On Tue, 23 Aug 2011 01:29:07 +0900
Akinobu Mita [off-list ref] 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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