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From:   Uriel Guajardo <urielguajardo@google.com>
Date:   Mon, 17 Aug 2020 16:00:59 -0500
Message-ID: <CAG30EecDS+yaW0k62Wu2sHoe2+amTe-=U4NAVHJcCfD2US+Enw@mail.gmail.com>
Subject: Re: [PATCH v3] kunit: added lockdep support
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Uriel Guajardo <urielguajardojr@gmail.com>,
        Brendan Higgins <brendanhiggins@google.com>, mingo@redhat.com,
        will@kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        KUnit Development <kunit-dev@googlegroups.com>,
        Alan Maguire <alan.maguire@oracle.com>
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On Sat, Aug 15, 2020 at 4:17 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Sat, Aug 15, 2020 at 10:30:29AM +0200, Ingo Molnar wrote:
> > >
> > > * Uriel Guajardo <urielguajardojr@gmail.com> wrote:
> > >
> > > > From: Uriel Guajardo <urielguajardo@google.com>
> > > >
> > > > 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.
> > >
> > > > --- a/lib/kunit/Makefile
> > > > +++ b/lib/kunit/Makefile
> > >
> > > > +void kunit_check_lockdep(struct kunit *test, struct kunit_lockdep *lockdep) {
> > > > + int saved_preempt_count = lockdep->preempt_count;
> > > > + bool saved_debug_locks = lockdep->debug_locks;
> > > > +
> > > > + if (DEBUG_LOCKS_WARN_ON(preempt_count() != saved_preempt_count))
> > > > +         preempt_count_set(saved_preempt_count);
> > > > +
> > > > +#ifdef CONFIG_TRACE_IRQFLAGS
> > > > + if (softirq_count())
> > > > +         current->softirqs_enabled = 0;
> > > > + else
> > > > +         current->softirqs_enabled = 1;
> > > > +#endif
> > > > +
> > > > + 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.");
> > > > + }
> > >
> > >
> > > So this basically duplicates what the boot-time locking self-tests do,
> > > in a poor fashion?
> >
> > No, it makes sure that any kunit based self-test fails when it messes up
> > it's locking.
>
> We have a flag for whether lockdep is running though, so is this
> basically a very complicated way to parse /proc/lockdep_debug? :-)
>

I may be missing something here, but what would be the advantage of
using another flag or using other means to find lockdep's status?

This patch is basically checking if debug_locks has changed after a
KUnit test case has executed. It's not sufficient to only check if
debug_locks is off, since it could have already been off many test
cases ago.

I imagine the only difference would be replacing "debug_locks" with
another flag or code checking lockdep's status, and I don't see that
as being any less complicated.