linux-kernel - Re: [PATCH 02/12] selftests/mm: fix unused variable warning in hugetlb-madvise.c

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Message-ID: <dd78871f-c95c-6691-9ceb-532271b1f453@redhat.com>
Date:   Fri, 2 Jun 2023 12:01:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/12] selftests/mm: fix unused variable warning in
 hugetlb-madvise.c

On 02.06.23 03:33, John Hubbard wrote:
> The dummy variable is required in order to make this work, so declare it
> as volatile in order to avoid the clang compiler warning.
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>   tools/testing/selftests/mm/hugetlb-madvise.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
> index 28426e30d9bc..3296ccaf7525 100644
> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
> @@ -65,7 +65,7 @@ void write_fault_pages(void *addr, unsigned long nr_pages)
>   
>   void read_fault_pages(void *addr, unsigned long nr_pages)
>   {
> -	unsigned long dummy = 0;
> +	volatile unsigned long dummy = 0;
>   	unsigned long i;
>   
>   	for (i = 0; i < nr_pages; i++)

The compiler can still decide to optimize it all out, because it's not a 
global variable.


Placing a

asm volatile("" : "+r" (dummy));

after the write tells the compiler that the value will be read and 
cannot be optimized out (we use that trick in the cow selftest and I've 
been using it in QEMU for the same purpose as well).

-- 
Thanks,

David / dhildenb