Documentation/filesystems/vfs.txt: document how ->i_link works

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/vfs.txt

@@ -463,6 +463,12 @@ otherwise noted.
 	argument.  If request can't be handled without leaving RCU mode,
 	have it return ERR_PTR(-ECHILD).
 
+	If the filesystem stores the symlink target in ->i_link, the
+	VFS may use it directly without calling ->get_link(); however,
+	->get_link() must still be provided.  ->i_link must not be
+	freed until after an RCU grace period.  Writing to ->i_link
+	post-iget() time requires a 'release' memory barrier.
+
   readlink: this is now just an override for use by readlink(2) for the
 	cases when ->get_link uses nd_jump_link() or object is not in
 	fact a symlink.  Normally filesystems should only implement