[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, 11 insertions, 1 deletions

diff --git a/fs/open.c b/fs/open.c
index e58382d57e72..b70e7666bb2c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -806,6 +806,8 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
                 error = __get_file_write_access(inode, mnt);
                 if (error)
                         goto cleanup_file;
+                if (!special_file(inode->i_mode))
+                        file_take_write(f);
         }
 
         f->f_mapping = inode->i_mapping;
@@ -847,8 +849,16 @@ cleanup_all:
         fops_put(f->f_op);
         if (f->f_mode & FMODE_WRITE) {
                 put_write_access(inode);
-                if (!special_file(inode->i_mode))
+                if (!special_file(inode->i_mode)) {
+                        /*
+                         * We don't consider this a real
+                         * mnt_want/drop_write() pair
+                         * because it all happenend right
+                         * here, so just reset the state.
+                         */
+                        file_reset_write(f);
                         mnt_drop_write(mnt);
+                }
         }
         file_kill(f);
         f->f_path.dentry = NULL;