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

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Date:   Tue, 26 May 2020 02:52:31 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, May 24, 2020 at 12:41:32AM +0200, Peter Zijlstra wrote:
> > On Sat, May 23, 2020 at 04:59:42PM +0200, Sebastian A. Siewior wrote:
> > >
> > > Any "static inline" in the header file using
> > > lockdep_assert_preemption_disabled() will tro to complain about missing
> > > current-> define. But yes, it will work otherwise.
> >
> > Because...? /me rummages around.. Ah you're proposing sticking this in
> > seqcount itself and then header hell.
> >
> > Moo.. ok I'll go have another look on Monday.
>
> How's this?
>

This will work for my case as current-> is no longer referenced by the
lockdep macros. Please continue below though.

...

> -#define lockdep_assert_irqs_enabled()	do {				\
> -		WARN_ONCE(debug_locks && !current->lockdep_recursion &&	\
> -			  !current->hardirqs_enabled,			\
> -			  "IRQs not enabled as expected\n");		\
> -	} while (0)
> +DECLARE_PER_CPU(int, hardirqs_enabled);
> +DECLARE_PER_CPU(int, hardirq_context);
>
> -#define lockdep_assert_irqs_disabled()	do {				\
> -		WARN_ONCE(debug_locks && !current->lockdep_recursion &&	\
> -			  current->hardirqs_enabled,			\
> -			  "IRQs not disabled as expected\n");		\
> -	} while (0)
> +#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.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH