Btrfs: send, fix race with transaction commits that create snapshots

From: Filipe Manana <[email protected]>

If we create a snapshot of a snapshot currently being used by a send operation, we can end up with send failing unexpectedly (returning -ENOENT error to user space for example). The following diagram shows how this happens.

CPU 1 CPU2 CPU3

btrfs_ioctl_send()
(...)
create_snapshot()
-> creates snapshot of a
root used by the send task
btrfs_commit_transaction()
create_pending_snapshot()
__get_inode_info()
btrfs_search_slot()

    btrfs_search_slot_get_root()
     down_read commit_root_sem

     get reference on eb of the
     commit root
      -> eb with bytenr == X

     up_read commit_root_sem

                                        btrfs_cow_block(root node)
                                         btrfs_free_tree_block()
                                          -> creates delayed ref to
                                             free the extent

                                       btrfs_run_delayed_refs()
                                        -> runs the delayed ref,
                                           adds extent to
                                           fs_info->pinned_extents

                                       btrfs_finish_extent_commit()
                                        unpin_extent_range()
                                         -> marks extent as free
                                            in the free space cache

                                      transaction commit finishes

                                                                       btrfs_start_transaction()
                                                                        (...)
                                                                        btrfs_cow_block()
                                                                         btrfs_alloc_tree_block()
                                                                          btrfs_reserve_extent()
                                                                           -> allocates extent at
                                                                              bytenr == X
                                                                          btrfs_init_new_buffer(bytenr X)
                                                                           btrfs_find_create_tree_block()
                                                                            alloc_extent_buffer(bytenr X)
                                                                             find_extent_buffer(bytenr X)
                                                                              -> returns existing eb,
                                                                                 which the send task got

                                                                        (...)
                                                                         -> modifies content of the
                                                                            eb with bytenr == X

    -> uses an eb that now
       belongs to some other
       tree and no more matches
       the commit root of the
       snapshot, resuts will be
       unpredictable

The consequences of this race can be various, and can lead to searches in the commit root performed by the send task failing unexpectedly (unable to find inode items, returning -ENOENT to user space, for example) or not failing because an inode item with the same number was added to the tree that reused the metadata extent, in which case send can behave incorrectly in the worst case or just fail later for some reason.

Fix this by performing a copy of the commit root's extent buffer when doing a search in the context of a send operation.

---
 fs/btrfs/ctree.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

index 539901fb5165..b3ab47cca384 100644
@@ -2587,11 +2587,12 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
 		/* The commit roots are read only so we always do read locks */
 		if (p->need_commit_sem)
 			down_read(&fs_info->commit_root_sem);
-		b = root->commit_root;
-		extent_buffer_get(b);
-		level = btrfs_header_level(b);
+		b = btrfs_clone_extent_buffer(root->commit_root);
 		if (p->need_commit_sem)
 			up_read(&fs_info->commit_root_sem);
+		if (!b)
+			return ERR_PTR(-ENOMEM);
+		level = btrfs_header_level(b);
 		/*
 		 * Ensure that all callers have set skip_locking when
 		 * p->search_commit_root = 1.

@@ -2717,6 +2718,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 again:
 	prev_cmp = -1;
 	b = btrfs_search_slot_get_root(root, p, write_lock_level);
+	if (IS_ERR(b)) {
+		ret = PTR_ERR(b);
+		goto done;
+	}
 
 	while (b) {
 		level = btrfs_header_level(b);

2.11.0

From: Filipe Manana <[email protected]>

If we create a snapshot of a snapshot currently being used by a send operation, we can end up with send failing unexpectedly (returning -ENOENT error to user space for example). The following diagram shows how this happens.

CPU 1 CPU2 CPU3

btrfs_ioctl_send()
(...)
create_snapshot()
-> creates snapshot of a
root used by the send task
btrfs_commit_transaction()
create_pending_snapshot()
__get_inode_info()
btrfs_search_slot()

    btrfs_search_slot_get_root()
     down_read commit_root_sem

     get reference on eb of the
     commit root
      -> eb with bytenr == X

     up_read commit_root_sem

                                        btrfs_cow_block(root node)
                                         btrfs_free_tree_block()
                                          -> creates delayed ref to
                                             free the extent

                                       btrfs_run_delayed_refs()
                                        -> runs the delayed ref,
                                           adds extent to
                                           fs_info->pinned_extents

                                       btrfs_finish_extent_commit()
                                        unpin_extent_range()
                                         -> marks extent as free
                                            in the free space cache

                                      transaction commit finishes

                                                                       btrfs_start_transaction()
                                                                        (...)
                                                                        btrfs_cow_block()
                                                                         btrfs_alloc_tree_block()
                                                                          btrfs_reserve_extent()
                                                                           -> allocates extent at
                                                                              bytenr == X
                                                                          btrfs_init_new_buffer(bytenr X)
                                                                           btrfs_find_create_tree_block()
                                                                            alloc_extent_buffer(bytenr X)
                                                                             find_extent_buffer(bytenr X)
                                                                              -> returns existing eb,
                                                                                 which the send task got

                                                                        (...)
                                                                         -> modifies content of the
                                                                            eb with bytenr == X

    -> uses an eb that now
       belongs to some other
       tree and no more matches
       the commit root of the
       snapshot, resuts will be
       unpredictable

The consequences of this race can be various, and can lead to searches in the commit root performed by the send task failing unexpectedly (unable to find inode items, returning -ENOENT to user space, for example) or not failing because an inode item with the same number was added to the tree that reused the metadata extent, in which case send can behave incorrectly in the worst case or just fail later for some reason.

Fix this by performing a copy of the commit root's extent buffer when doing a search in the context of a send operation.

---

V2: Updated comment and made it clone the extent buffer really only in the
send context, and not for every search on the commit root (all searches other from the ones done by send are well behaved hold the commit_root_sem).

 fs/btrfs/ctree.c | 29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

index 539901fb5165..99e7645ad94e 100644
@@ -2584,14 +2584,27 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
 	root_lock = BTRFS_READ_LOCK;
 
 	if (p->search_commit_root) {
-		/* The commit roots are read only so we always do read locks */
-		if (p->need_commit_sem)
+		/*
+		 * The commit roots are read only so we always do read locks,
+		 * and we always must hold the commit_root_sem when doing
+		 * searches on them, the only exception is send where we don't
+		 * want to block transaction commits for a long time, so
+		 * we need to clone the commit root in order to avoid races
+		 * with transaction commits that create a snapshot of one of
+		 * the roots used by a send operation.
+		 */
+		if (p->need_commit_sem) {
 			down_read(&fs_info->commit_root_sem);
-		b = root->commit_root;
-		extent_buffer_get(b);
-		level = btrfs_header_level(b);
-		if (p->need_commit_sem)
+			b = btrfs_clone_extent_buffer(root->commit_root);
 			up_read(&fs_info->commit_root_sem);
+			if (!b)
+				return ERR_PTR(-ENOMEM);
+
+		} else {
+			b = root->commit_root;
+			extent_buffer_get(b);
+		}
+		level = btrfs_header_level(b);
 		/*
 		 * Ensure that all callers have set skip_locking when
 		 * p->search_commit_root = 1.

@@ -2717,6 +2730,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 again:
 	prev_cmp = -1;
 	b = btrfs_search_slot_get_root(root, p, write_lock_level);
+	if (IS_ERR(b)) {
+		ret = PTR_ERR(b);
+		goto done;
+	}
 
 	while (b) {
 		level = btrfs_header_level(b);

2.11.0

Nice catch, patch added to misc-next, thanks.