linux-kernel - Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

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Message-Id: <20121029135935.bb8b0b2a.akpm@linux-foundation.org>
Date:	Mon, 29 Oct 2012 13:59:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Stefani Seibold <stefani@...bold.net>
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

On Fri, 26 Oct 2012 15:56:57 +0800
Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:

> Say, if we want to allocate a filo with size of 6 bytes, it would be safer
> to allocate 8 bytes instead of 4 bytes.
>
> ...
>
> --- a/kernel/kfifo.c
> +++ b/kernel/kfifo.c
> @@ -39,11 +39,11 @@ int __kfifo_alloc(struct __kfifo *fifo, unsigned int size,
>  		size_t esize, gfp_t gfp_mask)
>  {
>  	/*
> -	 * round down to the next power of 2, since our 'let the indices
> +	 * round up to the next power of 2, since our 'let the indices
>  	 * wrap' technique works only in this case.
>  	 */
>  	if (!is_power_of_2(size))
> -		size = rounddown_pow_of_two(size);
> +		size = roundup_pow_of_two(size);
>  
>  	fifo->in = 0;
>  	fifo->out = 0;
> @@ -84,7 +84,7 @@ int __kfifo_init(struct __kfifo *fifo, void *buffer,
>  	size /= esize;
>  
>  	if (!is_power_of_2(size))
> -		size = rounddown_pow_of_two(size);
> +		size = roundup_pow_of_two(size);
>  
>  	fifo->in = 0;
>  	fifo->out = 0;

hm, well, if the user asked for a 100-element fifo then it is a bit
strange and unexpected to give them a 128-element one.

If there's absolutely no prospect that the kfifo code will ever support
100-byte fifos then I guess we should rework the API so that the caller
has to pass in log2 of the size, not the size itself.  That way there
will be no surprises and no mistakes.

That being said, the power-of-2 limitation isn't at all intrinsic to a
fifo, so we shouldn't do this.  Ideally, we'd change the kfifo
implementation so it does what the caller asked it to do!
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