fs: add a kerneldoc header over lookup_fast

The lookup_fast helper in fs/namei.c has some subtlety in how dentries
are returned. Document them.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240802-openfast-v1-2-a1cff2a33063@kernel.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/namei.c

@@ -1639,6 +1639,20 @@ struct dentry *lookup_one_qstr_excl(const struct qstr *name,
 }
 EXPORT_SYMBOL(lookup_one_qstr_excl);
 
+/**
+ * lookup_fast - do fast lockless (but racy) lookup of a dentry
+ * @nd: current nameidata
+ *
+ * Do a fast, but racy lookup in the dcache for the given dentry, and
+ * revalidate it. Returns a valid dentry pointer or NULL if one wasn't
+ * found. On error, an ERR_PTR will be returned.
+ *
+ * If this function returns a valid dentry and the walk is no longer
+ * lazy, the dentry will carry a reference that must later be put. If
+ * RCU mode is still in force, then this is not the case and the dentry
+ * must be legitimized before use. If this returns NULL, then the walk
+ * will no longer be in RCU mode.
+ */
 static struct dentry *lookup_fast(struct nameidata *nd)
 {
 	struct dentry *dentry, *parent = nd->path.dentry;