From: Linus Torvalds torvalds@linux-foundation.org
commit e60cc61153e61e4e38bd983492df9959e82ae4dc upstream.
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.
Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Jimmy Tran jtoantran@google.com --- fs/dcache.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c index 4030c010a7682..82adee104f82c 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -100,9 +100,9 @@ static unsigned int d_hash_shift __read_mostly;
static struct hlist_bl_head *dentry_hashtable __read_mostly;
-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 @@ -2286,7 +2286,7 @@ 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;
@@ -2353,7 +2353,7 @@ 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;