linux-kernel - Re: [PATCH v4 7/8] lockdep: Change hardirq{s_enabled,

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Message-ID: <20200623161320.GA2996373@debian-buster-darwi.lab.linutronix.de>
Date:   Tue, 23 Jun 2020 18:13:21 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, will@...nel.org, tglx@...utronix.de,
        x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        bigeasy@...utronix.de, davem@...emloft.net,
        sparclinux@...r.kernel.org, mpe@...erman.id.au,
        linuxppc-dev@...ts.ozlabs.org, heiko.carstens@...ibm.com,
        linux-s390@...r.kernel.org, linux@...linux.org.uk
Subject: Re: [PATCH v4 7/8] lockdep: Change hardirq{s_enabled,_context} to
 per-cpu variables

On Tue, Jun 23, 2020 at 05:24:50PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 05:00:31PM +0200, Ahmed S. Darwish wrote:
> > On Tue, Jun 23, 2020 at 10:36:52AM +0200, Peter Zijlstra wrote:
> > ...
> > > -#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)
> > >
> >
> > Can we add a small comment on top of lockdep_off(), stating that lockdep
> > IRQ tracking will still be kept after a lockdep_off call?
>
> That would only legitimize lockdep_off(). The only comment I want to put
> on that is: "if you use this, you're doing it wrong'.
>

Well, freshly merged code is using it. For example, KCSAN:

    => f1bc96210c6a ("kcsan: Make KCSAN compatible with lockdep")
    => kernel/kcsan/report.c:

    void kcsan_report(...)
    {
	...
        /*
         * With TRACE_IRQFLAGS, lockdep's IRQ trace state becomes corrupted if
         * we do not turn off lockdep here; this could happen due to recursion
         * into lockdep via KCSAN if we detect a race in utilities used by
         * lockdep.
         */
        lockdep_off();
	...
    }

thanks,

--
Ahmed S. Darwish
Linutronix GmbH