[PATCH] media: vivid: work around high stack usage with clang

[PATCH] media: vivid: work around high stack usage with clang

[Arnd Bergmann]

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.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

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

[Nick Desaulniers]

On Thu, Jul 18, 2019 at 7:16 AM Arnd Bergmann <arnd@arndb.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@google.com>

>
> Signed-off-by: Arnd Bergmann <arnd@arndb.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