2 files changed, 51 insertions, 41 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 4d13bf50b7b1..d56a40b5a577 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1332,31 +1332,13 @@ EXPORT_SYMBOL(d_add_ci);
  * d_lookup - search for a dentry
  * @parent: parent dentry
  * @name: qstr of name we wish to find
+ * Returns: dentry, or NULL
  *
- * Searches the children of the parent dentry for the name in question. If
- * the dentry is found its reference count is incremented and the dentry
- * is returned. The caller must use dput to free the entry when it has
- * finished using it. %NULL is returned on failure.
- *
- * __d_lookup is dcache_lock free. The hash list is protected using RCU.
- * Memory barriers are used while updating and doing lockless traversal. 
- * To avoid races with d_move while rename is happening, d_lock is used.
- *
- * Overflows in memcmp(), while d_move, are avoided by keeping the length
- * and name pointer in one structure pointed by d_qstr.
- *
- * rcu_read_lock() and rcu_read_unlock() are used to disable preemption while
- * lookup is going on.
- *
- * The dentry unused LRU is not updated even if lookup finds the required dentry
- * in there. It is updated in places such as prune_dcache, shrink_dcache_sb,
- * select_parent and __dget_locked. This laziness saves lookup from dcache_lock
- * acquisition.
- *
- * d_lookup() is protected against the concurrent renames in some unrelated
- * directory using the seqlockt_t rename_lock.
+ * d_lookup searches the children of the parent dentry for the name in
+ * question. If the dentry is found its reference count is incremented and the
+ * dentry is returned. The caller must use dput to free the entry when it has
+ * finished using it. %NULL is returned if the dentry does not exist.
  */
-
 struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
 {
         struct dentry * dentry = NULL;
@@ -1372,6 +1354,21 @@ struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
 }
 EXPORT_SYMBOL(d_lookup);
 
+/*
+ * __d_lookup - search for a dentry (racy)
+ * @parent: parent dentry
+ * @name: qstr of name we wish to find
+ * Returns: dentry, or NULL
+ *
+ * __d_lookup is like d_lookup, however it may (rarely) return a
+ * false-negative result due to unrelated rename activity.
+ *
+ * __d_lookup is slightly faster by avoiding rename_lock read seqlock,
+ * however it must be used carefully, eg. with a following d_lookup in
+ * the case of failure.
+ *
+ * __d_lookup callers must be commented.
+ */
 struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
 {
         unsigned int len = name->len;
@@ -1382,6 +1379,19 @@ struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
         struct hlist_node *node;
         struct dentry *dentry;
 
+        /*
+         * The hash list is protected using RCU.
+         *
+         * Take d_lock when comparing a candidate dentry, to avoid races
+         * with d_move().
+         *
+         * It is possible that concurrent renames can mess up our list
+         * walk here and result in missing our dentry, resulting in the
+         * false-negative result. d_lookup() protects against concurrent
+         * renames using rename_lock seqlock.
+         *
+         * See Documentation/vfs/dcache-locking.txt for more details.
+         */
         rcu_read_lock();
         
         hlist_for_each_entry_rcu(dentry, node, head, d_hash) {
@@ -1396,8 +1406,8 @@ struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
 
                 /*
                  * Recheck the dentry after taking the lock - d_move may have
-                 * changed things.  Don't bother checking the hash because we're
-                 * about to compare the whole name anyway.
+                 * changed things. Don't bother checking the hash because
+                 * we're about to compare the whole name anyway.
                  */
                 if (dentry->d_parent != parent)
                         goto next;
diff --git a/fs/namei.c b/fs/namei.c
index b815a4d2e1d6..11de7c39ff76 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -735,6 +735,11 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
                         return err;
         }
 
+        /*
+         * Rename seqlock is not required here because in the off chance
+         * of a false negative due to a concurrent rename, we're going to
+         * do the non-racy lookup, below.
+         */
         dentry = __d_lookup(nd->path.dentry, name);
         if (!dentry)
                 goto need_lookup;
@@ -754,17 +759,13 @@ need_lookup:
         mutex_lock(&dir->i_mutex);
         /*
          * First re-do the cached lookup just in case it was created
-         * while we waited for the directory semaphore..
-         *
-         * FIXME! This could use version numbering or similar to
-         * avoid unnecessary cache lookups.
-         *
-         * The "dcache_lock" is purely to protect the RCU list walker
-         * from concurrent renames at this point (we mustn't get false
-         * negatives from the RCU list walk here, unlike the optimistic
-         * fast walk).
+         * while we waited for the directory semaphore, or the first
+         * lookup failed due to an unrelated rename.
          *
-         * so doing d_lookup() (with seqlock), instead of lockfree __d_lookup
+         * This could use version numbering or similar to avoid unnecessary
+         * cache lookups, but then we'd have to do the first lookup in the
+         * non-racy way. However in the common case here, everything should
+         * be hot in cache, so would it be a big win?
          */
         dentry = d_lookup(parent, name);
         if (likely(!dentry)) {
@@ -1136,13 +1137,12 @@ static struct dentry *__lookup_hash(struct qstr *name,
                         goto out;
         }
 
-        dentry = __d_lookup(base, name);
-
-        /* lockess __d_lookup may fail due to concurrent d_move()
-         * in some unrelated directory, so try with d_lookup
+        /*
+         * Don't bother with __d_lookup: callers are for creat as
+         * well as unlink, so a lot of the time it would cost
+         * a double lookup.
          */
-        if (!dentry)
-                dentry = d_lookup(base, name);
+        dentry = d_lookup(base, name);
 
         if (dentry && dentry->d_op && dentry->d_op->d_revalidate)
                 dentry = do_revalidate(dentry, nd);