On Tue, 23 Aug 2011 01:29:07 +0900
Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote:
> memchr_inv() is mainly used to check whether the whole buffer is filled
> with just a specified byte.
>
> The function name and prototype are stolen from logfs and the
> implementation is from SLUB.
>
> ...
>
> +/**
> + * memchr_inv - Find a character in an area of memory.
> + * @s: The memory area
> + * @c: The byte to search for
> + * @n: The size of the area.
This text seems to be stolen from memchr(). I guess it's close enough.
> + * returns the address of the first character other than @c, or %NULL
> + * if the whole buffer contains just @c.
> + */
> +void *memchr_inv(const void *start, int c, size_t bytes)
> +{
> + u8 value = c;
> + u64 value64;
> + unsigned int words, prefix;
> +
> + if (bytes <= 16)
> + return check_bytes8(start, value, bytes);
> +
> + value64 = value | value << 8 | value << 16 | value << 24;
> + value64 = (value64 & 0xffffffff) | value64 << 32;
> + prefix = 8 - ((unsigned long)start) % 8;
> +
> + if (prefix) {
> + u8 *r = check_bytes8(start, value, prefix);
> + if (r)
> + return r;
> + start += prefix;
> + bytes -= prefix;
> + }
> +
> + words = bytes / 8;
> +
> + while (words) {
> + if (*(u64 *)start != value64)
OK, problem. This will explode if passed a misaligned address on
certain (non-x86) architectures. This is nasty because people will
develop and test code on x86 and it works. Much later, the
alpha/ia64/etc guys discover the problem.
One fix would be to use get_unaligned(). This might be slow on some
architectures, I don't know. Another fix is to restrict the caller's
alignment freedom; document this and add a runtime WARN_ON().
> + return check_bytes8(start, value, 8);
> + start += 8;
> + words--;
> + }
> +
> + return check_bytes8(start, value, bytes % 8);
> +}
> +EXPORT_SYMBOL(memchr_inv);
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