path: root/lib
diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2012-05-26 10:43:17 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-05-26 11:33:40 -0700
commit36126f8f2ed8168eb13aa0662b9b9585cba100a9 (patch)
tree543f6b6ab60dd3e47af931142aa84f0ba7749d43 /lib
parent4ae73f2d53255c388d50bf83c1681112a6f9cba1 (diff)
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>
Diffstat (limited to 'lib')
1 files changed, 7 insertions, 40 deletions
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index c4c09b0e96b..bb2b201d6ad 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -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;
- if (mask >> 16)
- mask >>= 16;
- else
- byte += 2;
- return (mask >> 8) ? byte : byte + 1;
-#ifdef CONFIG_64BIT
- if (!((unsigned int) mask)) {
- mask >>= 32;
- byte = 4;
- }
- if (!(mask & 0xffff)) {
- mask >>= 16;
- byte += 2;
- }
- return (mask & 0xff) ? byte : byte + 1;
+#include <asm/word-at-a-time.h>
#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))))
- 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);