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vfs: dcache: move hashlen_hash() from callers into d_hash() 

Both __d_lookup_rcu() and __d_lookup_rcu_op_compare() have the full
'name_hash' value of the qstr that they want to look up, and mask it off
to just the low 32-bit hash before calling down to d_hash().

Other callers just load the 32-bit hash and pass it as the argument.

If we move the masking into d_hash() itself, it simplifies the two
callers that currently do the masking, and is a no-op for the other
cases.  It doesn't actually change the generated code since the compiler
will inline d_hash() and see that the end result is the same.

[ Technically, since the parse tree changes, the code generation may not
  be 100% the same, and for me on x86-64, this does result in gcc
  switching the operands around for one 'cmpl' instruction. So not
  necessarily the exact same code generation, but equivalent ]

However, this does encapsulate the 'd_hash()' operation more, and makes
the shift operation in particular be a "shift 32 bits right, return full
word".  Which matches the instruction semantics on both x86-64 and arm64
better, since a 32-bit shift will clear the upper bits.

That makes the next step of introducing a "shift by runtime constant"
more obvious and generates the shift with no extraneous type masking.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2024-06-10 11:47:06 -07:00
parent 6ba59ff422
commit e60cc61153
1 changed files with 4 additions and 4 deletions
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8
fs/dcache.c
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@ -100,9 +100,9 @@ static unsigned int d_hash_shift __ro_after_init;
static struct hlist_bl_head *dentry_hashtable __ro_after_init;
static inline struct hlist_bl_head *d_hash(unsigned int hash)
static inline struct hlist_bl_head *d_hash(unsigned long hashlen)
{
return dentry_hashtable + (hash >> d_hash_shift);
return dentry_hashtable + ((u32)hashlen >> d_hash_shift);
}
#define IN_LOOKUP_SHIFT 10
@ -2104,7 +2104,7 @@ static noinline struct dentry *__d_lookup_rcu_op_compare(
unsigned *seqp)
{
u64 hashlen = name->hash_len;
struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
struct hlist_bl_head *b = d_hash(hashlen);
struct hlist_bl_node *node;
struct dentry *dentry;
@ -2171,7 +2171,7 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
{
u64 hashlen = name->hash_len;
const unsigned char *str = name->name;
struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
struct hlist_bl_head *b = d_hash(hashlen);
struct hlist_bl_node *node;
struct dentry *dentry;