ext4: fix races in ext4_sync_parent()

Fix problems if fsync() races against a rename of a parent directory
as pointed out by Al Viro in his own inimitable way:

>While we are at it, could somebody please explain what the hell is ext4
>doing in
>static int ext4_sync_parent(struct inode *inode)
>{
>        struct writeback_control wbc;
>        struct dentry *dentry = NULL;
>        int ret = 0;
>
>        while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
>                ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
>                dentry = list_entry(inode->i_dentry.next,
>                                    struct dentry, d_alias);
>                if (!dentry || !dentry->d_parent || !dentry->d_parent->d_inode)
>                        break;
>                inode = dentry->d_parent->d_inode;
>                ret = sync_mapping_buffers(inode->i_mapping);
>                ...
>Note that dentry obviously can't be NULL there.  dentry->d_parent is never
>NULL.  And dentry->d_parent would better not be negative, for crying out
>loud!  What's worse, there's no guarantees that dentry->d_parent will
>remain our parent over that sync_mapping_buffers() *and* that inode won't
>just be freed under us (after rename() and memory pressure leading to
>eviction of what used to be our dentry->d_parent)......

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/fsync.c

@@ -129,15 +129,30 @@ static int ext4_sync_parent(struct inode *inode)
 {
 	struct writeback_control wbc;
 	struct dentry *dentry = NULL;
+	struct inode *next;
 	int ret = 0;
 
-	while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
+	if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
+		return 0;
+	inode = igrab(inode);
+	while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
 		ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
-		dentry = list_entry(inode->i_dentry.next,
-				    struct dentry, d_alias);
-		if (!dentry || !dentry->d_parent || !dentry->d_parent->d_inode)
+		dentry = NULL;
+		spin_lock(&inode->i_lock);
+		if (!list_empty(&inode->i_dentry)) {
+			dentry = list_first_entry(&inode->i_dentry,
+						  struct dentry, d_alias);
+			dget(dentry);
+		}
+		spin_unlock(&inode->i_lock);
+		if (!dentry)
+			break;
+		next = igrab(dentry->d_parent->d_inode);
+		dput(dentry);
+		if (!next)
 			break;
-		inode = dentry->d_parent->d_inode;
+		iput(inode);
+		inode = next;
 		ret = sync_mapping_buffers(inode->i_mapping);
 		if (ret)
 			break;
@@ -148,6 +163,7 @@ static int ext4_sync_parent(struct inode *inode)
 		if (ret)
 			break;
 	}
+	iput(inode);
 	return ret;
 }