linux-kernel - Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*

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Date:   Tue, 12 May 2020 21:09:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE
 variants

On Tue, May 12, 2020 at 08:38:39PM +0200, Marco Elver wrote:
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 741c93c62ecf..e902ca5de811 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -224,13 +224,16 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   * atomicity or dependency ordering guarantees. Note that this may result
>   * in tears!
>   */
> -#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
> +#define __READ_ONCE(x)							\
> +({									\
> +	kcsan_check_atomic_read(&(x), sizeof(x));			\
> +	data_race((*(const volatile __unqual_scalar_typeof(x) *)&(x))); \
> +})

NAK

This will actively insert instrumentation into __READ_ONCE() and I need
it to not have any.