[PATCH] r/o bind mounts: debugging for missed calls

There have been a few oopses caused by 'struct file's with NULL f_vfsmnts.
There was also a set of potentially missed mnt_want_write()s from
dentry_open() calls.

This patch provides a very simple debugging framework to catch these kinds of
bugs.  It will WARN_ON() them, but should stop us from having any oopses or
mnt_writer count imbalances.

I'm quite convinced that this is a good thing because it found bugs in the
stuff I was working on as soon as I wrote it.

[hch: made it conditional on a debug option.
      But it's still a little bit too ugly]

[hch: merged forced remount r/o fix from Dave and akpm's fix for the fix]

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 95de3102bc8..623ef24c238 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -427,6 +427,16 @@ config DEBUG_VM
 
           If unsure, say N.
 
+config DEBUG_WRITECOUNT
+        bool "Debug filesystem writers count"
+        depends on DEBUG_KERNEL
+        help
+          Enable this to catch wrong use of the writers count in struct
+          vfsmount.  This will increase the size of each file struct by
+          32 bits.
+
+          If unsure, say N.
+
 config DEBUG_LIST
         bool "Debug linked list manipulation"
         depends on DEBUG_KERNEL