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

Jürgen Groß
2020-08-07T10:35:36Z

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 think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.

Juergen