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Re: [Adeos-main] task_hijacked, prepare_arch_switch and irq disabling Jan Kiszka Mon Nov 09 07:00:10 2009

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! Thought is was an ipipe extension.

OK, thanks, clear now: prepare_task_switch pairs with enabling in
task_hijacked & ret_from_fork.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

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