Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

Marco Elver
2020-08-07T12:08:30Z

On Fri, 7 Aug 2020 at 14:04, Jürgen Groß <jgross@suse.com> wrote:
>
> On 07.08.20 13:38, Marco Elver wrote:
> > On Fri, Aug 07, 2020 at 12:35PM +0200, Jürgen Groß wrote:
> >> On 07.08.20 11:50, Marco Elver wrote:
> >>> On Fri, Aug 07, 2020 at 11:24AM +0200, Jürgen Groß wrote:
> >>>> On 07.08.20 11:01, Marco Elver wrote:
> >>>>> On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver@google.com> wrote:
> >>>>>> On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver@google.com> wrote:
> >>>>>>> On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@infradead.org wrote:
> >>>>>>>> On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:
> >>>>>>>>> Testing my hypothesis that raw then nested non-raw
> >>>>>>>>> local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
> >>>>>>>>> below. This is at least 1 case I can think of that we're bound to hit.
> >>>>>>> ...
> >>>>>>>>
> >>>>>>>> /me goes ponder things...
> >>>>>>>>
> >>>>>>>> How's something like this then?
> >>>>>>>>
> >>>>>>>> ---
> >>>>>>>> include/linux/sched.h | 3 ---
> >>>>>>>> kernel/kcsan/core.c | 62 ++++++++++++++++++++++++++++++++++++---------------
> >>>>>>>> 2 files changed, 44 insertions(+), 21 deletions(-)
> >>>>>>>
> >>>>>>> Thank you! That approach seems to pass syzbot (also with
> >>>>>>> CONFIG_PARAVIRT) and kcsan-test tests.
> >>>>>>>
> >>>>>>> I had to modify it some, so that report.c's use of the restore logic
> >>>>>>> works and not mess up the IRQ trace printed on KCSAN reports (with
> >>>>>>> CONFIG_KCSAN_VERBOSE).
> >>>>>>>
> >>>>>>> I still need to fully convince myself all is well now and we don't end
> >>>>>>> up with more fixes. :-) If it passes further testing, I'll send it as a
> >>>>>>> real patch (I want to add you as Co-developed-by, but would need your
> >>>>>>> Signed-off-by for the code you pasted, I think.)
> >>>>>
> >>>>> I let it run on syzbot through the night, and it's fine without
> >>>>> PARAVIRT (see below). I have sent the patch (need your Signed-off-by
> >>>>> as it's based on your code, thank you!):
> >>>>> https://lkml.kernel.org/r/20200807090031.3506555-1-elver@google.com
> >>>>>
> >>>>>> With CONFIG_PARAVIRT=y (without the notrace->noinstr patch), I still
> >>>>>> get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
> >>>>>> it takes longer for syzbot to hit them. But I think that's expected
> >>>>>> because we can still get the recursion that I pointed out, and will
> >>>>>> need that patch.
> >>>>>
> >>>>> Never mind, I get these warnings even if I don't turn on KCSAN
> >>>>> (CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
> >>>>> throws off IRQ state tracking. :-/
> >>>>
> >>>> What are the settings of CONFIG_PARAVIRT_XXL and
> >>>> CONFIG_PARAVIRT_SPINLOCKS in this case?
> >>>
> >>> I attached a config.
> >>>
> >>> $> grep PARAVIRT .config
> >>> CONFIG_PARAVIRT=y
> >>> CONFIG_PARAVIRT_XXL=y
> >>> # CONFIG_PARAVIRT_DEBUG is not set
> >>> CONFIG_PARAVIRT_SPINLOCKS=y
> >>> # CONFIG_PARAVIRT_TIME_ACCOUNTING is not set
> >>> CONFIG_PARAVIRT_CLOCK=y
> >>
> >> Anything special I need to do to reproduce the problem? Or would you be
> >> willing to do some more rounds with different config settings?
> >
> > I can only test it with syzkaller, but that probably doesn't help if you
> > don't already have it set up. It can't seem to find a C reproducer.
> >
> > I did some more rounds with different configs.
> >
> >> I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
> >> sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.
> >
> > Yes, PARAVIRT_XXL doesn't make a different. When disabling
> > PARAVIRT_SPINLOCKS, however, the warnings go away.
>
> Thanks for testing!
>
> I take it you are doing the tests in a KVM guest?

Yes, correct.

> If so I have a gut feeling that the use of local_irq_save() and
> local_irq_restore() in kvm_wait() might be fishy. I might be completely
> wrong here, though.

Happy to help debug more, although I might need patches or pointers
what to play with.

> BTW, I think Xen's variant of pv spinlocks is fine (no playing with IRQ
> on/off).
>
> Hyper-V seems to do the same as KVM, and kicking another vcpu could be
> problematic as well, as it is just using IPI.
>
>
> Juergen