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Date: Mon, 26 Sep 2016 16:32:26 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andrey Konovalov <andreyknvl@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
        Nicolai Stange <nicstange@gmail.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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        quentin.casasnovas@oracle.com, ryabinin.a.a@gmail.com,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] kcov: properly check if we are in an interrupt
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On Fri, 23 Sep 2016 16:51:13 +0200 Andrey Konovalov <andreyknvl@google.com> wrote:

> in_interrupt() returns a nonzero value when we are either in an
> interrupt or have bh disabled via local_bh_disable(). Since we are
> interested in only ignoring coverage from actual interrupts, do a
> proper check of whether we are really in an interrupt.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> It would look totally better to reuse in_irq(), in_serving_softirq() and
> in_nmi() instead of checking flags manually, but that leads to slower
> generated code (three separate tests for each of the flags). Would it be
> better to add another macro to preempt.h that would check if we're actually
> in interrupt and use it?

Yes please.  Is there anywhere else where such a macro can be used?

> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -54,7 +54,8 @@ void notrace __sanitizer_cov_trace_pc(void)
>  	 * We are interested in code coverage as a function of a syscall inputs,
>  	 * so we ignore code executed in interrupts.
>  	 */
> -	if (!t || in_interrupt())
> +	if (!t || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
> +							| NMI_MASK)))

Or include a prominent and very apologetic comment here explaining why
it is open-coded.  But I do agree that not open-coding it is better.