Re: 3.12 Regression: dcache: Translating dentry into pathname without taking rename_lock 232d2d60

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Nov 13, 2013 at 03:34:13AM -0800, Michael Marineau wrote:
> Greetings,
> 
> Commit 232d2d60aa5469bb097f55728f65146bd49c1d25 causes intermittent
> errors in /proc/*/fd/* where readlink returns "/" instead of the
> correct path. This can be reproduced by the script below which copies
> the kernel source directory structure while obsessively looking up
> directory fds in proc from another process. Reverting
> 232d2d60aa5469bb097f55728f65146bd49c1d25 after two related commits
> 48f5ec21d9c67e881ff35343988e290ef5cf933f
> 1812997720ab90d029548778c55d7315555e1fef fixes the issue.

Looking into it...  It seems that we are getting to the end of
prepend_path() with non-negative error and bptr == *buffer.
What the...

OK, I see what's going on.  We never reinitialize dentry, vfsmount and mnt
if we decide to restart.  See if the following helps:

diff --git a/fs/dcache.c b/fs/dcache.c
index ae6ebb8..89f9671 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2881,9 +2881,9 @@ static int prepend_path(const struct path *path,
 			const struct path *root,
 			char **buffer, int *buflen)
 {
-	struct dentry *dentry = path->dentry;
-	struct vfsmount *vfsmnt = path->mnt;
-	struct mount *mnt = real_mount(vfsmnt);
+	struct dentry *dentry;
+	struct vfsmount *vfsmnt;
+	struct mount *mnt;
 	int error = 0;
 	unsigned seq = 0;
 	char *bptr;
@@ -2893,6 +2893,9 @@ static int prepend_path(const struct path *path,
 restart:
 	bptr = *buffer;
 	blen = *buflen;
+	dentry = path->dentry;
+	vfsmnt = path->mnt;
+	mnt = real_mount(vfsmnt);
 	read_seqbegin_or_lock(&rename_lock, &seq);
 	while (dentry != root->dentry || vfsmnt != root->mnt) {
 		struct dentry * parent;