From f3e3d9cc35252a70a2fd698762c9687718268ec6 Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Mon, 13 Jul 2020 09:03:20 +0800 Subject: [PATCH] btrfs: avoid possible signal interruption of btrfs_drop_snapshot() on relocation tree [BUG] There is a bug report about bad signal timing could lead to read-only fs during balance: BTRFS info (device xvdb): balance: start -d -m -s BTRFS info (device xvdb): relocating block group 73001861120 flags metadata BTRFS info (device xvdb): found 12236 extents, stage: move data extents BTRFS info (device xvdb): relocating block group 71928119296 flags data BTRFS info (device xvdb): found 3 extents, stage: move data extents BTRFS info (device xvdb): found 3 extents, stage: update data pointers BTRFS info (device xvdb): relocating block group 60922265600 flags metadata BTRFS: error (device xvdb) in btrfs_drop_snapshot:5505: errno=-4 unknown BTRFS info (device xvdb): forced readonly BTRFS info (device xvdb): balance: ended with status: -4 [CAUSE] The direct cause is the -EINTR from the following call chain when a fatal signal is pending: relocate_block_group() |- clean_dirty_subvols() |- btrfs_drop_snapshot() |- btrfs_start_transaction() |- btrfs_delayed_refs_rsv_refill() |- btrfs_reserve_metadata_bytes() |- __reserve_metadata_bytes() |- wait_reserve_ticket() |- prepare_to_wait_event(); |- ticket->error = -EINTR; Normally this behavior is fine for most btrfs_start_transaction() callers, as they need to catch any other error, same for the signal, and exit ASAP. However for balance, especially for the clean_dirty_subvols() case, we're already doing cleanup works, getting -EINTR from btrfs_drop_snapshot() could cause a lot of unexpected problems. From the mentioned forced read-only report, to later balance error due to half dropped reloc trees. [FIX] Fix this problem by using btrfs_join_transaction() if btrfs_drop_snapshot() is called from relocation context. Since btrfs_join_transaction() won't get interrupted by signal, we can continue the cleanup. CC: stable@vger.kernel.org # 5.4+ Reviewed-by: Josef Bacik Signed-off-by: Qu Wenruo Reviewed-by: David Sterba 3 Signed-off-by: David Sterba --- fs/btrfs/extent-tree.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 9622381..61ede33 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -5298,7 +5298,14 @@ int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref, int for_reloc) goto out; } - trans = btrfs_start_transaction(tree_root, 0); + /* + * Use join to avoid potential EINTR from transaction start. See + * wait_reserve_ticket and the whole reservation callchain. + */ + if (for_reloc) + trans = btrfs_join_transaction(tree_root); + else + trans = btrfs_start_transaction(tree_root, 0); if (IS_ERR(trans)) { err = PTR_ERR(trans); goto out_free; -- 2.7.4