dect
/
linux-2.6
Archived
13
0
Fork 0
Code Releases Activity

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>
This commit is contained in:
Linus Torvalds 2012-05-26 10:43:17 -07:00
parent 4ae73f2d53
commit 36126f8f2e
6 changed files with 101 additions and 52 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/openrisc/include/asm/Kbuild
View File

@ -66,3 +66,4 @@ generic-y += topology.h
generic-y += types.h
generic-y += ucontext.h
generic-y += user.h
generic-y += word-at-a-time.h

1
arch/sparc/include/asm/Kbuild
View File

@ -21,3 +21,4 @@ generic-y += div64.h
generic-y += local64.h
generic-y += irq_regs.h
generic-y += local.h
generic-y += word-at-a-time.h

32
arch/x86/include/asm/word-at-a-time.h
View File

@ -10,6 +10,11 @@
* bit count instruction, that might be better than the multiply
* and shift, for example.
*/
struct word_at_a_time {
const unsigned long one_bits, high_bits;
};
#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) }
#ifdef CONFIG_64BIT
@ -37,10 +42,31 @@ static inline long count_masked_bytes(long mask)
#endif
/* Return the high bit set in the first byte that is a zero */
static inline unsigned long has_zero(unsigned long a)
/* Return nonzero if it has a zero */
static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c)
{
return ((a - REPEAT_BYTE(0x01)) & ~a) & REPEAT_BYTE(0x80);
unsigned long mask = ((a - c->one_bits) & ~a) & c->high_bits;
*bits = mask;
return mask;
}
static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits, const struct word_at_a_time *c)
{
return bits;
}
static inline unsigned long create_zero_mask(unsigned long bits)
{
bits = (bits - 1) & ~bits;
return bits >> 7;
}
/* The mask we created is directly usable as a bytemask */
#define zero_bytemask(mask) (mask)
static inline unsigned long find_zero(unsigned long mask)
{
return count_masked_bytes(mask);
}
/*

20
fs/namei.c
View File

@ -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);
b = a ^ REPEAT_BYTE('/');
} while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));
/* The mask *below* the first high bit set */
mask = (mask - 1) & ~mask;
mask >>= 7;
hash += a & mask;
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

52
include/asm-generic/word-at-a-time.h Normal file
View File

@ -0,0 +1,52 @@
#ifndef _ASM_WORD_AT_A_TIME_H
#define _ASM_WORD_AT_A_TIME_H
/*
* This says "generic", but it's actually big-endian only.
* Little-endian can use more efficient versions of these
* interfaces, see for example
* arch/x86/include/asm/word-at-a-time.h
* for those.
*/
#include <linux/kernel.h>
struct word_at_a_time {
const unsigned long high_bits, low_bits;
};
#define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0xfe) + 1, REPEAT_BYTE(0x7f) }
/* Bit set in the bytes that have a zero */
static inline long prep_zero_mask(unsigned long val, unsigned long rhs, const struct word_at_a_time *c)
{
unsigned long mask = (val & c->low_bits) + c->low_bits;
return ~(mask | rhs);
}
#define create_zero_mask(mask) (mask)
static inline long find_zero(unsigned long mask)
{
long byte = 0;
#ifdef CONFIG_64BIT
if (mask >> 32)
mask >>= 32;
else
byte = 4;
#endif
if (mask >> 16)
mask >>= 16;
else
byte += 2;
return (mask >> 8) ? byte : byte + 1;
}
static inline bool has_zero(unsigned long val, unsigned long *data, const struct word_at_a_time *c)
{
unsigned long rhs = val | c->low_bits;
*data = rhs;
return (val + c->high_bits) & ~rhs;
}
#endif /* _ASM_WORD_AT_A_TIME_H */

47
lib/strncpy_from_user.c
View File

@ -4,37 +4,7 @@
#include <linux/errno.h>
#include <asm/byteorder.h>
static inline long find_zero(unsigned long mask)
{
long byte = 0;
#ifdef __BIG_ENDIAN
#ifdef CONFIG_64BIT
if (mask >> 32)
mask >>= 32;
else
byte = 4;
#endif
if (mask >> 16)
mask >>= 16;
else
byte += 2;
return (mask >> 8) ? byte : byte + 1;
#else
#ifdef CONFIG_64BIT
if (!((unsigned int) mask)) {
mask >>= 32;
byte = 4;
}
#endif
if (!(mask & 0xffff)) {
mask >>= 16;
byte += 2;
}
return (mask & 0xff) ? byte : byte + 1;
#endif
}
#include <asm/word-at-a-time.h>
#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
#define IS_UNALIGNED(src, dst) 0
@ -51,8 +21,7 @@ static inline long find_zero(unsigned long mask)
*/
static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
{
const unsigned long high_bits = REPEAT_BYTE(0xfe) + 1;
const unsigned long low_bits = REPEAT_BYTE(0x7f);
const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
long res = 0;
/*
@ -66,18 +35,16 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
goto byte_at_a_time;
while (max >= sizeof(unsigned long)) {
unsigned long c, v, rhs;
unsigned long c, data;
/* Fall back to byte-at-a-time if we get a page fault */
if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
break;
rhs = c | low_bits;
v = (c + high_bits) & ~rhs;
*(unsigned long *)(dst+res) = c;
if (v) {
v = (c & low_bits) + low_bits;
v = ~(v | rhs);
return res + find_zero(v);
if (has_zero(c, &data, &constants)) {
data = prep_zero_mask(c, data, &constants);
data = create_zero_mask(data);
return res + find_zero(data);
}
res += sizeof(unsigned long);
max -= sizeof(unsigned long);