vfs: reimplement d_rcu_to_refcount() using lockref_get_or_lock()

This moves __d_rcu_to_refcount() from <linux/dcache.h> into fs/namei.c
and re-implements it using the lockref infrastructure instead.  It also
adds a lot of comments about what is actually going on, because turning
a dentry that was looked up using RCU into a long-lived reference
counted entry is one of the more subtle parts of the rcu walk.

We also used to be _particularly_ subtle in unlazy_walk() where we
re-validate both the dentry and its parent using the same sequence
count.  We used to do it by nesting the locks and then verifying the
sequence count just once.

That was silly, because nested locking is expensive, but the sequence
count check is not.  So this just re-validates the dentry and the parent
separately, avoiding the nested locking, and making the lockref lookup
possible.

Acked-by: Waiman Long <waiman.long@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 22 deletions

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index efdc94434c30..9169b91ea2d2 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -304,28 +304,6 @@ extern struct dentry *__d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *__d_lookup_rcu(const struct dentry *parent,
                                 const struct qstr *name, unsigned *seq);
 
-/**
- * __d_rcu_to_refcount - take a refcount on dentry if sequence check is ok
- * @dentry: dentry to take a ref on
- * @seq: seqcount to verify against
- * Returns: 0 on failure, else 1.
- *
- * __d_rcu_to_refcount operates on a dentry,seq pair that was returned
- * by __d_lookup_rcu, to get a reference on an rcu-walk dentry.
- */
-static inline int __d_rcu_to_refcount(struct dentry *dentry, unsigned seq)
-{
-        int ret = 0;
-
-        assert_spin_locked(&dentry->d_lock);
-        if (!read_seqcount_retry(&dentry->d_seq, seq)) {
-                ret = 1;
-                dentry->d_lockref.count++;
-        }
-
-        return ret;
-}
-
 static inline unsigned d_count(const struct dentry *dentry)
 {
         return dentry->d_lockref.count;