deal with deadlock in d_walk()

commit ca5358ef upstream.

... by not hitting rename_retry for reasons other than rename having
happened.  In other words, do _not_ restart when finding that
between unlocking the child and locking the parent the former got
into __dentry_kill().  Skip the killed siblings instead...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Moritz Muehlenhoff <jmm@inutil.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/dcache.c

@@ -466,7 +466,7 @@ static void __dentry_kill(struct dentry *dentry)
 	}
 	/* if it was on the hash then remove it */
 	__d_drop(dentry);
-	list_del(&dentry->d_child);
+	__list_del_entry(&dentry->d_child);
 	/*
 	 * Inform d_walk() that we are no longer attached to the
 	 * dentry tree
@@ -1120,33 +1120,31 @@ static void d_walk(struct dentry *parent, void *data,
 	/*
 	 * All done at this level ... ascend and resume the search.
 	 */
+	rcu_read_lock();
+ascend:
 	if (this_parent != parent) {
 		struct dentry *child = this_parent;
 		this_parent = child->d_parent;
 
-		rcu_read_lock();
 		spin_unlock(&child->d_lock);
 		spin_lock(&this_parent->d_lock);
 
-		/*
-		 * might go back up the wrong parent if we have had a rename
-		 * or deletion
-		 */
-		if (this_parent != child->d_parent ||
-			 (child->d_flags & DCACHE_DENTRY_KILLED) ||
-			 need_seqretry(&rename_lock, seq)) {
-			spin_unlock(&this_parent->d_lock);
-			rcu_read_unlock();
+		/* might go back up the wrong parent if we have had a rename. */
+		if (need_seqretry(&rename_lock, seq))
 			goto rename_retry;
+		next = child->d_child.next;
+		while (unlikely(child->d_flags & DCACHE_DENTRY_KILLED)) {
+			if (next == &this_parent->d_subdirs)
+				goto ascend;
+			child = list_entry(next, struct dentry, d_child);
+			next = next->next;
 		}
 		rcu_read_unlock();
-		next = child->d_child.next;
 		goto resume;
 	}
-	if (need_seqretry(&rename_lock, seq)) {
-		spin_unlock(&this_parent->d_lock);
+	if (need_seqretry(&rename_lock, seq))
 		goto rename_retry;
-	}
+	rcu_read_unlock();
 	if (finish)
 		finish(data);
 
@@ -1156,6 +1154,9 @@ static void d_walk(struct dentry *parent, void *data,
 	return;
 
 rename_retry:
+	spin_unlock(&this_parent->d_lock);
+	rcu_read_unlock();
+	BUG_ON(seq & 1);
 	if (!retry)
 		return;
 	seq = 1;