linux-kernel - Re: [PATCH] media: vivid: work around high stack usage with clang

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Date:   Thu, 18 Jul 2019 14:23:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Johan Korsnes <johan.korsnes@...il.com>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Gabriel Francisco Mandaji <gfmandaji@...il.com>,
        linux-media@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] media: vivid: work around high stack usage with clang

On Thu, Jul 18, 2019 at 7:16 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> Building a KASAN-enabled kernel with clang ends up in a case where too
> much is inlined into vivid_thread_vid_cap() and the stack usage grows
> a lot, possibly when the register allocation fails to produce efficient
> code and spills a lot of temporaries to the stack. This uses more
> than twice the amount of stack than the sum of the individual functions
> when they are not inlined:
>
> drivers/media/platform/vivid/vivid-kthread-cap.c:766:12: error: stack frame size of 2208 bytes in function 'vivid_thread_vid_cap' [-Werror,-Wframe-larger-than=]
>
> Marking two of the key functions in here as 'noinline_for_stack' avoids
> the pathological case in clang without any apparent downside for gcc.

Thanks for the patch, should be one less warning for CI.
Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> Not sure how much effort we want to put into fixing clang to not
> get into this case. I could open an llvm bug report if something
> thinks this has a chance of getting fixed there.
> ---
>  drivers/media/platform/vivid/vivid-kthread-cap.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/media/platform/vivid/vivid-kthread-cap.c b/drivers/media/platform/vivid/vivid-kthread-cap.c
> index 6cf495a7d5cc..4f94897e6303 100644
> --- a/drivers/media/platform/vivid/vivid-kthread-cap.c
> +++ b/drivers/media/platform/vivid/vivid-kthread-cap.c
> @@ -232,8 +232,8 @@ static void *plane_vaddr(struct tpg_data *tpg, struct vivid_buffer *buf,
>         return vbuf;
>  }
>
> -static int vivid_copy_buffer(struct vivid_dev *dev, unsigned p, u8 *vcapbuf,
> -               struct vivid_buffer *vid_cap_buf)
> +static noinline_for_stack int vivid_copy_buffer(struct vivid_dev *dev, unsigned p,
> +               u8 *vcapbuf, struct vivid_buffer *vid_cap_buf)
>  {
>         bool blank = dev->must_blank[vid_cap_buf->vb.vb2_buf.index];
>         struct tpg_data *tpg = &dev->tpg;
> @@ -670,7 +670,8 @@ static void vivid_cap_update_frame_period(struct vivid_dev *dev)
>         dev->cap_frame_period = f_period;
>  }
>
> -static void vivid_thread_vid_cap_tick(struct vivid_dev *dev, int dropped_bufs)
> +static noinline_for_stack void vivid_thread_vid_cap_tick(struct vivid_dev *dev,
> +                                                        int dropped_bufs)
>  {
>         struct vivid_buffer *vid_cap_buf = NULL;
>         struct vivid_buffer *vbi_cap_buf = NULL;
> --
> 2.20.0
>
> --
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-- 
Thanks,
~Nick Desaulniers