From 036a9348dcd0060e3df8f5b5db2a59e7d7eaf1f5 Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Mon, 23 Nov 2015 15:25:16 +0000 Subject: [PATCH] Btrfs: fix race between cleaner kthread and space cache writeout When a block group becomes unused and the cleaner kthread is currently running, we can end up getting the current transaction aborted with error -ENOENT when we try to commit the transaction, leading to the following trace: [59779.258768] WARNING: CPU: 3 PID: 5990 at fs/btrfs/extent-tree.c:3740 btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs]() [59779.272594] BTRFS: Transaction aborted (error -2) (...) [59779.291137] Call Trace: [59779.291621] [] dump_stack+0x4e/0x79 [59779.292543] [] warn_slowpath_common+0x9f/0xb8 [59779.293435] [] ? btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs] [59779.295000] [] warn_slowpath_fmt+0x48/0x50 [59779.296138] [] ? write_one_cache_group.isra.32+0x77/0x82 [btrfs] [59779.297663] [] btrfs_write_dirty_block_groups+0x17c/0x214 [btrfs] [59779.299141] [] commit_cowonly_roots+0x1de/0x261 [btrfs] [59779.300359] [] btrfs_commit_transaction+0x4c4/0x99c [btrfs] [59779.301805] [] btrfs_sync_fs+0x145/0x1ad [btrfs] [59779.302893] [] sync_filesystem+0x7f/0x93 (...) [59779.318186] ---[ end trace 577e2daff90da33a ]--- The following diagram illustrates a sequence of steps leading to this problem: CPU 1 CPU 2 adds bg A to list fs_info->unused_bgs adds bg B to list fs_info->unused_bgs cleaner kthread delete_unused_bgs() sees bg A in list fs_info->unused_bgs btrfs_start_transaction() deletes bg A update_block_group(bg C) --> adds bg C to list fs_info->unused_bgs deletes bg B sees bg C in the list fs_info->unused_bgs btrfs_remove_chunk(bg C) btrfs_remove_block_group(bg C) --> checks if the block group is in a dirty list, and because it isn't now, it does nothing --> the block group item is deleted from the extent tree --> adds bg C to list transaction->dirty_bgs some task calls btrfs_commit_transaction(t N + 1) commit_cowonly_roots() btrfs_write_dirty_block_groups() --> sees bg C in cur_trans->dirty_bgs --> calls write_one_cache_group() which returns -ENOENT because it did not find the block group item in the extent tree --> transaction aborte with -ENOENT because write_one_cache_group() returned that error So fix this by adding a block group to the list of dirty block groups before adding it to the list of unused block groups. This happened on a stress test using fsstress plus concurrent calls to fallocate 20G and truncate (releasing part of the space allocated with fallocate). Signed-off-by: Filipe Manana Signed-off-by: Chris Mason --- fs/btrfs/extent-tree.c | 29 ++++++++++++++++------------- 1 file changed, 16 insertions(+), 13 deletions(-) diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 8fd14b6f1d33..e563e5ab72b4 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -5915,19 +5915,6 @@ static int update_block_group(struct btrfs_trans_handle *trans, set_extent_dirty(info->pinned_extents, bytenr, bytenr + num_bytes - 1, GFP_NOFS | __GFP_NOFAIL); - /* - * No longer have used bytes in this block group, queue - * it for deletion. - */ - if (old_val == 0) { - spin_lock(&info->unused_bgs_lock); - if (list_empty(&cache->bg_list)) { - btrfs_get_block_group(cache); - list_add_tail(&cache->bg_list, - &info->unused_bgs); - } - spin_unlock(&info->unused_bgs_lock); - } } spin_lock(&trans->transaction->dirty_bgs_lock); @@ -5939,6 +5926,22 @@ static int update_block_group(struct btrfs_trans_handle *trans, } spin_unlock(&trans->transaction->dirty_bgs_lock); + /* + * No longer have used bytes in this block group, queue it for + * deletion. We do this after adding the block group to the + * dirty list to avoid races between cleaner kthread and space + * cache writeout. + */ + if (!alloc && old_val == 0) { + spin_lock(&info->unused_bgs_lock); + if (list_empty(&cache->bg_list)) { + btrfs_get_block_group(cache); + list_add_tail(&cache->bg_list, + &info->unused_bgs); + } + spin_unlock(&info->unused_bgs_lock); + } + btrfs_put_block_group(cache); total -= num_bytes; bytenr += num_bytes; -- 2.30.2