word-at-a-time: make the interfaces truly generic

This changes the interfaces in <asm/word-at-a-time.h> to be a bit more
complicated, but a lot more generic.

In particular, it allows us to really do the operations efficiently on
both little-endian and big-endian machines, pretty much regardless of
machine details.  For example, if you can rely on a fast population
count instruction on your architecture, this will allow you to make your
optimized <asm/word-at-a-time.h> file with that.

NOTE! The "generic" version in include/asm-generic/word-at-a-time.h is
not truly generic, it actually only works on big-endian.  Why? Because
on little-endian the generic algorithms are wasteful, since you can
inevitably do better. The x86 implementation is an example of that.

(The only truly non-generic part of the asm-generic implementation is
the "find_zero()" function, and you could make a little-endian version
of it.  And if the Kbuild infrastructure allowed us to pick a particular
header file, that would be lovely)

The <asm/word-at-a-time.h> functions are as follows:

 - WORD_AT_A_TIME_CONSTANTS: specific constants that the algorithm
   uses.

 - has_zero(): take a word, and determine if it has a zero byte in it.
   It gets the word, the pointer to the constant pool, and a pointer to
   an intermediate "data" field it can set.

   This is the "quick-and-dirty" zero tester: it's what is run inside
   the hot loops.

 - "prep_zero_mask()": take the word, the data that has_zero() produced,
   and the constant pool, and generate an *exact* mask of which byte had
   the first zero.  This is run directly *outside* the loop, and allows
   the "has_zero()" function to answer the "is there a zero byte"
   question without necessarily getting exactly *which* byte is the
   first one to contain a zero.

   If you do multiple byte lookups concurrently (eg "hash_name()", which
   looks for both NUL and '/' bytes), after you've done the prep_zero_mask()
   phase, the result of those can be or'ed together to get the "either
   or" case.

 - The result from "prep_zero_mask()" can then be fed into "find_zero()"
   (to find the byte offset of the first byte that was zero) or into
   "zero_bytemask()" (to find the bytemask of the bytes preceding the
   zero byte).

   The existence of zero_bytemask() is optional, and is not necessary
   for the normal string routines.  But dentry name hashing needs it, so
   if you enable DENTRY_WORD_AT_A_TIME you need to expose it.

This changes the generic strncpy_from_user() function and the dentry
hashing functions to use these modified word-at-a-time interfaces.  This
gets us back to the optimized state of the x86 strncpy that we lost in
the previous commit when moving over to the generic version.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 10 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 93ff12b..c651f02 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1452,7 +1452,8 @@ EXPORT_SYMBOL(full_name_hash);
  */
 static inline unsigned long hash_name(const char *name, unsigned int *hashp)
 {
-	unsigned long a, mask, hash, len;
+	unsigned long a, b, adata, bdata, mask, hash, len;
+	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
 
 	hash = a = 0;
 	len = -sizeof(unsigned long);
@@ -1460,17 +1461,18 @@ static inline unsigned long hash_name(const char *name, unsigned int *hashp)
 		hash = (hash + a) * 9;
 		len += sizeof(unsigned long);
 		a = load_unaligned_zeropad(name+len);
-		/* Do we have any NUL or '/' bytes in this word? */
-		mask = has_zero(a) | has_zero(a ^ REPEAT_BYTE('/'));
-	} while (!mask);
-
-	/* The mask *below* the first high bit set */
-	mask = (mask - 1) & ~mask;
-	mask >>= 7;
-	hash += a & mask;
+		b = a ^ REPEAT_BYTE('/');
+	} while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));
+
+	adata = prep_zero_mask(a, adata, &constants);
+	bdata = prep_zero_mask(b, bdata, &constants);
+
+	mask = create_zero_mask(adata | bdata);
+
+	hash += a & zero_bytemask(mask);
 	*hashp = fold_hash(hash);
 
-	return len + count_masked_bytes(mask);
+	return len + find_zero(mask);
 }
 
 #else