Re: [Adeos-main] task_hijacked, prepare_arch_switch and irq disabling

[Philippe Gerum <rpm@xenomai.org>]

On Mon, 2009-11-09 at 14:58 +0100, Jan Kiszka wrote:
> Philippe Gerum wrote:
> > On Mon, 2009-11-09 at 14:51 +0100, Jan Kiszka wrote:
> >> Philippe Gerum wrote:
> >>> On Mon, 2009-11-09 at 14:37 +0100, Philippe Gerum wrote:
> >>>> On Mon, 2009-11-09 at 14:00 +0100, Jan Kiszka wrote:
> >>>>> Hi Philippe,
> >>>>>
> >>>>> a customer just stumbled over some unclear spots in current I-pipe patches:
> >>>>>
> >>>>> Why is there local_irq_enable_hw in task_hijacked? Looks like it was
> >>>>> once paired with prepare_arch_switch, but that call is now only used by
> >>>>> legacy 2.4 PPC.
> >>>> It is still used in all trees, since we define it in asm/ipipe.h. We
> >>>> could not context switch properly on the linux side with hw interrupts
> >>>> on anyway, due to the conflicts that would raise with Xenomai's tasking
> >>>> code.
> >>>>
> >>>>>  Can we safely drop it from all other patches (it's in
> >>>>> the context switch fast-path...)?
> >>>> No, since prepare_arch_switch() is still applicable.
> >>>>
> >>>>>  Moreover, I bet the
> >>>>> ENABLE_INTERRUPTS_HW_COND in entry_32.S' ret_from_fork is related to
> >>>>> this as well, right?
> >>>> No, it's there to prevent the scheduling tail from running hw IRQs off,
> >>>> given that copy_thread() may set a copy for eflags which prevents
> >>>> preemption.
> >>> Forget about this one, this does not apply to x86 anymore. So the answer
> >>> to this question is rather: that used to be required on x86 a long time
> >>> ago due to the implementation of the task switching code in system.h
> >>> (2.4 era IIRC); but in any case, yes, this is still required for the
> >>> reasons explained above, since we must run the switch code with hw IRQs
> >>> off.  
> >> Still can't follow: Where is prepare_arch_switch referenced? 
> > 
> > prepare_task_switch(), kernel/sched.c
> 
> Ah, this is mainline!

Yep.

>  Thought is was an ipipe extension.
> 
> OK, thanks, clear now: prepare_task_switch pairs with enabling in
> task_hijacked & ret_from_fork.
> 
> Jan
> 


-- 
Philippe.