On 2015-02-20 19:59, Gilles Chanteperdrix wrote: > On Fri, Feb 20, 2015 at 07:57:51PM +0100, Jan Kiszka wrote: >> On 2015-02-20 19:53, Gilles Chanteperdrix wrote: >>> On Fri, Feb 20, 2015 at 07:51:19PM +0100, Jan Kiszka wrote: >>>> On 2015-02-20 19:38, Gilles Chanteperdrix wrote: >>>>> On Fri, Feb 20, 2015 at 07:03:14PM +0100, 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. >>>>>> - 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. >>>>>> >>>>>> Room for optimization: >>>>>> - ipipe_fault_entry is always called with hard IRQs off from >>>>>> do_page_fault and do_translation_fault. I suspect this applies to the >>>>>> remaining callers (do_DataAbort and do_PrefetchAbort ) as well. Thus >>>>>> the hard IRQ state is actually known at compile time, right? >>>> >>>> To follow up on this: do_DataAbort and do_PrefetchAbort are always >>>> invoked with hard IRQs disable when a regular exception takes us there. >>>> Only the ghost syscall cmpxchg simulates do_DataAbort without adjusting >>>> hardware interrupt. It's probably easier to adjust that than to account >>>> for hw irqs being potentially on an fault entry. >>>> >>>>>> >>>>>> I can hack up patches, but I'd like to confirm first that I'm not >>>>>> missing anything subtle or ARM-specific here. >>>>> >>>>> Just to explain the original hack. >>>>> >>>>> Some time ago, the faults handlers were executed irqs on ARM. The >>>>> irqs were enabled in entry.S before executing the handlers. >>>>> >>>>> At some point, this was removed in entry.S and fault handlers >>>>> started to be executed irqs off. On ARM, all faults relax to be >>>>> handled in secondary mode, actually there is an exception, the FPU, >>>>> but it goes through a completely different path which had always >>>>> been executed irqs off until recently where the irqs are reenabled >>>>> when accessing user-space to be able to handle faults without >>>>> lockups. >>>>> >>>>> My concern was that the code thus executed could have assertion >>>>> about the root domain being stalled which would be fail, so I added >>>>> code which stalled root and enabled hardware irqs on fault entry and >>>>> unstalled root and disabled hardware irqs on fault exit (which >>>>> always happen on root domain). This should have worked even if a fault >>>>> had happened to be handled in head domain, because then the >>>>> operation would have been a nop (simply stall/then unstall). >>>>> >>>>> But Philippe found this dumb approach to fail when working on LPAE, >>>>> IIRC. IIRC, namely, if the root domain happens to be stalled when >>>>> entering a fault over head domain, it would end up unstalled after >>>>> the operation. So, I believe the code he added saves the stall state >>>>> on fault entry and restores it on fault exit. I have checked >>>>> Philippe's code details at the time and did not find anything wrong. >>>> >>>> I suspect the LPAE scenario takes the do_page_fault path? Then it should >>>> rather be solved by providing the right information to or preventing >>>> the execution of >>>> >>>> /* Enable interrupts if they were enabled in the parent context. */ >>>> if (interrupts_enabled(regs)) >>>> local_irq_enable(); >>>> >>>> Now we unconditionally restore to the root state on entry, overwriting >>>> what may happen to it during the handler execution - specifically via >>>> the snippet above. >>> >>> This code is part of the mainline kernel. >> >> Correct. But we can adjust it to take interrupt virtualization and >> domain migration into account. > > I do not think we should. Everything should appear to the kernel as > if interrupts are NOT virtualized. So, local_irq_enable() should in > fact do an unstall root, and everything around should be made so > that it works.
We are on the same page. I guess I just didn't get what you wanted to state with the previous statement. My point is: we have to ensure that interrupts_enabled(regs) decides about enabling irqs for root based on the correct information. And we must not overwrite this decision afterward. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux _______________________________________________ Xenomai mailing list [email protected] http://www.xenomai.org/mailman/listinfo/xenomai
