Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>> On Sat, 2010-01-23 at 11:09 +0100, Jan Kiszka wrote:
>>> Philippe Gerum wrote:
>>>> On Fri, 2010-01-22 at 19:08 +0100, Philippe Gerum wrote:
>>>>> On Fri, 2010-01-22 at 19:03 +0100, Jan Kiszka wrote:
>>>>>> Philippe Gerum wrote:
>>>>>>> On Fri, 2010-01-22 at 18:41 +0100, Jan Kiszka wrote:
>>>>>>>> Gilles Chanteperdrix wrote:
>>>>>>>>> Philippe Gerum wrote:
>>>>>>>>>> On Fri, 2010-01-22 at 17:58 +0100, Jan Kiszka wrote:
>>>>>>>>>>> Hi guys,
>>>>>>>>>>>
>>>>>>>>>>> we are currently trying to catch an ugly Linux pipeline state
>>>>>>>>>>> corruption
>>>>>>>>>>> on x86-64.
>>>>>>>>>>>
>>>>>>>>>>> Conceptual question: If a Xenomai task causes a fault, we enter
>>>>>>>>>>> ipipe_trap_notify over the primary domain and leave it over the root
>>>>>>>>>>> domain, right? Now, if the root domain happened to be stalled when
>>>>>>>>>>> the
>>>>>>>>>>> exception happened, where should it normally be unstalled again,
>>>>>>>>>>> *for_that_task*? Our problem is that we generate a code path where
>>>>>>>>>>> this
>>>>>>>>>>> does not happen.
>>>>>>>>>> xnhadow_relax -> ipipe_reenter_root -> finish_task_switch ->
>>>>>>>>>> finish_lock_switch -> unstall
>>>>>>>>>>
>>>>>>>>>> Since xnshadow_relax is called on behalf the event dispatcher, we
>>>>>>>>>> should
>>>>>>>>>> expect it to return with the root domain unstalled after a domain
>>>>>>>>>> downgrade, from primary to root.
>>>>>>>>> Ok, but what about local_irq_restore_nosync at the end of the
>>>>>>>>> function ?
>>>>>>>>>
>>>>>>>> That is, IMO, our problem: It replays the root state on fault entry,
>>>>>>>> but
>>>>>>>> that one is totally unrelated to the (Xenomai) task that caused the
>>>>>>>> fault.
>>>>>>> The code seems fishy. Try restoring only when the incoming domain was
>>>>>>> the root one. Indeed.
>>>>>>>
>>>>>> Something like this?
>>>>>>
>>>>>> diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c
>>>>>> index 4442d96..0558ea3 100644
>>>>>> --- a/arch/x86/kernel/ipipe.c
>>>>>> +++ b/arch/x86/kernel/ipipe.c
>>>>>> @@ -702,19 +702,21 @@ static int __ipipe_xlate_signo[] = {
>>>>>>
>>>>>> int __ipipe_handle_exception(struct pt_regs *regs, long error_code, int
>>>>>> vector)
>>>>>> {
>>>>>> + bool restore_flags = false;
>>>>>> unsigned long flags;
>>>>>>
>>>>>> - /* Pick up the root domain state of the interrupted context. */
>>>>>> - local_save_flags(flags);
>>>>>> + if (ipipe_root_domain_p && irqs_disabled_hw()) {
>>>>>> + /* Pick up the root domain state of the interrupted
>>>>>> context. */
>>>>>> + local_save_flags(flags);
>>>>>>
>>>>>> - if (ipipe_root_domain_p) {
>>>>>> /*
>>>>>> * Replicate hw interrupt state into the virtual mask
>>>>>> before
>>>>>> * calling the I-pipe event handler over the root
>>>>>> domain. Also
>>>>>> * required later when calling the Linux exception
>>>>>> handler.
>>>>>> */
>>>>>> - if (irqs_disabled_hw())
>>>>>> - local_irq_disable();
>>>>>> + local_irq_disable();
>>>>>> +
>>>>>> + restore_flags = true;
>>>>>> }
>>>>>> #ifdef CONFIG_KGDB
>>>>>> /* catch exception KGDB is interested in over non-root domains
>>>>>> */
>>>>>> @@ -725,7 +727,8 @@ int __ipipe_handle_exception(struct pt_regs *regs,
>>>>>> long error_code, int vector)
>>>>>> #endif /* CONFIG_KGDB */
>>>>>>
>>>>>> if (unlikely(ipipe_trap_notify(vector, regs))) {
>>>>>> - local_irq_restore_nosync(flags);
>>>>>> + if (restore_flags)
>>>>>> + local_irq_restore_nosync(flags);
>>>>>> return 1;
>>>>>> }
>>>>>>
>>>>>> @@ -770,7 +773,8 @@ int __ipipe_handle_exception(struct pt_regs *regs,
>>>>>> long error_code, int vector)
>>>>>> * Relevant for 64-bit: Restore root domain state as the
>>>>>> low-level
>>>>>> * return code will not align it to regs.flags.
>>>>>> */
>>>>>> - local_irq_restore_nosync(flags);
>>>>>> + if (restore_flags)
>>>>>> + local_irq_restore_nosync(flags);
>>>>>>
>>>>>> return 0;
>>>>>> }
>>>>>>
>>>>>>
>>>>>> We are currently not able to test this on the system that triggers it,
>>>>>> but we'll do so tomorrow (yeah...).
>>>>>>
>>>>> Should work. Famous last words.
>>>>>
>>>> Strike that. This won't work, because the fixup code will use the saved
>>>> flags even when root is not the incoming domain and/or hw IRQs are on on
>>>> entry. In short, local_save_flags() must be done unconditionally, as
>>>> previously.
>>> It will accidentally work for 64-bit where __fixup_if is empty. And for
>>> 32-bit, I would say we need to make it depend on restore_flags as well.
>>>
>> AFAIK, Gilles is working on this. We just need to avoid stepping on
>> 32bit toes to fix 64.
>
> The arguably less ambitious following patch works for me on x86_32:
>
> diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c
> index 4442d96..a7e1241 100644
> --- a/arch/x86/kernel/ipipe.c
> +++ b/arch/x86/kernel/ipipe.c
> @@ -703,6 +703,7 @@ static int __ipipe_xlate_signo[] = {
> int __ipipe_handle_exception(struct pt_regs *regs, long error_code, int
> vector)
> {
> unsigned long flags;
> + unsigned root_on_entry = 0;
>
> /* Pick up the root domain state of the interrupted context. */
> local_save_flags(flags);
> @@ -715,6 +716,7 @@ int __ipipe_handle_exception(struct pt_regs *regs, long
> error_code, int vector)
> */
> if (irqs_disabled_hw())
> local_irq_disable();
> + root_on_entry = 1;
> }
> #ifdef CONFIG_KGDB
> /* catch exception KGDB is interested in over non-root domains */
> @@ -725,7 +727,8 @@ int __ipipe_handle_exception(struct pt_regs *regs, long
> error_code, int vector)
> #endif /* CONFIG_KGDB */
>
> if (unlikely(ipipe_trap_notify(vector, regs))) {
> - local_irq_restore_nosync(flags);
> + if (root_on_entry)
> + local_irq_restore_nosync(flags);
> return 1;
> }
>
> @@ -734,7 +737,8 @@ int __ipipe_handle_exception(struct pt_regs *regs, long
> error_code, int vector)
> * handler, restore the original IF from exception entry as the
> * low-level return code will evaluate it.
> */
> - __fixup_if(raw_irqs_disabled_flags(flags), regs);
> + if (root_on_entry)
> + __fixup_if(raw_irqs_disabled_flags(flags), regs);
See my other mail, this is conflicting with what was once documented as
the purpose of this fixup. I think we need to pick up the current root
state here and push it into regs - if we are running over root.
>
> if (unlikely(!ipipe_root_domain_p)) {
> /* Detect unhandled faults over non-root domains. */
> @@ -770,7 +774,8 @@ int __ipipe_handle_exception(struct pt_regs *regs, long
> error_code, int vector)
> * Relevant for 64-bit: Restore root domain state as the low-level
> * return code will not align it to regs.flags.
> */
> - local_irq_restore_nosync(flags);
> + if (root_on_entry)
> + local_irq_restore_nosync(flags);
>
> return 0;
> }
>
>
Jan
--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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