NFS: Add dentry materialisation op

The attached patch adds a new directory cache management function that prepares
a disconnected anonymous function to be connected into the dentry tree. The
anonymous dentry is transferred the name and parentage from another dentry.

The following changes were made in [try #2]:

 (*) d_materialise_dentry() now switches the parentage of the two nodes around
     correctly when one or other of them is self-referential.

The following changes were made in [try #7]:

 (*) d_instantiate_unique() has had the interior part split out as function
     __d_instantiate_unique(). Callers of this latter function must be holding
     the appropriate locks.

 (*) _d_rehash() has been added as a wrapper around __d_rehash() to call it
     with the most obvious hash list (the one from the name). d_rehash() now
     calls _d_rehash().

 (*) d_materialise_dentry() is now __d_materialise_dentry() and is static.

 (*) d_materialise_unique() added to perform the combination of d_find_alias(),
     d_materialise_dentry() and d_add_unique() that the NFS client was doing
     twice, all within a single dcache_lock critical section. This reduces the
     number of times two different spinlocks were being accessed.

The following further changes were made:

 (*) Add the dentries onto their parents d_subdirs lists.

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 150 insertions, 14 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 1b4a3a34ec57..17b392a2049e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -828,17 +828,19 @@ void d_instantiate(struct dentry *entry, struct inode * inode)
  * (or otherwise set) by the caller to indicate that it is now
  * in use by the dcache.
  */
-struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
+static struct dentry *__d_instantiate_unique(struct dentry *entry,
+                                             struct inode *inode)
 {
         struct dentry *alias;
         int len = entry->d_name.len;
         const char *name = entry->d_name.name;
         unsigned int hash = entry->d_name.hash;
 
-        BUG_ON(!list_empty(&entry->d_alias));
-        spin_lock(&dcache_lock);
-        if (!inode)
-                goto do_negative;
+        if (!inode) {
+                entry->d_inode = NULL;
+                return NULL;
+        }
+
         list_for_each_entry(alias, &inode->i_dentry, d_alias) {
                 struct qstr *qstr = &alias->d_name;
 
@@ -851,19 +853,35 @@ struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
                 if (memcmp(qstr->name, name, len))
                         continue;
                 dget_locked(alias);
-                spin_unlock(&dcache_lock);
-                BUG_ON(!d_unhashed(alias));
-                iput(inode);
                 return alias;
         }
+
         list_add(&entry->d_alias, &inode->i_dentry);
-do_negative:
         entry->d_inode = inode;
         fsnotify_d_instantiate(entry, inode);
-        spin_unlock(&dcache_lock);
-        security_d_instantiate(entry, inode);
         return NULL;
 }
+
+struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
+{
+        struct dentry *result;
+
+        BUG_ON(!list_empty(&entry->d_alias));
+
+        spin_lock(&dcache_lock);
+        result = __d_instantiate_unique(entry, inode);
+        spin_unlock(&dcache_lock);
+
+        if (!result) {
+                security_d_instantiate(entry, inode);
+                return NULL;
+        }
+
+        BUG_ON(!d_unhashed(result));
+        iput(inode);
+        return result;
+}
+
 EXPORT_SYMBOL(d_instantiate_unique);
 
 /**
@@ -1235,6 +1253,11 @@ static void __d_rehash(struct dentry * entry, struct hlist_head *list)
         hlist_add_head_rcu(&entry->d_hash, list);
 }
 
+static void _d_rehash(struct dentry * entry)
+{
+        __d_rehash(entry, d_hash(entry->d_parent, entry->d_name.hash));
+}
+
 /**
  * d_rehash     - add an entry back to the hash
  * @entry: dentry to add to the hash
@@ -1244,11 +1267,9 @@ static void __d_rehash(struct dentry * entry, struct hlist_head *list)
  
 void d_rehash(struct dentry * entry)
 {
-        struct hlist_head *list = d_hash(entry->d_parent, entry->d_name.hash);
-
         spin_lock(&dcache_lock);
         spin_lock(&entry->d_lock);
-        __d_rehash(entry, list);
+        _d_rehash(entry);
         spin_unlock(&entry->d_lock);
         spin_unlock(&dcache_lock);
 }
@@ -1386,6 +1407,120 @@ already_unhashed:
         spin_unlock(&dcache_lock);
 }
 
+/*
+ * Prepare an anonymous dentry for life in the superblock's dentry tree as a
+ * named dentry in place of the dentry to be replaced.
+ */
+static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
+{
+        struct dentry *dparent, *aparent;
+
+        switch_names(dentry, anon);
+        do_switch(dentry->d_name.len, anon->d_name.len);
+        do_switch(dentry->d_name.hash, anon->d_name.hash);
+
+        dparent = dentry->d_parent;
+        aparent = anon->d_parent;
+
+        dentry->d_parent = (aparent == anon) ? dentry : aparent;
+        list_del(&dentry->d_u.d_child);
+        if (!IS_ROOT(dentry))
+                list_add(&dentry->d_u.d_child, &dentry->d_parent->d_subdirs);
+        else
+                INIT_LIST_HEAD(&dentry->d_u.d_child);
+
+        anon->d_parent = (dparent == dentry) ? anon : dparent;
+        list_del(&anon->d_u.d_child);
+        if (!IS_ROOT(anon))
+                list_add(&anon->d_u.d_child, &anon->d_parent->d_subdirs);
+        else
+                INIT_LIST_HEAD(&anon->d_u.d_child);
+
+        anon->d_flags &= ~DCACHE_DISCONNECTED;
+}
+
+/**
+ * d_materialise_unique - introduce an inode into the tree
+ * @dentry: candidate dentry
+ * @inode: inode to bind to the dentry, to which aliases may be attached
+ *
+ * Introduces an dentry into the tree, substituting an extant disconnected
+ * root directory alias in its place if there is one
+ */
+struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
+{
+        struct dentry *alias, *actual;
+
+        BUG_ON(!d_unhashed(dentry));
+
+        spin_lock(&dcache_lock);
+
+        if (!inode) {
+                actual = dentry;
+                dentry->d_inode = NULL;
+                goto found_lock;
+        }
+
+        /* See if a disconnected directory already exists as an anonymous root
+         * that we should splice into the tree instead */
+        if (S_ISDIR(inode->i_mode) && (alias = __d_find_alias(inode, 1))) {
+                spin_lock(&alias->d_lock);
+
+                /* Is this a mountpoint that we could splice into our tree? */
+                if (IS_ROOT(alias))
+                        goto connect_mountpoint;
+
+                if (alias->d_name.len == dentry->d_name.len &&
+                    alias->d_parent == dentry->d_parent &&
+                    memcmp(alias->d_name.name,
+                           dentry->d_name.name,
+                           dentry->d_name.len) == 0)
+                        goto replace_with_alias;
+
+                spin_unlock(&alias->d_lock);
+
+                /* Doh! Seem to be aliasing directories for some reason... */
+                dput(alias);
+        }
+
+        /* Add a unique reference */
+        actual = __d_instantiate_unique(dentry, inode);
+        if (!actual)
+                actual = dentry;
+        else if (unlikely(!d_unhashed(actual)))
+                goto shouldnt_be_hashed;
+
+found_lock:
+        spin_lock(&actual->d_lock);
+found:
+        _d_rehash(actual);
+        spin_unlock(&actual->d_lock);
+        spin_unlock(&dcache_lock);
+
+        if (actual == dentry) {
+                security_d_instantiate(dentry, inode);
+                return NULL;
+        }
+
+        iput(inode);
+        return actual;
+
+        /* Convert the anonymous/root alias into an ordinary dentry */
+connect_mountpoint:
+        __d_materialise_dentry(dentry, alias);
+
+        /* Replace the candidate dentry with the alias in the tree */
+replace_with_alias:
+        __d_drop(alias);
+        actual = alias;
+        goto found;
+
+shouldnt_be_hashed:
+        spin_unlock(&dcache_lock);
+        BUG();
+        goto shouldnt_be_hashed;
+}
+
 /**
  * d_path - return the path of a dentry
  * @dentry: dentry to report
@@ -1784,6 +1919,7 @@ EXPORT_SYMBOL(d_instantiate);
 EXPORT_SYMBOL(d_invalidate);
 EXPORT_SYMBOL(d_lookup);
 EXPORT_SYMBOL(d_move);
+EXPORT_SYMBOL_GPL(d_materialise_unique);
 EXPORT_SYMBOL(d_path);
 EXPORT_SYMBOL(d_prune_aliases);
 EXPORT_SYMBOL(d_rehash);