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

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Message-ID: <20200526081350.GI317569@hirez.programming.kicks-ass.net>
Date:   Tue, 26 May 2020 10:13:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 07/25] lockdep: Add preemption disabled assertion API

On Tue, May 26, 2020 at 02:52:31AM +0200, Ahmed S. Darwish wrote:
> Peter Zijlstra <peterz@...radead.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.