From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 23 Feb 2015 17:37:43 +0100 From: Gilles Chanteperdrix Message-ID: <20150223163743.GA22377@hermes.click-hack.org> References: <54E776E2.2030501@siemens.com> <54E77A52.4010806@siemens.com> <54E78EB8.4060204@xenomai.org> <54E78F62.9040505@xenomai.org> <54E79086.8030801@xenomai.org> <54EB5021.3030508@siemens.com> <54EB5638.3050805@xenomai.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54EB5638.3050805@xenomai.org> 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: Philippe Gerum Cc: Jan Kiszka , Xenomai 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... -- Gilles.