linux-kernel - [PATCH 02/12] vfs: More precise tests in d

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Message-ID: <87sir75yck.fsf_-_@xmission.com>
Date:	Tue, 25 Feb 2014 01:35:23 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: [PATCH 02/12] 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).

The truth is that for remote filesystems the failure of d_revalidate
and d_weak_revalidate prevents us from populating or otherwise
inappropriately using a directory.  For local filesystems and for
local directory removals the setting of S_DEAD prevents us from
populating or otherwise inappropriate using a directory.

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@...redi.hu>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/dcache.c |   38 ++++----------------------------------
 1 files changed, 4 insertions(+), 34 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c71c86f7780e..b0add629f5fe 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -613,9 +613,8 @@ 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.
  */
@@ -630,38 +629,9 @@ 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);
 
-- 
1.7.5.4

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