tree e6c0866fb277d403bd0913456dfec2f9f8283db9
parent 863d5709790f3dac5155cd5480803471be031746
author Miklos Szeredi <miklos@szeredi.hu> 1347913410 +0200
committer chrmhoffmann <chrmhoffmann@gmail.com> 1532289005 +0200

vfs: dcache: fix deadlock in tree traversal

commit 8110e16d42d587997bcaee0c864179e6d93603fe upstream.

IBM reported a deadlock in select_parent().  This was found to be caused
by taking rename_lock when already locked when restarting the tree
traversal.

There are two cases when the traversal needs to be restarted:

 1) concurrent d_move(); this can only happen when not already locked,
    since taking rename_lock protects against concurrent d_move().

 2) racing with final d_put() on child just at the moment of ascending
    to parent; rename_lock doesn't protect against this rare race, so it
    can happen when already locked.

Because of case 2, we need to be able to handle restarting the traversal
when rename_lock is already held.  This patch fixes all three callers of
try_to_ascend().

IBM reported that the deadlock is gone with this patch.

[ I rewrote the patch to be smaller and just do the "goto again" if the
  lock was already held, but credit goes to Miklos for the real work.
   - Linus ]

Change-Id: I5252e2ca993b49bc87c7bb01c3f6c1665ffca9f7
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>