linux-kernel - Re: [RFC] _

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Message-ID: <20090422204633.GE570@1wt.eu>
Date:	Wed, 22 Apr 2009 22:46:33 +0200
From:	Willy Tarreau <w@....eu>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] __ffs64()

Hi Steve,

On Wed, Apr 22, 2009 at 04:46:32PM +0100, Steven Whitehouse wrote:
> Hi,
> 
> I'd like to add a new bitop, __ffs64() which I need in order to fix a
> bug in GFS2. The question is, where should it go?
> 
> On 64 bit arches, __ffs64() would be a synonym for __ffs(), but on 32
> bit arches it degenerates to a conditional plus a call to __ffs(). I'm
> assuming that there would not be a lot of point in optimising this
> operation on 32 bit arches even if such an instruction was available, so
> that I should do something like the below patch.
> 
> Does that seem reasonable, or should I give it a separate header file
> under asm-generic/bitops/ like some of the similar operations? It looks
> like I'd have to touch a lot of other files if I were to go that route,

While I have no reply to your questions above, I have one minor comment :

> +/**
> + * __ffs64 - find first set bit in a 64 bit word
> + * @word: The 64 bit word
> + *
> + * On 64 bit arches this is a synomyn for __ffs
> + */

IMHO you should remind here that the result is undefined when word==0
and the caller must check against it first.

> +static inline unsigned long __ffs64(u64 word)
> +{
> +#if BITS_PER_LONG == 32
> +	if (((u32)word) == 0UL)
> +		return __ffs((u32)(word >> 32)) + 32;
> +#elif BITS_PER_LONG != 64
> +#error BITS_PER_LONG not 32 or 64
> +#endif
> +	return __ffs((unsigned long)word);
> +}
> +
>  #ifdef __KERNEL__
>  #ifdef CONFIG_GENERIC_FIND_FIRST_BIT

Regards,
Willy

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