4.20-stable review patch. If anyone has any objections, please let me know.
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From: Filipe Manana fdmanana@suse.com
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@vger.kernel.org # 4.4.x: 1fc28d8e2e9: Btrfs: move get root out of btrfs_search_slot to a helper CC: stable@vger.kernel.org # 4.4.x: f9ddfd0592a: Btrfs: remove unused check of skip_locking CC: stable@vger.kernel.org # 4.4.x Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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);