From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <54EB8CC3.5050005@xenomai.org> Date: Mon, 23 Feb 2015 21:25:39 +0100 From: Philippe Gerum MIME-Version: 1.0 References: <54E77A52.4010806@siemens.com> <54E78EB8.4060204@xenomai.org> <54E78F62.9040505@xenomai.org> <54E79086.8030801@xenomai.org> <54EB5021.3030508@siemens.com> <54EB5638.3050805@xenomai.org> <20150223163743.GA22377@hermes.click-hack.org> <54EB5A45.9000002@siemens.com> <20150223165549.GC22377@hermes.click-hack.org> <54EB5D02.8080105@xenomai.org> <20150223171250.GE22377@hermes.click-hack.org> In-Reply-To: <20150223171250.GE22377@hermes.click-hack.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] ipipe: issues with ARM exception handling List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Jan Kiszka , Xenomai On 02/23/2015 06:12 PM, Gilles Chanteperdrix wrote: > 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. > How is a user-space task supposed to enter a fault with the stall bit set? -- Philippe.