[PATCH] Add lookup_instantiate_filp usage warning

I think it would be nice to put an usage warning in header of
lookup_instantiate_filp() to indicate it is unsafe to use it on anything
but regular files (even that is potentially unsafe, but there your ->open()
is usually in your hands anyway), so that others won't fall into the same
trap I did.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/open.c

diff --git a/fs/open.c b/fs/open.c
index 1091dad..7d02d19 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -890,6 +890,10 @@
  * a fully instantiated struct file to the caller.
  * This function is meant to be called from within a filesystem's
  * lookup method.
+ * Beware of calling it for non-regular files! Those ->open methods might block
+ * (e.g. in fifo_open), leaving you with parent locked (and in case of fifo,
+ * leading to a deadlock, as nobody can open that fifo anymore, because
+ * another process to open fifo will block on locked parent when doing lookup).
  * Note that in case of error, nd->intent.open.file is destroyed, but the
  * path information remains valid.
  * If the open callback is set to NULL, then the standard f_op->open()