vfs: More precise tests in d_invalidate

The current comments in d_invalidate about what and why it is doing
what it is doing are wildly off-base.  Which is not surprising as
the comments date back to last minute bug fix of the 2.2 kernel.

The big fat lie of a comment said: If it's a directory, we can't drop
it for fear of somebody re-populating it with children (even though
dropping it would make it unreachable from that root, we still might
repopulate it if it was a working directory or similar).

[AV] What we really need to avoid is multiple dentry aliases of the
same directory inode; on all filesystems that have ->d_revalidate()
we either declare all positive dentries always valid (and thus never
fed to d_invalidate()) or use d_materialise_unique() and/or d_splice_alias(),
which take care of alias prevention.

The current rules are:
- To prevent mount point leaks dentries that are mount points or that
  have childrent that are mount points may not be be unhashed.
- All dentries may be unhashed.
- Directories may be rehashed with d_materialise_unique

check_submounts_and_drop implements this already for well maintained
remote filesystems so implement the current rules in d_invalidate
by just calling check_submounts_and_drop.

The one difference between d_invalidate and check_submounts_and_drop
is that d_invalidate must respect it when a d_revalidate method has
earlier called d_drop so preserve the d_unhashed check in
d_invalidate.

Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 4 insertions, 34 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 1f8e6acb0ea4..8150e4e9e88b 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -650,9 +650,8 @@ EXPORT_SYMBOL(dput);
  * @dentry: dentry to invalidate
  *
  * Try to invalidate the dentry if it turns out to be
- * possible. If there are other dentries that can be
- * reached through this one we can't delete it and we
- * return -EBUSY. On success we return 0.
+ * possible. If there are reasons not to delete it
+ * return -EBUSY. On success return 0.
  *
  * no dcache lock.
  */
@@ -667,38 +666,9 @@ int d_invalidate(struct dentry * dentry)
                 spin_unlock(&dentry->d_lock);
                 return 0;
         }
-        /*
-         * Check whether to do a partial shrink_dcache
-         * to get rid of unused child entries.
-         */
-        if (!list_empty(&dentry->d_subdirs)) {
-                spin_unlock(&dentry->d_lock);
-                shrink_dcache_parent(dentry);
-                spin_lock(&dentry->d_lock);
-        }
-
-        /*
-         * Somebody else still using it?
-         *
-         * If it's a directory, we can't drop it
-         * for fear of somebody re-populating it
-         * with children (even though dropping it
-         * would make it unreachable from the root,
-         * we might still populate it if it was a
-         * working directory or similar).
-         * We also need to leave mountpoints alone,
-         * directory or not.
-         */
-        if (dentry->d_lockref.count > 1 && dentry->d_inode) {
-                if (S_ISDIR(dentry->d_inode->i_mode) || d_mountpoint(dentry)) {
-                        spin_unlock(&dentry->d_lock);
-                        return -EBUSY;
-                }
-        }
-
-        __d_drop(dentry);
         spin_unlock(&dentry->d_lock);
-        return 0;
+
+        return check_submounts_and_drop(dentry);
 }
 EXPORT_SYMBOL(d_invalidate);