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Date:   Thu, 13 Aug 2020 11:44:03 -0500
From:   Uriel Guajardo <urielguajardo@...gle.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Uriel Guajardo <urielguajardojr@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        will@...nel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, kunit-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kunit: added lockdep support

On Thu, Aug 13, 2020 at 4:11 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> On Wed, 12 Aug 2020, Uriel Guajardo wrote:
>
> > KUnit will fail tests upon observing a lockdep failure. Because lockdep
> > turns itself off after its first failure, only fail the first test and
> > warn users to not expect any future failures from lockdep.
> >
> > Similar to lib/locking-selftest [1], we check if the status of
> > debug_locks has changed after the execution of a test case. However, we
> > do not reset lockdep afterwards.
> >
> > Like the locking selftests, we also fix possible preemption count
> > corruption from lock bugs.
> >
> > Depends on kunit: support failure from dynamic analysis tools [2]
> >
> > [1] https://elixir.bootlin.com/linux/v5.7.12/source/lib/locking-selftest.c#L1137
> >
> > [2] https://lore.kernel.org/linux-kselftest/20200806174326.3577537-1-urielguajardojr@gmail.com/
> >
> > Signed-off-by: Uriel Guajardo <urielguajardo@...gle.com>
> > ---
> > v2 Changes:
> > - Removed lockdep_reset
> >
> > - Added warning to users about lockdep shutting off
> > ---
> >  lib/kunit/test.c | 27 ++++++++++++++++++++++++++-
> >  1 file changed, 26 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/kunit/test.c b/lib/kunit/test.c
> > index d8189d827368..7e477482457b 100644
> > --- a/lib/kunit/test.c
> > +++ b/lib/kunit/test.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/kref.h>
> >  #include <linux/sched/debug.h>
> >  #include <linux/sched.h>
> > +#include <linux/debug_locks.h>
> >
> >  #include "debugfs.h"
> >  #include "string-stream.h"
> > @@ -22,6 +23,26 @@ void kunit_fail_current_test(void)
> >               kunit_set_failure(current->kunit_test);
> >  }
> >
> > +static void kunit_check_locking_bugs(struct kunit *test,
> > +                                  unsigned long saved_preempt_count,
> > +                                  bool saved_debug_locks)
> > +{
> > +     preempt_count_set(saved_preempt_count);
> > +#ifdef CONFIG_TRACE_IRQFLAGS
> > +     if (softirq_count())
> > +             current->softirqs_enabled = 0;
> > +     else
> > +             current->softirqs_enabled = 1;
> > +#endif
> > +#if IS_ENABLED(CONFIG_LOCKDEP)
> > +     if (saved_debug_locks && !debug_locks) {
> > +             kunit_set_failure(test);
> > +             kunit_warn(test, "Dynamic analysis tool failure from LOCKDEP.");
> > +             kunit_warn(test, "Further tests will have LOCKDEP disabled.");
> > +     }
> > +#endif
> > +}
>
> Nit: I could be wrong but the general approach for this sort of
> feature is to do conditional compilation combined with "static inline"
> definitions to handle the case where the feature isn't enabled.
> Could we tidy this up a bit and haul this stuff out into a
> conditionally-compiled (if CONFIG_LOCKDEP) kunit lockdep.c file?

Sure! Apologies if this isn't convention.

> Then in kunit's lockdep.h we'd have
>
> struct kunit_lockdep {
>         int preempt_count;
>         bool debug_locks;
> };
>
> #if IS_ENABLED(CONFIG_LOCKDEP)
> void kunit_test_init_lockdep(struct kunit_test *test, struct
>                              kunit_lockdep *lockdep);
> void kunit_test_check_lockdep(struct kunit_test *test,
>                               struct kunit_lockdep *lockdep);
> #else
> static inline void kunit_init_lockdep(struct kunit_test *test,
>                                       struct kunit_lockdep *lockdep) { }
> static inline void kunit_check_lockdep(struct kunit_test *test,
>                                        struct kunit_lockdep *lockdep) { }
> #endif
>
>
> The test execution code could then call
>
>         struct kunit_lockdep lockdep;
>
>         kunit_test_init_lockdep(test, &lockdep);
>
>         kunit_test_check_lockdep(test, &lockdep);
>

Thanks for these helpful tips. I agree that it'll be cleaner this way.
I'll implement this in the next version of the patch.

> If that approach makes sense, we could go a bit further
> and we might benefit from a bit more generalization
> here.  _If_ the pattern of needing pre- and post- test
> actions is sustained across multiple analysis tools,
> could we add generic hooks for this? That would allow any
> additional dynamic analysis tools to utilize them.  So

I think this is a great idea. Right now I'm a little hesitant to
generalize beyond lockdep, since most analysis tools I've seen don't
seem to require this. For most tools, they fail, they report to KUnit,
then they continue working without us needing to clean state. Perhaps
the generic hooks could prove useful in other ways that I'm not
considering..

In any case, I will go ahead and work on the lockdep-specific hook for
KUnit. If you or anyone else thinks it could be useful in other ways
in the future, we can make it generic!

> kunit_try_run_case() would then cycle through the registered
> pre- hooks prior to running the case and post- hooks after,
> failing if any of the latter returned a failure value.
>
> I'm thinking something like
>
>   kunit_register_external_test("lockdep", lockdep_pre, lockdep_post,
>                                &kunit_lockdep);
>
> (or we could define a kunit_external_test struct for
> better extensibility).
>
> A void * would be passed to pre/post, in this case it'd
> be a pointer to a struct containing the saved preempt
> count/debug locks, and the registration could be called during
> kunit initialization.  This doesn't need to be done with your
> change of course but I wanted to float the idea as in addition
> to uncluttering the test case execution code, it might allow
> us to build facilities on top of that generic tool support for
> situations like "I'd like to see if the test passes absent
> any lockdep issues, so I'd like to disable lockdep-based failure".
> Such situations are more likely to arise in a world where
> kunit+tests are built as modules and run multiple times within
> a single system boot admittedly, but worth considering I think.

Interesting!

>
> For that we'd need a way to select which dynamic tools kunit
> enables(kernel/module parameters or debugfs could do
> this), but a generic approach might help that sort of thing.
>
> An external test under this model wouldn't have to necessarily
> be external to the area under test; the general criteria for
> such things would be "something I want to track across multiple
> test case execution".
>
> Again I'm not trying to put you on the hook for any of
> the above suggestions (having lockdep support like this is
> fantastic!), but I think it'd be good to see if there's a
> pattern here we could potentially exploit in other use cases.

No worries, thanks for putting these suggestions out there.

>
> Thanks!
>
> Alan

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