fs: Use rename lock and RCU for multi-step operations

The remaining usages for dcache_lock is to allow atomic, multi-step read-side
operations over the directory tree by excluding modifications to the tree.
Also, to walk in the leaf->root direction in the tree where we don't have
a natural d_lock ordering.

This could be accomplished by taking every d_lock, but this would mean a
huge number of locks and actually gets very tricky.

Solve this instead by using the rename seqlock for multi-step read-side
operations, retry in case of a rename so we don't walk up the wrong parent.
Concurrent dentry insertions are not serialised against.  Concurrent deletes
are tricky when walking up the directory: our parent might have been deleted
when dropping locks so also need to check and retry for that.

We can also use the rename lock in cases where livelock is a worry (and it
is introduced in subsequent patch).

Signed-off-by: Nick Piggin <[email protected]>

drivers/staging/pohmelfs/path_entry.c

@@ -83,10 +83,11 @@ int pohmelfs_construct_path_string(struct pohmelfs_inode *pi, void *data, int le
 int pohmelfs_path_length(struct pohmelfs_inode *pi)
 {
 	struct dentry *d, *root, *first;
-	int len = 1; /* Root slash */
+	int len;
+	unsigned seq;
 
-	first = d = d_find_alias(&pi->vfs_inode);
-	if (!d) {
+	first = d_find_alias(&pi->vfs_inode);
+	if (!first) {
 		dprintk("%s: ino: %llu, mode: %o.\n", __func__, pi->ino, pi->vfs_inode.i_mode);
 		return -ENOENT;
 	}
@@ -95,6 +96,11 @@ int pohmelfs_path_length(struct pohmelfs_inode *pi)
 	root = dget(current->fs->root.dentry);
 	spin_unlock(&current->fs->lock);
 
+rename_retry:
+	len = 1; /* Root slash */
+	d = first;
+	seq = read_seqbegin(&rename_lock);
+	rcu_read_lock();
 	spin_lock(&dcache_lock);
 
 	if (!IS_ROOT(d) && d_unhashed(d))
@@ -105,6 +111,9 @@ int pohmelfs_path_length(struct pohmelfs_inode *pi)
 		d = d->d_parent;
 	}
 	spin_unlock(&dcache_lock);
+	rcu_read_unlock();
+	if (read_seqretry(&rename_lock, seq))
+		goto rename_retry;
 
 	dput(root);
 	dput(first);

fs/autofs4/waitq.c

@@ -186,16 +186,25 @@ static int autofs4_getpath(struct autofs_sb_info *sbi,
 {
 	struct dentry *root = sbi->sb->s_root;
 	struct dentry *tmp;
-	char *buf = *name;
+	char *buf;
 	char *p;
-	int len = 0;
-
+	int len;
+	unsigned seq;
+
+rename_retry:
+	buf = *name;
+	len = 0;
+	seq = read_seqbegin(&rename_lock);
+	rcu_read_lock();
 	spin_lock(&dcache_lock);
 	for (tmp = dentry ; tmp != root ; tmp = tmp->d_parent)
 		len += tmp->d_name.len + 1;
 
 	if (!len || --len > NAME_MAX) {
 		spin_unlock(&dcache_lock);
+		rcu_read_unlock();
+		if (read_seqretry(&rename_lock, seq))
+			goto rename_retry;
 		return 0;
 	}
 
@@ -209,6 +218,9 @@ static int autofs4_getpath(struct autofs_sb_info *sbi,
 		strncpy(p, tmp->d_name.name, tmp->d_name.len);
 	}
 	spin_unlock(&dcache_lock);
+	rcu_read_unlock();
+	if (read_seqretry(&rename_lock, seq))
+		goto rename_retry;
 
 	return len;
 }