linux-kernel - Re: [PATCH 1/2] dma/pool: trivial: add semicolon after label attributes

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Date:   Fri, 1 Sep 2023 09:59:20 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Chunhui He <hchunhui@...l.ustc.edu.cn>
Cc:     hch@....de, m.szyprowski@...sung.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dma/pool: trivial: add semicolon after label
 attributes

On 2023-09-01 09:56, Robin Murphy wrote:
> On 2023-08-31 12:59, Chunhui He wrote:
>>
>> On Tue, 29 Aug 2023 16:28:05 +0100, Robin Murphy 
>> <robin.murphy@....com> wrote:
>>> On 29/08/2023 4:12 pm, Christoph Hellwig wrote:
>>>> On Tue, Aug 29, 2023 at 03:22:22PM +0100, Robin Murphy wrote:
>>>>> AFAICS, what that clearly says is that *C++* label attributes can be
>>>>> ambiguous. This is not C++ code. Even in C11, declarations still
>>>>> cannot be
>>>>> labelled, so it should still be the case that, per the same GCC
>>>>> documentation, "the ambiguity does not arise". And even if the
>>>>> language did
>>>>> allow it, an inline declaration at that point at the end of a function
>>>>> would be downright weird and against the kernel coding style anyway.
>>>>>
>>>>> So, I don't really see what's "better" about cluttering up C code with
>>>>> unnecessary C++isms; it's just weird noise to me. The only thing I
>>>>> think it
>>>>> *does* achieve is introduce the chance that the static checker brigade
>>>>> eventually identifies a redundant semicolon and we get more patches to
>>>>> remove it again.
>>
>> Inline declaration is a GNU C extension, so the ambiguity may arise.
>> Adding ';' makes the compiler easier to parse correctly, so I say
>> "better". The commit 13a453c241b78934a945b1af572d0533612c9bd1
>> (sched/fair: Add ';' after label attributes) also says the same.
> 
> And that commit was also wrong. Nobody suggested C11 doesn't support 
> inline declarations - it demonstrably does - the fact in question is 
> that *attributes* on declarations is a C++ thing and not valid in C:

Argh, sorry, s/attributes/labels/

/me goes to make more coffee...

Robin.

> ~/src/linux$ git diff
> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> index 1acec2e22827..e1354235cb9c 100644
> --- a/kernel/dma/pool.c
> +++ b/kernel/dma/pool.c
> @@ -137,7 +137,8 @@ static int atomic_pool_expand(struct gen_pool *pool, 
> size_t pool_size,
>          dma_common_free_remap(addr, pool_size);
>   #endif
>   free_page: __maybe_unused
> -       __free_pages(page, order);
> +       int x = order;
> +       __free_pages(page, x);
>   out:
>          return ret;
>   }
> ~/src/linux$ make -j32
>    CALL    scripts/checksyscalls.sh
>    CC      kernel/dma/pool.o
> kernel/dma/pool.c: In function ‘atomic_pool_expand’:
> kernel/dma/pool.c:140:2: error: a label can only be part of a statement 
> and a declaration is not a statement
>    140 |  int x = order;
>        |  ^~~
> make[4]: *** [scripts/Makefile.build:243: kernel/dma/pool.o] Error 1
> make[3]: *** [scripts/Makefile.build:480: kernel/dma] Error 2
> make[2]: *** [scripts/Makefile.build:480: kernel] Error 2
> make[2]: *** Waiting for unfinished jobs....
> make[1]: *** [/home/robmur01/src/linux/Makefile:2032: .] Error 2
> make: *** [Makefile:234: __sub-make] Error 2
> 
> 
> Thanks,
> Robin.
>