Re: [PATCH v1 07/25] lockdep: Add preemption disabled assertion API

Ahmed S. Darwish
2020-05-26T09:46:03Z

On Tue, May 26, 2020 at 10:13:50AM +0200, Peter Zijlstra wrote:
> On Tue, May 26, 2020 at 02:52:31AM +0200, Ahmed S. Darwish wrote:
> > Peter Zijlstra <peterz@infradead.org> wrote:
>
> > > +#define lockdep_assert_irqs_enabled() \
> > > +do { \
> > > + WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirqs_enabled)); \
> > > +} while (0)
> > >
> >
> > Given that lockdep_off() is defined at lockdep.c as:
> >
> > void lockdep_off(void)
> > {
> > current->lockdep_recursion += LOCKDEP_OFF;
> > }
> >
> > This would imply that all of the macros:
> >
> > - lockdep_assert_irqs_enabled()
> > - lockdep_assert_irqs_disabled()
> > - lockdep_assert_in_irq()
> > - lockdep_assert_preemption_disabled()
> > - lockdep_assert_preemption_enabled()
> >
> > will do the lockdep checks *even if* lockdep_off() was called.
> >
> > This doesn't sound right. Even if all of the above macros call sites
> > didn't care about lockdep_off()/on(), it is semantically incoherent.
>
> lockdep_off() is an abomination and really should not exist.
>
> That dm-cache-target.c thing, for example, is atrocious shite that will
> explode on -rt. Whoever wrote that needs a 'medal'.
>
> People using it deserve all the pain they get.
>
> Also; IRQ state _should_ be tracked irrespective of tracking lock
> dependencies -- I see that that currently isn't entirely the case, lemme
> go fix that.
>

Exactly, currently all the lockdep IRQ checks gets nullified if
lockdep_off() is called. That was the source of my confusion.

If you'll have any extra patches on this, I can also queue them in the
next iteration of this series, before this patch.

Thanks a lot,

--
Ahmed S. Darwish
Linutronix GmbH