Re: [Xenomai-core] [Xenomai-git] Jan Kiszka : nucleus: Fix interrupt handler tails

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 06/20/2011 10:41 PM, Jan Kiszka wrote:
> xnarch_switch_to is the central entry point for everyone. It may decide
> to branch to switch_to or __switch_to, or it simply handles all on its
> own - that's depending on the arch.

No, the Linux kernel does not know anything about xnarch_switch_to, so
the schedule() function continues to use switch_to happily.
xnarch_switch_to is only used to switch from xnthread_t to xnthread_t,
by __xnpod_schedule(). Now, that some architecture (namely x86) decide
that xnarch_switch_to should use switch_to (or more likely an inner
__switch_to) when the xnthread_t has a non NULL user_task member is an
implementation detail.

> Can you point out where in those paths irqs are disabled again (after
> entering xnarch_switch_to)

They are not disabled again after xnarch_switch_to, they are disabled
when starting switch_to.

 and left off for each of the unlocked
> switching archs? I'm still skeptical that the need for disable irqs
> during thread switch on some archs also leads to unconditionally
> disabled hard irqs when returning from xnarch_switch_to.
> 
> But even if that's all the case today, we would better set this
> requirement in stone:
> 
> diff --git a/ksrc/nucleus/pod.c b/ksrc/nucleus/pod.c
> index f2fc2ab..c4c5807 100644
> --- a/ksrc/nucleus/pod.c
> +++ b/ksrc/nucleus/pod.c
> @@ -2273,6 +2273,8 @@ reschedule:
> 
>  	xnpod_switch_to(sched, prev, next);
> 
> +	XENO_BUGON(NUCLEUS, !irqs_disabled_hw());
> +

You misunderstand me: only after the second half context switch in the
case of xnshadow_relax are the interrupts disabled. Because this second
half-switch started as a switch_to and not an xnarch_switch_to, so,
started as:

#define switch_to(prev,next,last)                                       \
do {                                                                    \
        local_irq_disable_hw_cond();                                    \
        last = __switch_to(prev,task_thread_info(prev),
task_thread_info(next));        \
        local_irq_enable_hw_cond();                                     \
} while (0)

(On ARM for instance).

But that is true, we could assert this in the shadow epilogue case.

-- 
                                                                Gilles.