[Xenomai-core] Domain switch during page fault handling

[Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.

In other words, and now it's starting to become a pure ipipe topic:

If we migrated during ipipe_trap_notify, shouldn't we restore the root
domain state afterwards from the state of the previously active domain
on entry of __ipipe_handle_exception? I bet that would fix our case, but
I'm yet unsure about side effects and the intended semantic behind this.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

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.

Normally.

> 
> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Gilles Chanteperdrix]

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 ?

-- 
					    Gilles.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

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.

> 
> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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...).

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

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.

> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

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.

> > Jan
> > 
> 
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

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.

> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Gilles Chanteperdrix]

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);
 
 	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;
 }


-- 
					    Gilles.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> 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.
> 

What about this (draft, not yet tested)?

diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c
index 4442d96..99c5346 100644
--- a/arch/x86/kernel/ipipe.c
+++ b/arch/x86/kernel/ipipe.c
@@ -702,19 +702,17 @@ 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) {
+	if (ipipe_root_domain_p && irqs_disabled_hw()) {
 		/*
 		 * 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_save(flags);
+		restore_flags = true;
 	}
 #ifdef CONFIG_KGDB
 	/* catch exception KGDB is interested in over non-root domains */
@@ -725,18 +723,20 @@ 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;
 	}
 
-	/*
-	 * 32-bit: In case we migrated to root domain inside the event
-	 * 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 (unlikely(!ipipe_root_domain_p)) {
+	if (likely(ipipe_root_domain_p)) {
+		/*
+		 * 32-bit: In case we migrated to root domain inside the event
+		 * handler, align regs.flags with the root domain state as the
+		 * low-level return code will evaluate it.
+		 */
+		__fixup_if(test_bit(IPIPE_STALL_FLAG,
+				    &ipipe_root_cpudom_var(status)), regs);
+	} else {
 		/* Detect unhandled faults over non-root domains. */
 		struct ipipe_domain *ipd = ipipe_current_domain;
 
@@ -770,21 +770,26 @@ 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;
 }
 
 int __ipipe_divert_exception(struct pt_regs *regs, int vector)
 {
+	bool restore_flags = false;
 	unsigned long flags;
 
-	/* Same root state handling as in __ipipe_handle_exception. */
-	local_save_flags(flags);
-
 	if (ipipe_root_domain_p) {
 		if (irqs_disabled_hw())
-			local_irq_disable();
+			/*
+			 * Same root state handling as in
+			 * __ipipe_handle_exception.
+			 */
+			local_irq_save(flags);
+			restore_flags = true;
+		}
 	}
 #ifdef CONFIG_KGDB
 	/* catch int1 and int3 over non-root domains */
@@ -804,16 +809,22 @@ int __ipipe_divert_exception(struct pt_regs *regs, 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;
 	}
 
+
+	if (likely(ipipe_root_domain_p)) {
+		/* see __ipipe_handle_exception */
+		__fixup_if(test_bit(IPIPE_STALL_FLAG,
+				    &ipipe_root_cpudom_var(status)), regs);
+	}
+
 	/*
-	 * 32-bit: Due to possible migration inside the event handler, we have
-	 * to restore IF so that low-level return code sets the root domain
-	 * state correctly.
+	 * No need to restore root state in the 64-bit case, the Linux handler
+	 * and the return code will take care of it.
 	 */
-	__fixup_if(raw_irqs_disabled_flags(flags), regs);
 
 	return 0;
 }

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.
> 

OK.

Just realized that my suggestion would conflict with the comment above
__fixup_if. So I think we first need to clarify the various scenarios
again to avoid breaking one while fixing another.

Entry over non-root, exit over non-root:
 - no need to fiddle with the root state

Entry over root, exit over root, !irqs_disabled_hw():
 - no need to fiddle with the root state
 - 32 bit: regs fixup required?

Entry over root, exit over root, irqs_disabled_hw():
 - save root state & disable root IRQs on entry
 - 32 bit: replicate saved state into regs.eflags before calling linux
   handler
 - restore saved state on exit

Entry over non-root, exit over root:
 - ?

I tend to think that, for the 32-bit cases, we should pick up the flags
from the root state _after_ returning from ipipe_trap_notify and only if
we are truly running in the root domain then. That should be the value
the migration left behind, so the correct one, right?

Any scenario missing?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

On Sat, 2010-01-23 at 11:33 +0100, Jan Kiszka 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.
> > 
> 
> OK.
> 
> Just realized that my suggestion would conflict with the comment above
> __fixup_if. So I think we first need to clarify the various scenarios
> again to avoid breaking one while fixing another.
> 
> Entry over non-root, exit over non-root:
>  - no need to fiddle with the root state
> 

Correct.

> Entry over root, exit over root, !irqs_disabled_hw():
>  - no need to fiddle with the root state
>  - 32 bit: regs fixup required?
> 

The iret emulation via iret_root needs proper fixup to have taken place,
so doing the fixup is mandatory in any case.

> Entry over root, exit over root, irqs_disabled_hw():
>  - save root state & disable root IRQs on entry
>  - 32 bit: replicate saved state into regs.eflags before calling linux
>    handler
>  - restore saved state on exit

Correct.

> 
> Entry over non-root, exit over root:
>  - ?
> 
> I tend to think that, for the 32-bit cases, we should pick up the flags
> from the root state _after_ returning from ipipe_trap_notify and only if
> we are truly running in the root domain then. That should be the value
> the migration left behind, so the correct one, right?
> 

Yes.

> Any scenario missing?
> 

Should be ok. But the more I think of it, the more I'm convinced that we
should make the 32 and 64 bit implementation converge to the x86_64 one.
iret_root emulation is required because we virtualize the interrupt
masking in the assembly-written portions for x86_32, which we don't for
x86_64. 

Hyste^Horically, there was a strong requirement to do that with legacy
2.4 kernels the 32 bit implementation was designed for, because lengthy
masked sections would raise the latency above the top. With current
2.6/x86 kernels, I don't think virtualizing interrupt masking in the
assembly-written portions makes a lot of sense anymore, since
significant work took place upstream over time, to allow for
fine-grained preemption there.

> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

[-- Attachment #1: Type: text/plain, Size: 7039 bytes --]

Philippe Gerum wrote:
> On Sat, 2010-01-23 at 11:33 +0100, Jan Kiszka 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.
>>>
>> OK.
>>
>> Just realized that my suggestion would conflict with the comment above
>> __fixup_if. So I think we first need to clarify the various scenarios
>> again to avoid breaking one while fixing another.
>>
>> Entry over non-root, exit over non-root:
>>  - no need to fiddle with the root state
>>
> 
> Correct.
> 
>> Entry over root, exit over root, !irqs_disabled_hw():
>>  - no need to fiddle with the root state
>>  - 32 bit: regs fixup required?
>>
> 
> The iret emulation via iret_root needs proper fixup to have taken place,
> so doing the fixup is mandatory in any case.
> 
>> Entry over root, exit over root, irqs_disabled_hw():
>>  - save root state & disable root IRQs on entry
>>  - 32 bit: replicate saved state into regs.eflags before calling linux
>>    handler
>>  - restore saved state on exit
> 
> Correct.
> 
>> Entry over non-root, exit over root:
>>  - ?
>>
>> I tend to think that, for the 32-bit cases, we should pick up the flags
>> from the root state _after_ returning from ipipe_trap_notify and only if
>> we are truly running in the root domain then. That should be the value
>> the migration left behind, so the correct one, right?
>>
> 
> Yes.
> 
>> Any scenario missing?
>>
> 
> Should be ok. But the more I think of it, the more I'm convinced that we
> should make the 32 and 64 bit implementation converge to the x86_64 one.
> iret_root emulation is required because we virtualize the interrupt
> masking in the assembly-written portions for x86_32, which we don't for
> x86_64. 
> 
> Hyste^Horically, there was a strong requirement to do that with legacy
> 2.4 kernels the 32 bit implementation was designed for, because lengthy
> masked sections would raise the latency above the top. With current
> 2.6/x86 kernels, I don't think virtualizing interrupt masking in the
> assembly-written portions makes a lot of sense anymore, since
> significant work took place upstream over time, to allow for
> fine-grained preemption there.

I'm all for this, but I think it should happen gradually. In step 1, we
should fix the current situation. Step 2 would obsolete __fixup_if by
moving x86-32 towards the 64-bit scheme.

So a discussion of my last patch would be warmly welcomed.

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 257 bytes --]

Re: [Xenomai-core] Domain switch during page fault handling

[Philippe Gerum]

On Sun, 2010-01-24 at 11:45 +0100, Jan Kiszka wrote:
> Philippe Gerum wrote:
> > On Sat, 2010-01-23 at 11:33 +0100, Jan Kiszka 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.
> >>>
> >> OK.
> >>
> >> Just realized that my suggestion would conflict with the comment above
> >> __fixup_if. So I think we first need to clarify the various scenarios
> >> again to avoid breaking one while fixing another.
> >>
> >> Entry over non-root, exit over non-root:
> >>  - no need to fiddle with the root state
> >>
> > 
> > Correct.
> > 
> >> Entry over root, exit over root, !irqs_disabled_hw():
> >>  - no need to fiddle with the root state
> >>  - 32 bit: regs fixup required?
> >>
> > 
> > The iret emulation via iret_root needs proper fixup to have taken place,
> > so doing the fixup is mandatory in any case.
> > 
> >> Entry over root, exit over root, irqs_disabled_hw():
> >>  - save root state & disable root IRQs on entry
> >>  - 32 bit: replicate saved state into regs.eflags before calling linux
> >>    handler
> >>  - restore saved state on exit
> > 
> > Correct.
> > 
> >> Entry over non-root, exit over root:
> >>  - ?
> >>
> >> I tend to think that, for the 32-bit cases, we should pick up the flags
> >> from the root state _after_ returning from ipipe_trap_notify and only if
> >> we are truly running in the root domain then. That should be the value
> >> the migration left behind, so the correct one, right?
> >>
> > 
> > Yes.
> > 
> >> Any scenario missing?
> >>
> > 
> > Should be ok. But the more I think of it, the more I'm convinced that we
> > should make the 32 and 64 bit implementation converge to the x86_64 one.
> > iret_root emulation is required because we virtualize the interrupt
> > masking in the assembly-written portions for x86_32, which we don't for
> > x86_64. 
> > 
> > Hyste^Horically, there was a strong requirement to do that with legacy
> > 2.4 kernels the 32 bit implementation was designed for, because lengthy
> > masked sections would raise the latency above the top. With current
> > 2.6/x86 kernels, I don't think virtualizing interrupt masking in the
> > assembly-written portions makes a lot of sense anymore, since
> > significant work took place upstream over time, to allow for
> > fine-grained preemption there.
> 
> I'm all for this, but I think it should happen gradually. In step 1, we
> should fix the current situation. Step 2 would obsolete __fixup_if by
> moving x86-32 towards the 64-bit scheme.

I'm not saying anything else. But I want to stress the fact that we are
fixing the current mess for the last time; next time, let's kick out
this legacy.

> 
> So a discussion of my last patch would be warmly welcomed.
> 
> Jan
> 


-- 
Philippe.

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

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.

That all happens as expected. But then __ipipe_handle_exception - as it
last duty - ruins our day by replaying an unrelated root state.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Jan Kiszka]

Jan Kiszka 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.
> 
> That all happens as expected. But then __ipipe_handle_exception - as it
> last duty - ruins our day by replaying an unrelated root state.
> 

To illustrate what goes wrong (maybe it already fixes the issue, but it
may have side effects and is surely not clean):

diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c
index 4442d96..b6a3720 100644
--- a/arch/x86/kernel/ipipe.c
+++ b/arch/x86/kernel/ipipe.c
@@ -702,11 +702,14 @@ static int __ipipe_xlate_signo[] = {
 
 int __ipipe_handle_exception(struct pt_regs *regs, long error_code, int vector)
 {
-	unsigned long flags;
+	struct ipipe_domain *entry_domain = ipipe_current_domain;
+	unsigned long flags, entry_dom_flags;
 
 	/* Pick up the root domain state of the interrupted context. */
 	local_save_flags(flags);
 
+	entry_dom_flags = ipipe_test_pipeline() ? 0 : X86_EFLAGS_IF;
+
 	if (ipipe_root_domain_p) {
 		/*
 		 * Replicate hw interrupt state into the virtual mask before
@@ -729,6 +732,9 @@ int __ipipe_handle_exception(struct pt_regs *regs, long error_code, int vector)
 		return 1;
 	}
 
+	if (entry_domain != ipipe_current_domain)
+		flags = entry_dom_flags;
+
 	/*
 	 * 32-bit: In case we migrated to root domain inside the event
 	 * handler, restore the original IF from exception entry as the


Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

Re: [Xenomai-core] Domain switch during page fault handling

[Gilles Chanteperdrix]

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.

I have spent a few hours on a similar problem on x86_32. The difference 
on x86_32 is that the stall bit is used as user-space interrupt flag, so
the effect is visible on latencies. I have to say that understanding 
__ipipe_handle_exception requires more time than I have currently spent,
but I intend to elucidate this sooner or later. Here are the kind of 
traces I get:

:|  #*event   tick@domain.hid   -188+   3.035  xntimer_next_local_shot+0x85 (xntimer_t
(...)
:    +func                -170+   1.797  up_read+0x3 (do_page_fault+0x136)
:    #func                -168    0.279  __ipipe_unstall_iret_root+0x4 (restore_ret+0x0)
:|   #begin   0x80000000  -168    0.326  __ipipe_unstall_iret_root+0x7b (restore_ret+0x0)
:|   #end     0x8000000d  -167+   1.572  __ipipe_unstall_iret_root+0x64 (restore_ret+0x0)
:|   #func                -166    0.239  __ipipe_syscall_root+0xf (system_call+0x2d)
:|   #func                -166    0.323  __ipipe_dispatch_event+0x9 (__ipipe_syscall_root+0x40)
:|  +*func                -165    0.859  hisyscall_event+0xf (__ipipe_dispatch_event+0xd3)
:|   #func                -164+   1.388  losyscall_event+0x9 (__ipipe_dispatch_event+0xd3)
:|   #func                -163    0.870  sys_time+0x11 (syscall_call+0x7)
:|   #func                -162    0.278  get_seconds+0x3 (sys_time+0x1e)
:    #func                -162    0.271  __ipipe_unstall_iret_root+0x4 (restore_ret+0x0)
:|   #begin   0x80000000  -162    0.575  __ipipe_unstall_iret_root+0x7b (restore_ret+0x0)
:|   #end     0x8000000d  -161!  64.984  __ipipe_unstall_iret_root+0x64 (restore_ret+0x0)
:|   #func                 -96    0.275  __ipipe_syscall_root+0xf (system_call+0x2d)
:|   #func                 -96    0.304  __ipipe_dispatch_event+0x9 (__ipipe_syscall_root+0x40)
:|  +*func                 -95    0.482  hisyscall_event+0xf (__ipipe_dispatch_event+0xd3)
:|   #func                 -95    0.758  losyscall_event+0x9 (__ipipe_dispatch_event+0xd3)
:|   #func                 -94    0.971  sys_write+0x8 (syscall_call+0x7)


:|  #*event   tick@domain.hid   -228    0.546  xntimer_next_local_shot+0x89 (xntimer_t
(...)
:    +func                -181    0.647  up_read+0x3 (do_page_fault+0x126)
:    #func                -181+   1.282  __ipipe_unstall_iret_root+0x3 (restore_ret+0x0)
:|   #func                -179    0.356  __ipipe_syscall_root+0x11 (system_call+0x2d)
:|   #func                -179    0.317  __ipipe_dispatch_event+0x9 (__ipipe_syscall_root+0x42)
:|  +*func                -179    0.712  hisyscall_event+0xf (__ipipe_dispatch_event+0xb7)
:|   #func                -178+   1.415  losyscall_event+0x9 (__ipipe_dispatch_event+0xb7)
:|   #func                -177    0.763  sys_time+0x11 (syscall_call+0x7)
:|   #func                -176    0.350  get_seconds+0x3 (sys_time+0x1e)
:    #func                -176!  71.098  __ipipe_unstall_iret_root+0x3 (restore_ret+0x0)
:|   #func                -104    0.309  __ipipe_syscall_root+0x11 (system_call+0x2d)
:|   #func                -104    0.839  __ipipe_dispatch_event+0x9 (__ipipe_syscall_root+0x42)
:|  +*func                -103    0.400  hisyscall_event+0xf (__ipipe_dispatch_event+0xb7)
:|   #func                -103+   1.362  losyscall_event+0x9 (__ipipe_dispatch_event+0xb7)
:|   #func                -102    0.520  sys_write+0x8 (syscall_call+0x7)


-- 
					    Gilles.