Re: [Xenomai] ipipe: issues with ARM exception handling

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

On Mon, Feb 23, 2015 at 06:01:54PM +0100, Philippe Gerum wrote:
> On 02/23/2015 05:55 PM, Gilles Chanteperdrix wrote:
> > On Mon, Feb 23, 2015 at 05:50:13PM +0100, Jan Kiszka wrote:
> >> On 2015-02-23 17:37, Gilles Chanteperdrix wrote:
> >>> On Mon, Feb 23, 2015 at 05:32:56PM +0100, Philippe Gerum wrote:
> >>>> On 02/23/2015 05:06 PM, Jan Kiszka wrote:
> >>>>> On 2015-02-20 20:52, Philippe Gerum wrote:
> >>>>>> On 02/20/2015 08:47 PM, Philippe Gerum wrote:
> >>>>>>> On 02/20/2015 08:44 PM, Philippe Gerum wrote:
> >>>>>>>> On 02/20/2015 07:17 PM, Jan Kiszka wrote:
> >>>>>>>>> On 2015-02-20 19:03, Jan Kiszka wrote:
> >>>>>>>>>> Hi Gilles,
> >>>>>>>>>>
> >>>>>>>>>> analyzing a lockdep warning on 3.16 with I-pipe enabled, I dug deeper
> >>>>>>>>>> into the hard and virtual interrupt state management during exception
> >>>>>>>>>> handling on ARM. I think there are several issues:
> >>>>>>>>>>
> >>>>>>>>>> - ipipe_fault_entry should not fiddle with the root irq state if run
> >>>>>>>>>>   over head, only when invoked over root.
> >>>>>>>>>> - ipipe_fault_exit must not change the root state unless we entered over
> >>>>>>>>>>   head and are about to leave over root - see x86. The current code may
> >>>>>>>>>>   keep root incorrectly stalled after an exception, though this will
> >>>>>>>>>>   probably be fixed up again in practice quickly.
> >>>>>>>>>
> >>>>>>>>> And the adjustment of the root irq state after migration has to happen
> >>>>>>>>> before Linux starts to handle the event. It would basically be a late
> >>>>>>>>> ipipe_fault_entry.
> >>>>>>>>>
> >>>>>>>>>> - do_sect_fault is only called by do_DataAbort and do_PrefetchAbort,
> >>>>>>>>>>   in both cases already wrapped in ipipe_fault_entry/exit, thus it
> >>>>>>>>>>   shouldn't invoke them once again.
> >>>>>>>>>
> >>>>>>>>> Sorry, this was a misinterpretation - do_sect_fault is invoked before
> >>>>>>>>> ipipe_fault_entry.
> >>>>>>>>>
> >>>>>>>>> What I need to add, though:
> >>>>>>>>>
> >>>>>>>>> - do_DataAbort and do_PrefetchAbort call __ipipe_report_trap after
> >>>>>>>>>   ipipe_fault_entry, thus with hard IRQs on.
> >>>>>>>>
> >>>>>>>> This would break LPAE with the Xenomai nucleus as a module on 2.6.x, by
> >>>>>>>> treading over a non-linear kernel mapping before the page table could be
> >>>>>>>> fixed up. do_translation_fault() must run via the fsr handler
> >>>>>>>> indirection before any non-linear access.
> >>>>>>>>
> >>>>>>>
> >>>>>>> Sorry, if you do that _after_ the fault entry notification, then it's ok
> >>>>>>> in theory. However, I don't understand why we would need to notify when
> >>>>>>> only a minor fixup is required, that does not entail a mode migration.
> >>>>>>>
> >>>>>>
> >>>>>> To be clearer, do you intend to report the minor fault upon
> >>>>>> do_translation_fault() returning zero, or are you referring to a
> >>>>>> different context?
> >>>>>
> >>>>> No, I'm just talking about this potential change:
> >>>>>
> >>>>> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> >>>>> index 38834c6..b42632a 100644
> >>>>> --- a/arch/arm/mm/fault.c
> >>>>> +++ b/arch/arm/mm/fault.c
> >>>>> @@ -629,10 +629,10 @@ do_DataAbort(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> >>>>>  	if (!inf->fn(addr, fsr & ~FSR_LNX_PF, regs))
> >>>>>  		return;
> >>>>>  
> >>>>> -	irqflags = ipipe_fault_entry();
> >>>>> -
> >>>>>  	if (__ipipe_report_trap(IPIPE_TRAP_UNKNOWN, regs))
> >>>>> -		goto out;
> >>>>> +		return;
> >>>>> +
> >>>>> +	irqflags = ipipe_fault_entry();
> >>>>>  
> >>>>>  	printk(KERN_ALERT "Unhandled fault: %s (0x%03x) at 0x%08lx\n",
> >>>>>  		inf->name, fsr, addr);
> >>>>> @@ -642,7 +642,7 @@ do_DataAbort(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> >>>>>  	info.si_code  = inf->code;
> >>>>>  	info.si_addr  = (void __user *)addr;
> >>>>>  	arm_notify_die("", regs, &info, fsr, 0);
> >>>>> -out:
> >>>>> +
> >>>>>  	ipipe_fault_exit(irqflags);
> >>>>>  }
> >>>>>  
> >>>>> @@ -669,10 +669,10 @@ do_PrefetchAbort(unsigned long addr, unsigned int ifsr, struct pt_regs *regs)
> >>>>>  	if (!inf->fn(addr, ifsr | FSR_LNX_PF, regs))
> >>>>>  		return;
> >>>>>  
> >>>>> -	irqflags = ipipe_fault_entry();
> >>>>> -
> >>>>>  	if (__ipipe_report_trap(IPIPE_TRAP_UNKNOWN, regs))
> >>>>> -		goto out;
> >>>>> +		return;
> >>>>> +
> >>>>> +	irqflags = ipipe_fault_entry();
> >>>>>  
> >>>>>  	printk(KERN_ALERT "Unhandled prefetch abort: %s (0x%03x) at 0x%08lx\n",
> >>>>>  		inf->name, ifsr, addr);
> >>>>> @@ -682,7 +682,7 @@ do_PrefetchAbort(unsigned long addr, unsigned int ifsr, struct pt_regs *regs)
> >>>>>  	info.si_code  = inf->code;
> >>>>>  	info.si_addr  = (void __user *)addr;
> >>>>>  	arm_notify_die("", regs, &info, ifsr, 0);
> >>>>> -out:
> >>>>> +
> >>>>>  	ipipe_fault_exit(irqflags);
> >>>>>  }
> >>>>>  
> >>>>>
> >>>>> This seems more consistent - if not more correct - as it now does the
> >>>>> reporting with hard irqs off, like in the other cases.
> >>>>>
> >>>>
> >>>> Ack, definitely. The pattern is to cause any migration first if need be,
> >>>> _then_ flip the virtual IRQ state, so that ipipe_fault_restore() always
> >>>> reinstates the interrupt state in effect after the caller has migrated
> >>>> to the root domain.
> >>>
> >>> Is it even useful ? After a relax, the state of the root thread
> >>> stall bit and irq flags are well known...
> >>
> >> We still need to disable IRQs for root. HW IRQs are likely already on,
> >> right?
> >>
> >> And, again, we should refrain from restoring any root irq state on
> >> return - it belongs to Linux (once we migrated and synchronized the state).
> > 
> > The ipipe_fault_exit in my tree is:
> > 
> > static inline void ipipe_fault_exit(unsigned long x)
> > {
> > 	if (!arch_demangle_irq_bits(&x))
> > 		local_irq_enable();
> > 	else
> > 		hard_local_irq_restore(x);
> > }
> > 
> > And I must say I am not sure I understand how it works. To me it
> > seems:
> 
> It mangles both the real and virtual states in one word.
> 
> > hard_local_irq_disable() should always be called in case entry.S
> > expects us to return as we entered: with hw irqs off
> 
> Which is what ipipe_fault_exit() does by testing the mangled state. If
> the fault entered with virtual IRQs on, then you must exit with both the
> stall bit and CPSR_I bit cleared.

Absolutely not. Imagine a Linux task, with root unstalled
experiencing a fault. entry.S is entered root is still unstalled,
with hardware irqs off. On fault entry, we must reflect this
hardware irq state on the stall bit and enable hw irqs. Then when
the fault is handled, undo that, unstall the root stage, disable hw
irqs and return to entry.S, so that it may resume the execution of
the Linux task. If it returns quickly to user-space, a stalled root
at this point would be a disaster, because nothing, certainly not
entry.S will unstall the root stage.

Then there is the case where Linux re-enables irqs in the course of
the handler, in that case, we should not return to entry.S with hw
irqs off.

-- 
					    Gilles.