linux-kernel - Re: [PATCH v2] kcov: load acquire coverage count in user-space code

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Message-ID: <CANpmjNNvsJ_u7ky+d1tiXtwc-T3z6VB4SiMqpo6aKWBBFO3ERA@mail.gmail.com>
Date: Mon, 4 Aug 2025 08:00:00 +0200
From: Marco Elver <elver@...gle.com>
To: Soham Bagchi <soham.bagchi@...h.edu>
Cc: akpm@...ux-foundation.org, andreyknvl@...il.com, arnd@...db.de, 
	corbet@....net, dvyukov@...gle.com, glider@...gle.com, 
	kasan-dev@...glegroups.com, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, sohambagchi@...look.com, tglx@...utronix.de, 
	workflows@...r.kernel.org
Subject: Re: [PATCH v2] kcov: load acquire coverage count in user-space code

On Sun, 3 Aug 2025 at 20:06, Soham Bagchi <soham.bagchi@...h.edu> wrote:
>
> Updating the KCOV documentation to use a load-acquire
> operation for the first element of the shared memory
> buffer between kernel-space and user-space.
>
> The load-acquire pairs with the write memory barrier
> used in kcov_move_area()
>
> Signed-off-by: Soham Bagchi <soham.bagchi@...h.edu>

Reviewed-by: Marco Elver <elver@...gle.com>

> ---
>
> Changes in v2:

Btw, it is customary to send out the whole patch series on a version
bump, even if only one of the patches changed.
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#explicit-in-reply-to-headers

> - note for load-acquire shifted to block comment
>   in code rather than in the preceding paragraphs
> ---
>  Documentation/dev-tools/kcov.rst | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 6611434e2dd..40a4b500073 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -361,7 +361,12 @@ local tasks spawned by the process and the global task that handles USB bus #1:
>          */
>         sleep(2);
>
> -       n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> +        /*
> +         * The load to the coverage count should be an acquire to pair with
> +         * pair with the corresponding write memory barrier (smp_wmb()) on
> +         * the kernel-side in kcov_move_area().
> +         */
> +       n = __atomic_load_n(&cover[0], __ATOMIC_ACQUIRE);
>         for (i = 0; i < n; i++)
>                 printf("0x%lx\n", cover[i + 1]);
>         if (ioctl(fd, KCOV_DISABLE, 0))
> --
> 2.34.1
>