[PATCH] Fix rpc dentry list usage

This fixes the problems with NFS that got triggered by the list
poisoning.

The problem is that whereas using the dentry->d_hash list for private
purposes was previously harmless, it is not allowed with the RCU lists,
because we depend on the unhashed node continuing to point back to the
original list.

So keep a small vector of dentries around instead of using the d_hash
list to keep track of them.

net/sunrpc/rpc_pipe.c

@@ -472,30 +472,37 @@ static void
 rpc_depopulate(struct dentry *parent)
 {
 	struct inode *dir = parent->d_inode;
-	HLIST_HEAD(head);
 	struct list_head *pos, *next;
-	struct dentry *dentry;
+	struct dentry *dentry, *dvec[10];
+	int n = 0;
 
 	down(&dir->i_sem);
+repeat:
 	spin_lock(&dcache_lock);
 	list_for_each_safe(pos, next, &parent->d_subdirs) {
 		dentry = list_entry(pos, struct dentry, d_child);
+		spin_lock(&dentry->d_lock);
 		if (!d_unhashed(dentry)) {
 			dget_locked(dentry);
 			__d_drop(dentry);
-			hlist_add_head(&dentry->d_hash, &head);
-		}
+			spin_unlock(&dentry->d_lock);
+			dvec[n++] = dentry;
+			if (n == ARRAY_SIZE(dvec))
+				break;
+		} else
+			spin_unlock(&dentry->d_lock);
 	}
 	spin_unlock(&dcache_lock);
-	while (!hlist_empty(&head)) {
-		dentry = list_entry(head.first, struct dentry, d_hash);
-		/* Private list, so no dcache_lock needed and use __d_drop */
-		__d_drop(dentry);
+	if (n) {
+		do {
+			dentry = dvec[--n];
 			if (dentry->d_inode) {
 				rpc_inode_setowner(dentry->d_inode, NULL);
 				simple_unlink(dir, dentry);
 			}
 			dput(dentry);
+		} while (n);
+		goto repeat;
 	}
 	up(&dir->i_sem);
 }