linux-kernel - Re: [PATCH v8 1/4] bitops: Introduce the for_each_set

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Date:   Tue, 16 Jun 2020 11:14:28 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     linus.walleij@...aro.org, akpm@...ux-foundation.org,
        vilhelm.gray@...il.com, arnd@...db.de, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 1/4] bitops: Introduce the for_each_set_clump macro

On Mon, Jun 15, 2020 at 06:21:18PM +0530, Syed Nayyar Waris wrote:
> This macro iterates for each group of bits (clump) with set bits,
> within a bitmap memory region. For each iteration, "start" is set to
> the bit offset of the found clump, while the respective clump value is
> stored to the location pointed by "clump". Additionally, the
> bitmap_get_value and bitmap_set_value functions are introduced to
> respectively get and set a value of n-bits in a bitmap memory region.
> The n-bits can have any size less than or equal to BITS_PER_LONG.
> Moreover, during setting value of n-bit in bitmap, if a situation arise
> that the width of next n-bit is exceeding the word boundary, then it
> will divide itself such that some portion of it is stored in that word,
> while the remaining portion is stored in the next higher word. Similar
> situation occurs while retrieving value of n-bits from bitmap.

On the second view...

> +static inline unsigned long bitmap_get_value(const unsigned long *map,
> +					      unsigned long start,
> +					      unsigned long nbits)
> +{
> +	const size_t index = BIT_WORD(start);
> +	const unsigned long offset = start % BITS_PER_LONG;

> +	const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);

This perhaps should use round_up()

> +	const unsigned long space = ceiling - start;

And I think I see a scenario to complain.

If start == 0, then ceiling will be 64.
space == 64. Not good.

> +	unsigned long value_low, value_high;
> +
> +	if (space >= nbits)
> +		return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> +	else {
> +		value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
> +		value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
> +		return (value_low >> offset) | (value_high << space);
> +	}
> +}

...

> +/**
> + * bitmap_set_value - set n-bit value within a memory region
> + * @map: address to the bitmap memory region
> + * @value: value of nbits
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits
> + */
> +static inline void bitmap_set_value(unsigned long *map,
> +				    unsigned long value,
> +				    unsigned long start, unsigned long nbits)
> +{
> +	const size_t index = BIT_WORD(start);
> +	const unsigned long offset = start % BITS_PER_LONG;

> +	const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
> +	const unsigned long space = ceiling - start;

Ditto for both lines.

> +	value &= GENMASK(nbits - 1, 0);
> +
> +	if (space >= nbits) {
> +		map[index] &= ~(GENMASK(nbits + offset - 1, offset));
> +		map[index] |= value << offset;
> +	} else {
> +		map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
> +		map[index] |= value << offset;
> +		map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> +		map[index + 1] |= (value >> space);
> +	}
> +}

-- 
With Best Regards,
Andy Shevchenko