Messages in this thread | | | Subject | Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers | From | Jürgen Groß <> | Date | Fri, 7 Aug 2020 14:04:22 +0200 |
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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?
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.
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
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