Unionfs: document lockdep issues

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/Documentation/filesystems/unionfs/issues.txt b/Documentation/filesystems/unionfs/issues.txt
index 6101ebf1fbe0dd37d2c582142cff4406f0042089..c3a0aef23bc932d57bdc787f2182b56403edae85 100644 (file)
--- a/Documentation/filesystems/unionfs/issues.txt
+++ b/Documentation/filesystems/unionfs/issues.txt
@@ -8,5 +8,17 @@ KNOWN Unionfs 2.1 ISSUES:
    currently introducing VFS changes to fs/namei.c's do_path_lookup() to
    allow proper file lookup and opening in stackable file systems.
 
+2. Lockdep (a debugging feature) isn't aware of stacking, and so it
+   incorrectly complains about locking problems.  The problem boils down to
+   this: Lockdep considers all objects of a certain type to be in the same
+   class, for example, all inodes.  Lockdep doesn't like to see a lock held
+   on two inodes within the same task, and warns that it could lead to a
+   deadlock.  However, stackable file systems do precisely that: they lock
+   an upper object, and then a lower object, in a strict order to avoid
+   locking problems; in addition, Unionfs, as a fan-out file system, may
+   have to lock several lower inodes.  We are currently looking into Lockdep
+   to see how to make it aware of stackable file systems.  In the mean time,
+   if you get any warnings from Lockdep, you can safely ignore them (or feel
+   free to report them to the Unionfs maintainers, just to be sure).
 
 For more information, see <http://unionfs.filesystems.org/>.