linux-kernel - Re: [PATCH 1/2] drm/exec: use unique instead of local label

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Message-ID: <CAKwvOdmRm=vOjNXGWeuKw-h78CXMrkcwc5vnCRVqFxMxWzhCcg@mail.gmail.com>
Date:   Tue, 1 Aug 2023 13:35:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     boris.brezillon@...labora.com, maarten.lankhorst@...ux.intel.com,
        mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com,
        daniel@...ll.ch, trix@...hat.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        naresh.kamboju@...aro.org, dakr@...hat.com
Subject: Re: [PATCH 1/2] drm/exec: use unique instead of local label

On Mon, Jul 31, 2023 at 5:36 AM Christian König
<ckoenig.leichtzumerken@...il.com> wrote:
>
> GCC forbids to jump to labels in loop conditions and a new clang
> check stumbled over this.
>
> So instead using a local label inside the loop condition use an
> unique label outside of it.
>
> Fixes: commit 09593216bff1 ("drm: execution context for GEM buffers v7")
> Link: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
> Link: https://github.com/ClangBuiltLinux/linux/issues/1890
> Link: https://github.com/llvm/llvm-project/commit/20219106060208f0c2f5d096eb3aed7b712f5067
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> CC: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Christian König <christian.koenig@....com>

Works for me; thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

I suspect it's possible to change the indirect goto into a direct goto
with some further refactoring (macros can take block statements; if
drm_exec_until_all_locked accepted a block statement arg then you
could introduce a new scope, and a new local label to that scope, then
just use direct goto), but this will probably apply cleaner. (oh, is
09593216bff1 only in next at the moment? The AuthorDate threw me.)

There are some curious cases where __attribute__((cleanup())) doesn't
mesh well with indirect gotos.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37722

May not ever be a problem here...

> ---
>  include/drm/drm_exec.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> index 73205afec162..e0462361adf9 100644
> --- a/include/drm/drm_exec.h
> +++ b/include/drm/drm_exec.h
> @@ -3,6 +3,7 @@
>  #ifndef __DRM_EXEC_H__
>  #define __DRM_EXEC_H__
>
> +#include <linux/compiler.h>

If you wanted to be more specific (if this addition is due to
__PASTE), then `compiler_types.h` is more precise.

>  #include <linux/ww_mutex.h>
>
>  #define DRM_EXEC_INTERRUPTIBLE_WAIT    BIT(0)
> @@ -74,13 +75,12 @@ struct drm_exec {
>   * Since labels can't be defined local to the loops body we use a jump pointer
>   * to make sure that the retry is only used from within the loops body.
>   */
> -#define drm_exec_until_all_locked(exec)                                \
> -       for (void *__drm_exec_retry_ptr; ({                     \
> -               __label__ __drm_exec_retry;                     \
> -__drm_exec_retry:                                              \
> -               __drm_exec_retry_ptr = &&__drm_exec_retry;      \
> -               (void)__drm_exec_retry_ptr;                     \
> -               drm_exec_cleanup(exec);                         \
> +#define drm_exec_until_all_locked(exec)                                        \
> +__PASTE(__drm_exec_, __LINE__):                                                \
> +       for (void *__drm_exec_retry_ptr; ({                             \
> +               __drm_exec_retry_ptr = &&__PASTE(__drm_exec_, __LINE__);\
> +               (void)__drm_exec_retry_ptr;                             \
> +               drm_exec_cleanup(exec);                                 \
>         });)
>
>  /**
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers