namei: Improve hash mixing if CONFIG_DCACHE_WORD_ACCESS

The hash mixing between adding the next 64 bits of name
was just a bit weak.

Replaced with a still very fast but slightly more effective
mixing function.

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/namei.c b/fs/namei.c
index 30145f8f21ed5b47e47fba7e29e0434c76e858d0..42f8ca038254078f4cb37d1827f9c3b2da8c85ba 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1794,30 +1794,49 @@ static inline unsigned int fold_hash(unsigned long hash)
        return hash_64(hash, 32);
 }
 
+/*
+ * This is George Marsaglia's XORSHIFT generator.
+ * It implements a maximum-period LFSR in only a few
+ * instructions.  It also has the property (required
+ * by hash_name()) that mix_hash(0) = 0.
+ */
+static inline unsigned long mix_hash(unsigned long hash)
+{
+       hash ^= hash << 13;
+       hash ^= hash >> 7;
+       hash ^= hash << 17;
+       return hash;
+}
+
 #else  /* 32-bit case */
 
 #define fold_hash(x) (x)
 
+static inline unsigned long mix_hash(unsigned long hash)
+{
+       hash ^= hash << 13;
+       hash ^= hash >> 17;
+       hash ^= hash << 5;
+       return hash;
+}
+
 #endif
 
 unsigned int full_name_hash(const unsigned char *name, unsigned int len)
 {
-       unsigned long a, mask;
-       unsigned long hash = 0;
+       unsigned long a, hash = 0;
 
        for (;;) {
                a = load_unaligned_zeropad(name);
                if (len < sizeof(unsigned long))
                        break;
-               hash += a;
-               hash *= 9;
+               hash = mix_hash(hash + a);
                name += sizeof(unsigned long);
                len -= sizeof(unsigned long);
                if (!len)
                        goto done;
        }
-       mask = bytemask_from_count(len);
-       hash += mask & a;
+       hash += a & bytemask_from_count(len);
 done:
        return fold_hash(hash);
 }
@@ -1835,7 +1854,7 @@ static inline u64 hash_name(const char *name)
        hash = a = 0;
        len = -sizeof(unsigned long);
        do {
-               hash = (hash + a) * 9;
+               hash = mix_hash(hash + a);
                len += sizeof(unsigned long);
                a = load_unaligned_zeropad(name+len);
                b = a ^ REPEAT_BYTE('/');