Commit bafc9b75 authored Feb 13, 2014 by Eric W. Biederman Committed by Al Viro Oct 09, 2014

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>

parent 3ccb354d

fs/dcache.c

+4 −34

Original line number	Original line	Diff line number	Diff line
	@@ -650,9 +650,8 @@ EXPORT_SYMBOL(dput);
	* @dentry: dentry to invalidate		* @dentry: dentry to invalidate
	*		*
	* Try to invalidate the dentry if it turns out to be		* Try to invalidate the dentry if it turns out to be
	* possible. If there are other dentries that can be		* possible. If there are reasons not to delete it
	* reached through this one we can't delete it and we		* return -EBUSY. On success return 0.
	* return -EBUSY. On success we return 0.
	*		*
	* no dcache lock.		* no dcache lock.
	*/		*/
	@@ -667,38 +666,9 @@ int d_invalidate(struct dentry * dentry)
	spin_unlock(&dentry->d_lock);		spin_unlock(&dentry->d_lock);
	return 0;		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);		spin_unlock(&dentry->d_lock);
	shrink_dcache_parent(dentry);
	spin_lock(&dentry->d_lock);
	}

	/*		return check_submounts_and_drop(dentry);
	* 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;
	}		}
	EXPORT_SYMBOL(d_invalidate);		EXPORT_SYMBOL(d_invalidate);