linux-kernel - Re: [PATCH] kunit: added lockdep support

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Date:   Tue, 11 Aug 2020 17:22:26 -0500
From:   Uriel Guajardo <urielguajardo@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Uriel Guajardo <urielguajardojr@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>, 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] kunit: added lockdep support

On Tue, Aug 11, 2020 at 2:05 PM <peterz@...radead.org> wrote:
>
> On Tue, Aug 11, 2020 at 12:03:51PM -0500, Uriel Guajardo wrote:
> > On Mon, Aug 10, 2020 at 4:43 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Mon, Aug 10, 2020 at 09:32:57PM +0000, Uriel Guajardo wrote:
> > > > +static inline void kunit_check_locking_bugs(struct kunit *test,
> > > > +                                         unsigned long 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 IS_ENABLED(CONFIG_LOCKDEP)
> > > > +     local_irq_disable();
> > > > +     if (!debug_locks) {
> > > > +             kunit_set_failure(test);
> > > > +             lockdep_reset();
> > > > +     }
> > > > +     local_irq_enable();
> > > > +#endif
> > > > +}
> > >
> > > Unless you can guarantee this runs before SMP brinup, that
> > > lockdep_reset() is terminally broken.
> >
> > Good point. KUnit is initialized after SMP is set up, and KUnit can
> > also be built as a module, so it's not a guarantee that we can make.
>
> Even if you could, there's still the question of wether throwing out all
> the dependencies learned during boot is a sensible idea.
>
> > Is there any other way to turn lockdep back on after we detect a
> > failure? It would be ideal if lockdep could still run in the next test
> > case after a failure in a previous one.
>
> Not really; the moment lockdep reports a failure it turns off all
> tracking and we instantly loose state.
>
> You'd have to:
>
>  - delete the 'mistaken' dependency from the graph such that we loose
>    the cycle, otherwise it will continue to find and report the cycle.
>
>  - put every task through a known empty state which turns the tracking
>    back on.
>
> Bart implemented most of what you need for the first item last year or
> so, but the remaining bit and the second item would still be a fair
> amount of work.
>
> Also, I'm really not sure it's worth it, the kernel should be free of
> lock cycles, so just fix one, reboot and continue.
>
> > I suppose we could only display the first failure that occurs, similar
> > to how lockdep does it. But it could also be useful to developers if
> > they saw failures in subsequent test cases, with the knowledge that
> > those failures may be unreliable.
>
> People already struggle with lockdep reports enough; I really don't want
> to given them dodgy report to worry about.

Ah, ok! Fair enough, thanks for the info. Although resetting lockdep
would be nice to have in the future, I think it's enough to only
report the first failure and warn the user that further test cases
will have lockdep disabled. People can then fix the issue and then
re-run it. I'll follow up with a patch that does this.