From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 23 Feb 2015 18:02:51 +0100 From: Gilles Chanteperdrix Message-ID: <20150223170251.GD22377@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> <20150223163743.GA22377@hermes.click-hack.org> <54EB5A45.9000002@siemens.com> <20150223165549.GC22377@hermes.click-hack.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150223165549.GC22377@hermes.click-hack.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: Jan Kiszka Cc: Xenomai On Mon, Feb 23, 2015 at 05:55:49PM +0100, 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: > hard_local_irq_disable() should always be called in case entry.S > expects us to return as we entered: with hw irqs off Well, unless linux called local_irq_enable(). So, in fact the hw irqs state should be modeled after the current state of the stall bit, it should not depend on the flags upon entry. -- Gilles.