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We'd like to make more use of the highly-optimized dcache hash functions throughout the kernel, rather than have every subsystem create its own, and a function that hashes basic null-terminated strings is required for that. (The name is to emphasize that it returns both hash and length.) It's actually useful in the dcache itself, specifically d_alloc_name(). Other uses in the next patch. full_name_hash() is also tweaked to make it more generally useful: 1) Take a "char *" rather than "unsigned char *" argument, to be consistent with hash_name(). 2) Handle zero-length inputs. If we want more callers, we don't want to make them worry about corner cases. Signed-off-by: George Spelvin <linux@sciencehorizons.net>
77 lines
2.4 KiB
C
77 lines
2.4 KiB
C
#ifndef __LINUX_STRINGHASH_H
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#define __LINUX_STRINGHASH_H
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#include <linux/compiler.h> /* For __pure */
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#include <linux/types.h> /* For u32, u64 */
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/*
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* Routines for hashing strings of bytes to a 32-bit hash value.
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*
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* These hash functions are NOT GUARANTEED STABLE between kernel
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* versions, architectures, or even repeated boots of the same kernel.
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* (E.g. they may depend on boot-time hardware detection or be
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* deliberately randomized.)
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*
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* They are also not intended to be secure against collisions caused by
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* malicious inputs; much slower hash functions are required for that.
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*
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* They are optimized for pathname components, meaning short strings.
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* Even if a majority of files have longer names, the dynamic profile of
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* pathname components skews short due to short directory names.
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* (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
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*/
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/*
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* Version 1: one byte at a time. Example of use:
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*
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* unsigned long hash = init_name_hash;
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* while (*p)
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* hash = partial_name_hash(tolower(*p++), hash);
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* hash = end_name_hash(hash);
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*
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* Although this is designed for bytes, fs/hfsplus/unicode.c
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* abuses it to hash 16-bit values.
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*/
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/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
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#define init_name_hash() 0
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/* partial hash update function. Assume roughly 4 bits per character */
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static inline unsigned long
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partial_name_hash(unsigned long c, unsigned long prevhash)
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{
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return (prevhash + (c << 4) + (c >> 4)) * 11;
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}
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/*
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* Finally: cut down the number of bits to a int value (and try to avoid
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* losing bits)
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*/
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static inline unsigned long end_name_hash(unsigned long hash)
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{
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return (unsigned int)hash;
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}
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/*
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* Version 2: One word (32 or 64 bits) at a time.
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* If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
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* exists, which describes major Linux platforms like x86 and ARM), then
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* this computes a different hash function much faster.
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*
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* If not set, this falls back to a wrapper around the preceding.
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*/
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extern unsigned int __pure full_name_hash(const char *, unsigned int);
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/*
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* A hash_len is a u64 with the hash of a string in the low
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* half and the length in the high half.
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*/
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#define hashlen_hash(hashlen) ((u32)(hashlen))
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#define hashlen_len(hashlen) ((u32)((hashlen) >> 32))
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#define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
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/* Return the "hash_len" (hash and length) of a null-terminated string */
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extern u64 __pure hashlen_string(const char *name);
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#endif /* __LINUX_STRINGHASH_H */
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