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vfs: More precise tests in d_invalidate

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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>
This commit is contained in:
Eric W. Biederman 2014-02-13 07:54:28 -08:00 committed by Al Viro
parent 3ccb354d64
commit bafc9b754f
1 changed files with 4 additions and 34 deletions
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36
fs/dcache.c
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@ -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);