netdev - Re: [PATCH] linux/log2.h: enclose macro arg in parens

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Message-ID: <20200623135959.5c4eda49@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 23 Jun 2020 13:59:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH] linux/log2.h: enclose macro arg in parens

On Tue, 23 Jun 2020 10:07:39 -0700 Jonathan Lemon wrote:
> From: Jonathan Lemon <bsd@...com>
> 
> roundup_pow_of_two uses its arg without enclosing it in parens.
> 
> A call of the form:
> 
>    roundup_pow_of_two(boolval ? PAGE_SIZE : frag_size)
> 
> resulted in an compile warning:
> 
> warning: ?: using integer constants in boolean context [-Wint-in-bool-context]
>               PAGE_SIZE :
> ../include/linux/log2.h:176:4: note: in definition of macro ‘roundup_pow_of_two’
>    (n == 1) ? 1 :  \
>     ^
> And the resulting code used '1' as the result of the operation.

I'd have thought that this warning is harmless, since the expression
clearly is not a constant, interesting.

> Fixes: 312a0c170945 ("[PATCH] LOG2: Alter roundup_pow_of_two() so that it can use a ilog2() on a constant")
> 

Please no empty line between fixes and other tags.

> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> ---
>  include/linux/log2.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index 83a4a3ca3e8a..c619ec6eff4a 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -173,7 +173,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>  #define roundup_pow_of_two(n)			\
>  (						\
>  	__builtin_constant_p(n) ? (		\
> -		(n == 1) ? 1 :			\
> +		((n) == 1) ? 1 :		\
>  		(1UL << (ilog2((n) - 1) + 1))	\
>  				   ) :		\
>  	__roundup_pow_of_two(n)			\