already 0. We end up avoiding a big chunk of code that would 
> > have been useful for a really strange corner case.
> > 
> 
> Here comes version 2:
> diff --git a/include/nucleus/sched-idle.h b/include/nucleus/sched-idle.h
> index 6399a17..417170f 100644
> --- a/include/nucleus/sched-idle.h
> +++ b/include/nucleus/sched-idle.h
> @@ -39,6 +39,8 @@ extern struct xnsched_class xnsched_class_idle;
>  static inline void __xnsched_idle_setparam(struct xnthread *thread,
>  					   const union xnsched_policy_param *p)
>  {
> +	if (xnthread_test_state(thread, XNSHADOW))
> +		xnthread_clear_state(thread, XNOTHER);
>  	thread->cprio = p->idle.prio;
>  }
>  
> diff --git a/include/nucleus/sched-rt.h b/include/nucleus/sched-rt.h
> index 71f655c..cc1cefa 100644
> --- a/include/nucleus/sched-rt.h
> +++ b/include/nucleus/sched-rt.h
> @@ -86,6 +86,12 @@ static inline void __xnsched_rt_setparam(struct xnthread *thread,
>  					 const union xnsched_policy_param *p)
>  {
>  	thread->cprio = p->rt.prio;
> +	if (xnthread_test_state(thread, XNSHADOW)) {
> +		if (thread->cprio)
> +			xnthread_clear_state(thread, XNOTHER);
> +		else
> +			xnthread_set_state(thread, XNOTHER);
> +	}
>  }
>  
>  static inline void __xnsched_rt_getparam(struct xnthread *thread,
> diff --git a/ksrc/nucleus/pod.c b/ksrc/nucleus/pod.c
> index 9a02e80..d19999f 100644
> --- a/ksrc/nucleus/pod.c
> +++ b/ksrc/nucleus/pod.c
> @@ -1896,16 +1896,6 @@ int __xnpod_set_thread_schedparam(struct xnthread *thread,
>  		xnsched_putback(thread);
>  
>  #ifdef CONFIG_XENO_OPT_PERVASIVE
> -	/*
> -	 * A non-real-time shadow may upgrade to real-time FIFO
> -	 * scheduling, but the latter may never downgrade to
> -	 * SCHED_NORMAL Xenomai-wise. In the valid case, we clear
> -	 * XNOTHER to reflect the change. Note that we keep handling
> -	 * non real-time shadow specifics in higher code layers, not
> -	 * to pollute the core scheduler with peculiarities.
> -	 */
> -	if (sched_class == &xnsched_class_rt && sched_param->rt.prio > 0)
> -		xnthread_clear_state(thread, XNOTHER);
>  	if (propagate) {
>  		if (xnthread_test_state(thread, XNRELAX))
>  			xnshadow_renice(thread);
> diff --git a/ksrc/nucleus/sched-sporadic.c b/ksrc/nucleus/sched-sporadic.c
> index fd37c21..ffc9bab 100644
> --- a/ksrc/nucleus/sched-sporadic.c
> +++ b/ksrc/nucleus/sched-sporadic.c
> @@ -258,6 +258,8 @@ static void xnsched_sporadic_setparam(struct xnthread *thread,
>  		}
>  	}
>  
> +	if (xnthread_test_state(thread, XNSHADOW))
> +		xnthread_clear_state(thread, XNOTHER);
>  	thread->cprio = p->pss.current_prio;
>  }
>  
> diff --git a/ksrc/nucleus/sched-tp.c b/ksrc/nucleus/sched-tp.c
> index 43a548e..a2af1d3 100644
> --- a/ksrc/nucleus/sched-tp.c
> +++ b/ksrc/nucleus/sched-tp.c
> @@ -100,6 +100,8 @@ static void xnsched_tp_setparam(struct xnthread *thread,
>  {
>  	struct xnsched *sched = thread->sched;
>  
> +	if (xnthread_test_state(thread, XNSHADOW))
> +		xnthread_clear_state(thread, XNOTHER);
>  	thread->tps = &sched->tp.partitions[p->tp.ptid];
>  	thread->cprio = p->tp.prio;
>  }
> diff --git a/ksrc/nucleus/synch.c b/ksrc/nucleus/synch.c
> index b956e46..47bc0c5 100644
> --- a/ksrc/nucleus/synch.c
> +++ b/ksrc/nucleus/synch.c
> @@ -684,9 +684,13 @@ xnsynch_release_thread(struct xnsynch *synch, struct xnthread *lastowner)
>  
>  	XENO_BUGON(NUCLEUS, !testbits(synch->status, XNSYNCH_OWNER));
>  
> -	if (xnthread_test_state(lastowner, XNOTHER))
> -		xnthread_dec_rescnt(lastowner);
> -	XENO_BUGON(NUCLEUS, xnthread_get_rescnt(lastowner) < 0);
> +	if (xnthread_test_state(lastowner, XNOTHER)) {
> +		if (xnthread_get_rescnt(lastowner) == 0)
> +			xnshadow_send_sig(lastowner, SIGDEBUG,
> +					  SIGDEBUG_MIGRATE_PRIOINV, 1);
> +		else
> +			xnthread_dec_rescnt(lastowner);
> +	}
>  	lastownerh = xnthread_handle(lastowner);
>  
>  	if (use_fastlock &&
> 
> 
> 

Agreed, the logic of this patch sounds right. Switching from -rt to -nrt
while holding a -rt mutex is pathological behavior. We don't have to
support apps implementing voluntary priority inversion.

-- 
Philippe.