vfs: dcache: use DCACHE_DENTRY_KILLED instead of DCACHE_DISCONNECTED in d_kill()

IBM reported a soft lockup after applying the fix for the rename_lock
deadlock.  Commit c83ce989 ("VFS: Fix the nfs sillyrename regression
in kernel 2.6.38") was found to be the culprit.

The nfs sillyrename fix used DCACHE_DISCONNECTED to indicate that the
dentry was killed.  This flag can be set on non-killed dentries too,
which results in infinite retries when trying to traverse the dentry
tree.

This patch introduces a separate flag: DCACHE_DENTRY_KILLED, which is
only set in d_kill() and makes try_to_ascend() test only this flag.

IBM reported successful test results with this patch.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -389,7 +389,7 @@ static struct dentry *d_kill(struct dentry *dentry, struct dentry *parent)
 	 * Inform try_to_ascend() that we are no longer attached to the
 	 * dentry tree
 	 */
-	dentry->d_flags |= DCACHE_DISCONNECTED;
+	dentry->d_flags |= DCACHE_DENTRY_KILLED;
 	if (parent)
 		spin_unlock(&parent->d_lock);
 	dentry_iput(dentry);
@@ -1048,7 +1048,7 @@ static struct dentry *try_to_ascend(struct dentry *old, int locked, unsigned seq
 	 * or deletion
 	 */
 	if (new != old->d_parent ||
-		 (old->d_flags & DCACHE_DISCONNECTED) ||
+		 (old->d_flags & DCACHE_DENTRY_KILLED) ||
 		 (!locked && read_seqretry(&rename_lock, seq))) {
 		spin_unlock(&new->d_lock);
 		new = NULL;

include/linux/dcache.h

@@ -206,6 +206,8 @@ struct dentry_operations {
 #define DCACHE_MANAGED_DENTRY \
 	(DCACHE_MOUNTED|DCACHE_NEED_AUTOMOUNT|DCACHE_MANAGE_TRANSIT)
 
+#define DCACHE_DENTRY_KILLED	0x100000
+
 extern seqlock_t rename_lock;
 
 static inline int dname_external(struct dentry *dentry)