Philippe Gerum wrote: > On Sat, 2010-01-23 at 11:33 +0100, Jan Kiszka wrote: >> Philippe Gerum wrote: >>> On Sat, 2010-01-23 at 11:09 +0100, Jan Kiszka wrote: >>>> Philippe Gerum wrote: >>>>> On Fri, 2010-01-22 at 19:08 +0100, Philippe Gerum wrote: >>>>>> On Fri, 2010-01-22 at 19:03 +0100, Jan Kiszka wrote: >>>>>>> Philippe Gerum wrote: >>>>>>>> On Fri, 2010-01-22 at 18:41 +0100, Jan Kiszka wrote: >>>>>>>>> Gilles Chanteperdrix wrote: >>>>>>>>>> Philippe Gerum wrote: >>>>>>>>>>> On Fri, 2010-01-22 at 17:58 +0100, Jan Kiszka wrote: >>>>>>>>>>>> Hi guys, >>>>>>>>>>>> >>>>>>>>>>>> we are currently trying to catch an ugly Linux pipeline state >>>>>>>>>>>> corruption >>>>>>>>>>>> on x86-64. >>>>>>>>>>>> >>>>>>>>>>>> Conceptual question: If a Xenomai task causes a fault, we enter >>>>>>>>>>>> ipipe_trap_notify over the primary domain and leave it over the >>>>>>>>>>>> root >>>>>>>>>>>> domain, right? Now, if the root domain happened to be stalled when >>>>>>>>>>>> the >>>>>>>>>>>> exception happened, where should it normally be unstalled again, >>>>>>>>>>>> *for_that_task*? Our problem is that we generate a code path where >>>>>>>>>>>> this >>>>>>>>>>>> does not happen. >>>>>>>>>>> xnhadow_relax -> ipipe_reenter_root -> finish_task_switch -> >>>>>>>>>>> finish_lock_switch -> unstall >>>>>>>>>>> >>>>>>>>>>> Since xnshadow_relax is called on behalf the event dispatcher, we >>>>>>>>>>> should >>>>>>>>>>> expect it to return with the root domain unstalled after a domain >>>>>>>>>>> downgrade, from primary to root. >>>>>>>>>> Ok, but what about local_irq_restore_nosync at the end of the >>>>>>>>>> function ? >>>>>>>>>> >>>>>>>>> That is, IMO, our problem: It replays the root state on fault entry, >>>>>>>>> but >>>>>>>>> that one is totally unrelated to the (Xenomai) task that caused the >>>>>>>>> fault. >>>>>>>> The code seems fishy. Try restoring only when the incoming domain was >>>>>>>> the root one. Indeed. >>>>>>>> >>>>>>> Something like this? >>>>>>> >>>>>>> diff --git a/arch/x86/kernel/ipipe.c b/arch/x86/kernel/ipipe.c >>>>>>> index 4442d96..0558ea3 100644 >>>>>>> --- a/arch/x86/kernel/ipipe.c >>>>>>> +++ b/arch/x86/kernel/ipipe.c >>>>>>> @@ -702,19 +702,21 @@ static int __ipipe_xlate_signo[] = { >>>>>>> >>>>>>> int __ipipe_handle_exception(struct pt_regs *regs, long error_code, >>>>>>> int vector) >>>>>>> { >>>>>>> + bool restore_flags = false; >>>>>>> unsigned long flags; >>>>>>> >>>>>>> - /* Pick up the root domain state of the interrupted context. */ >>>>>>> - local_save_flags(flags); >>>>>>> + if (ipipe_root_domain_p && irqs_disabled_hw()) { >>>>>>> + /* Pick up the root domain state of the interrupted >>>>>>> context. */ >>>>>>> + local_save_flags(flags); >>>>>>> >>>>>>> - if (ipipe_root_domain_p) { >>>>>>> /* >>>>>>> * Replicate hw interrupt state into the virtual mask >>>>>>> before >>>>>>> * calling the I-pipe event handler over the root >>>>>>> domain. Also >>>>>>> * required later when calling the Linux exception >>>>>>> handler. >>>>>>> */ >>>>>>> - if (irqs_disabled_hw()) >>>>>>> - local_irq_disable(); >>>>>>> + local_irq_disable(); >>>>>>> + >>>>>>> + restore_flags = true; >>>>>>> } >>>>>>> #ifdef CONFIG_KGDB >>>>>>> /* catch exception KGDB is interested in over non-root domains >>>>>>> */ >>>>>>> @@ -725,7 +727,8 @@ int __ipipe_handle_exception(struct pt_regs *regs, >>>>>>> long error_code, int vector) >>>>>>> #endif /* CONFIG_KGDB */ >>>>>>> >>>>>>> if (unlikely(ipipe_trap_notify(vector, regs))) { >>>>>>> - local_irq_restore_nosync(flags); >>>>>>> + if (restore_flags) >>>>>>> + local_irq_restore_nosync(flags); >>>>>>> return 1; >>>>>>> } >>>>>>> >>>>>>> @@ -770,7 +773,8 @@ int __ipipe_handle_exception(struct pt_regs *regs, >>>>>>> long error_code, int vector) >>>>>>> * Relevant for 64-bit: Restore root domain state as the >>>>>>> low-level >>>>>>> * return code will not align it to regs.flags. >>>>>>> */ >>>>>>> - local_irq_restore_nosync(flags); >>>>>>> + if (restore_flags) >>>>>>> + local_irq_restore_nosync(flags); >>>>>>> >>>>>>> return 0; >>>>>>> } >>>>>>> >>>>>>> >>>>>>> We are currently not able to test this on the system that triggers it, >>>>>>> but we'll do so tomorrow (yeah...). >>>>>>> >>>>>> Should work. Famous last words. >>>>>> >>>>> Strike that. This won't work, because the fixup code will use the saved >>>>> flags even when root is not the incoming domain and/or hw IRQs are on on >>>>> entry. In short, local_save_flags() must be done unconditionally, as >>>>> previously. >>>> It will accidentally work for 64-bit where __fixup_if is empty. And for >>>> 32-bit, I would say we need to make it depend on restore_flags as well. >>>> >>> AFAIK, Gilles is working on this. We just need to avoid stepping on >>> 32bit toes to fix 64. >>> >> OK. >> >> Just realized that my suggestion would conflict with the comment above >> __fixup_if. So I think we first need to clarify the various scenarios >> again to avoid breaking one while fixing another. >> >> Entry over non-root, exit over non-root: >> - no need to fiddle with the root state >> > > Correct. > >> Entry over root, exit over root, !irqs_disabled_hw(): >> - no need to fiddle with the root state >> - 32 bit: regs fixup required? >> > > The iret emulation via iret_root needs proper fixup to have taken place, > so doing the fixup is mandatory in any case. > >> Entry over root, exit over root, irqs_disabled_hw(): >> - save root state & disable root IRQs on entry >> - 32 bit: replicate saved state into regs.eflags before calling linux >> handler >> - restore saved state on exit > > Correct. > >> Entry over non-root, exit over root: >> - ? >> >> I tend to think that, for the 32-bit cases, we should pick up the flags >> from the root state _after_ returning from ipipe_trap_notify and only if >> we are truly running in the root domain then. That should be the value >> the migration left behind, so the correct one, right? >> > > Yes. > >> Any scenario missing? >> > > Should be ok. But the more I think of it, the more I'm convinced that we > should make the 32 and 64 bit implementation converge to the x86_64 one. > iret_root emulation is required because we virtualize the interrupt > masking in the assembly-written portions for x86_32, which we don't for > x86_64. > > Hyste^Horically, there was a strong requirement to do that with legacy > 2.4 kernels the 32 bit implementation was designed for, because lengthy > masked sections would raise the latency above the top. With current > 2.6/x86 kernels, I don't think virtualizing interrupt masking in the > assembly-written portions makes a lot of sense anymore, since > significant work took place upstream over time, to allow for > fine-grained preemption there.
I'm all for this, but I think it should happen gradually. In step 1, we should fix the current situation. Step 2 would obsolete __fixup_if by moving x86-32 towards the 64-bit scheme. So a discussion of my last patch would be warmly welcomed. Jan
signature.asc
Description: OpenPGP digital signature
_______________________________________________ Xenomai-core mailing list Xenomai-core@gna.org https://mail.gna.org/listinfo/xenomai-core