4.20-stable review patch. If anyone has any objections, please let me know.
------------------
From: Filipe Manana <fdmanana@xxxxxxxx>
commit be6821f82c3cc36e026f5afd10249988852b35ea upstream.
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.
CC: stable@xxxxxxxxxxxxxxx # 4.4.x: 1fc28d8e2e9: Btrfs: move get root out of btrfs_search_slot to a helper
CC: stable@xxxxxxxxxxxxxxx # 4.4.x: f9ddfd0592a: Btrfs: remove unused check of skip_locking
CC: stable@xxxxxxxxxxxxxxx # 4.4.x
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/btrfs/ctree.c | 29 +++++++++++++++++++++++------
1 file changed, 23 insertions(+), 6 deletions(-)
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2584,14 +2584,27 @@ static struct extent_buffer *btrfs_searc
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
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);
